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Canon EF 800mm f5.6L IS USM – DSLR Camera Lens

R238,238.00

Canon EF 600mm f4L IS III USM – DSLR Camera Lens

R236,188.00

Canon RF 400mm f/2.8L IS USM – Mirrorless Camera Lens

R220,798.00

Canon EF 400mm f2.8L IS III USM – DSLR Camera Lens

R220,798.00

Canon RF 600mm f/4L IS USM – Mirrorless Camera Lens

R214,578.00

Canon EF 200-400mm f4 L IS USM + Extender 1.4x – DSLR Camera Lens

R195,998.00

Canon EF 500mm f4 L IS II USM – DSLR Camera Lens

R169,508.00

Canon EF 400mm f4 DO IS II USM – DSLR Camera Lens

R117,738.00

Canon EF 300mm f2.8L IS II USM – DSLR Camera Lens

R110,278.00

Fujifilm XF 200mm f2 R LM OIS WR + XF 1.4x TC F2 WR Teleconverter – Mirrorless Camera Lens

R100,558.00

Canon CN-E18-80 T4.4L IS – Cinema Camera Lens

R90,504.00

Sigma 500mm F4 DG OS HSM Nikon Sport – DSLR Camera Lens

R88,158.00

Sigma 500mm F4 DG OS HSM Sport For Canon – DSLR Camera Lens

R88,158.00

Canon RF 85mm F1.2 L USM DS – Mirrorless Camera Lens

R65,978.00

Canon RF 28-70mm f2L USM – Mirrorless Camera Lens

R60,258.00

Canon EF 11-24mm f4 L USM – DSLR Camera Lens

R60,258.00

Canon RF 85mm f1.2L USM – Mirrorless Camera Lens

R55,978.00

Canon RF 100-500mm f/4.5-7.1L IS USM – Mirrorless Camera Lens

R55,968.00

Canon RF 70-200mm F2.8 L IS USM – Mirrorless Camera Lens

R54,548.00

Sigma 120-300mm f2.8 DG OS HSM For Nikon – DSLR Camera Lens

R54,098.00

Camera Lenses
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A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

There is no major difference in principle between a lens used for a still camera, a video camera, a telescope, a microscope, or other apparatus, but the details of design and construction are different. A lens might be permanently fixed to a camera, or it might be interchangeable with lenses of different focal lengths, apertures, and other properties.

While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these and produce a design that is suitable for photographic use and possibly mass production.

Interchangeable lenses

Canon EF and Nikon F lens mounts are rivalling
Canon EF
Nikon F
Most SLR and DSLR cameras provide the option of changing the lens. This enables the use of lens that are best suited for the current photographic need, and allows the attachment of specialized lenses. Film SLR cameras have existed since the late 1950s, and over the years a very large number of different lenses have been produced, both by camera manufacturers (who typically only make lenses intended for their own camera bodies) and by third-party optics companies who may make lenses for several different camera lines.

DSLRs became affordable around the mid-1990s, and have become extremely popular in recent years. Some manufacturers, for example Minolta, Canon and Nikon, chose to make their DSLRs 100% compatible with their existing SLR lenses in the beginning, allowing owners of new DSLRs to continue to use their existing lenses and get a longer lifespan from their investment. Others, for example Olympus, chose to create a completely new lens mount and series of lenses for their DSLRs. The Pentax SLR camera K-mount system is backward compatible to all previous lens generations from Pentax, including the latest digital SLRs like the K-3 and K-50. A Pentax K-mount lens from the early 70s can be used on the newest Pentax DSLR although it may not provide features that are included in newer lenses (e.g. autofocus). There are a few exceptions from the MZ and ZX series of Pentax film cameras that do not work with some of the older lenses.[1]

As implied by the above, lenses are only directly interchangeable within the “mount system” for which they are built. Mixing mounting systems requires an adapter, and most often results in compromises such as loss of functionality (e.g. lack of autofocus or automatic aperture control). Further, in some cases the adapter will require an additional optical element to correct for varied registration distances (the distance from the rear of the mount to the focal plane on the image sensor or film). Adapters may not be available to bridge every combination of lens mount and camera mount.

Aperture and depth of field

Large (1) and small (2) aperture
Main articles: Aperture, f-number, and Depth of field
The aperture of a lens is the opening that regulates the amount of light that passes through the lens. It is controlled by a diaphragm inside the lens, which is in turn controlled either manually or by the exposure circuitry in the camera body.

The relative aperture is specified as an f-number, the ratio of the lens focal length to its effective aperture diameter. A small f-number like f/2.0 indicates a large aperture (more light passes through), while a large f-number like f/22 indicates a small aperture (little light passes through). Aperture settings are usually not continuously variable; instead the diaphragm has typically 5–10 discrete settings. The normal “full-stop” f-number scale for modern lenses is as follows: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, but many lenses also allow setting to half-stop or third-stop increments. A “slow” lens (one that is not capable of passing a lot of light through) might have a maximum aperture from 5.6 to 11, while a “fast” lens (one that can pass more light through) might have a maximum aperture from 1 to 4. Fast lenses are by definition larger than slow lenses (for comparable focal length), and typically cost more.[2] The aperture affects not only the amount of light that passes through the lens, but also the depth of field of the resulting image: a larger aperture (a smaller f-number, e.g. f/2.0) will have a shallow depth of field, while a smaller aperture (a larger f-number, e.g. f/11) will have a greater depth of field.[3]

Focal length and angle of view
Main articles: Focal length and Angle of view
The focal length of a lens, together with the size of the image sensor in the camera (or size of the 35 mm film), determines the angle of view. A lens is considered to be a “normal lens”, in terms of its angle of view on a camera, when its focal length is approximately equal to the diagonal dimension of the film format or image sensor format.[4] The resulting diagonal angle of view of about 53 degrees is often said to approximate the angle of human vision; since the angle of view of a human eye is at least 140 degrees,[5] more careful authors will qualify that, for example as “similar to the angle of crisp human vision.”[6] A wide-angle lens has a shorter focal length, and includes more of the viewed scene than a normal lens; a telephoto lens has a longer focal length, and images a small portion of the scene, making it seem closer.

Lenses are not labeled or sold according to their angle of view, but rather by their focal length, usually expressed in millimeters. But this specification is insufficient to compare lenses for different cameras because field of view also depends on the sensor size. For example, a 50 mm lens mounted on a Nikon D3 (a full-frame camera) provides approximately the same field of view as a 32 mm lens mounted on a Sony α100 (an APS-C camera). Conversely, the same lens can produce different fields of view when mounted on different cameras. For example, a 35 mm lens mounted on a Canon EOS 5D (full-frame) provides a slightly wide-angle view, while the same lens mounted on a Canon EOS 400D (APS-C) provides a “normal” or slightly telephoto view.

In order to make it easier to compare lens–camera pairs, it is common to talk about their 35 mm equivalent focal length. For example, when talking about a 14 mm lens for a Four Thirds System camera, one would not only indicate that it had a focal length of 14 mm, but also that its “35 mm equivalent focal length” is 28 mm. This way of talking about lenses is not just limited to SLR and DSLR lenses; it is very common to see this focal length equivalency in the specification of the lens on a digicam.

Values in the following table are approximate, and apply to rectilinear lenses only, not to fisheye lenses.
Types of lenses
Zoom lenses

Nikkor 28–200 mm zoom lens, extended to 200 mm at left and collapsed to 28 mm focal length at right
Main article: Zoom lens
The focal length of a zoom lens is not fixed; instead it can be varied between a specified minimum and maximum value. Modern lens technology is such that the loss of image quality in zoom lenses (relative to non-zoom lenses) is minimal, and zoom lenses have become the standard lenses for SLRs and DSLRs. This is different from the late 1980s when, due to image quality concerns, most professional photographers still relied primarily on standard non-zoom lenses. However, zoom lenses still typically have a lower maximum aperture than fixed-focal (“prime”) lenses for the same weight and cost, especially for shorter focal lengths.

Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or “4×” zoom. Typical zoom lenses cover a 3.5× range, for example 24–90 mm (standard zoom) or 60–200 mm (telephoto zoom). “Super-zoom” lenses with a range of 10× or even 14× are becoming more common, although the image quality does typically suffer a bit compared with the more traditional zooms.

The maximum aperture for a zoom lens may be the same (constant) for all focal lengths, but it is more common that the maximum aperture is greater at the wide-angle end than at the telephoto end of the zoom range. For example, a 100 mm to 400 mm lens may have a maximum aperture of f/4.0 at the 100 mm end but will diminish to only f/5.6 at the 400 mm end of the zoom range. Zoom lenses with constant maximum apertures (such as f/2.8 for a 24–70mm lens) are usually reserved for lenses with higher build quality and are thus more expensive than those with variable maximum apertures.

Macro lenses
Main article: Macro photography
Macro lenses are designed for extreme closeup work. Such lenses are popular for nature shooting such as small flowers, as well as for many technical applications. As most of these lenses can also focus to infinity and tend to be quite sharp, many are used as general-purpose optics.

Special purpose lenses
See also: Photographic lens § Special-purpose

Special-purpose perspective control lens for architectural photographs
Most users of SLR and DSLR cameras stick to using zoom lenses, while a few of the more adventurous amateurs and many professional photographers also invest in a few prime lenses. Special purpose lenses are, as the designation implies, for special purposes, and are not so common.

There are many different kinds of special purpose lenses, the most popular being fisheye lenses, which are extreme wide-angle lenses with an angle of view of up to 180 degrees or more, with very noticeable (sometimes intended) distortion. Perspective control lenses and soft-focus lenses, were more popular with film SLRs but are less popular for DSLRs because the same or similar results can be obtained with post-processing software.

Automatic focus

Electronics of a Canon EF-S lens
Main article: Autofocus
Almost all modern lenses for SLRs and DSLRs provide automatic focus. The autofocus sensor(s) and electronics are actually in the camera body, and this circuitry provides electrical power and signals to a motor inside the lens that adjusts the focus. (Some older autofocus systems are based on a motor in the camera body and using a mechanical connection to the focus mechanism in the lens.)

There are two different kinds of in-lens electronic focus drive motors currently in use, the traditional servo motor and the more modern “ultrasonic” drive systems. These ultrasonic drives go by different names according to the manufacturer, for example USM (Canon), AF-S/Silent Wave (Nikon), Super Sonicwave Motor/SSM (Sony), Supersonic Wave Drive (Olympus), Extra Silent Motor (Panasonic/Leica), Supersonic Drive Motor (Pentax), and Hypersonic Motor/HSM (Sigma). These ultrasonic focus drives typically provide faster focusing than the non-ultrasonic drives, as well as being practically silent and using less battery power.[7]

Image stabilization
Image stabilization is a technique used to reduce image blur caused by the camera not being held steady. There are two kinds of image stabilization used in SLR and DSLR cameras and their lenses:

In-body image stabilization is implemented by moving the image sensor in an attempt to counteract the sensed motion of the camera. The advantage of this technique is that it works for all lenses mounted on the camera, at least if the camera electronics are aware of the lens’ focal length. This is most commonly done automatically, but some cameras (such as all Olympus bodies with IS) allow the user to input the focal length manually for use with lenses with no electronic coupling. In-body image stabilization is used in modern Olympus, Sony, and Pentax cameras.
In-lens image stabilization, also known as optical image stabilization, is implemented in the lens itself and moves the lens elements in an attempt to counteract the sensed motion of the camera. The inherent advantage of this kind of image stabilization is that it steadies the viewfinder image, allowing for more accurate framing and autofocus. The disadvantage is that you have to pay the extra cost for every lens you buy for which you want image stabilization.[8] Panasonic, Canon, and Nikon use lens-based image stabilization. Some third-party lenses from Sigma and Tamron also have lens-based IS systems.
Modern advanced cameras use not one image stabilization, but more than one and it is called as Hybrid System Image Stabization. One for fast focusing, one for accurate good image quality and the other one for take video while walking.

The effectiveness of image stabilization systems varies somewhat from implementation to another, but there seems to be no inherent superiority to either lens-based or sensor-based systems [9] as far as the actual improvement in captured images.

Modest image stabilization systems can degrade image quality if the photographer is intentionally panning (as the system tries to negate the panning motion), or if the camera is mounted on a very sturdy tripod (the system drifts around slowly due to spurious measurements over the course of a long exposure). Some more recent IS systems can automatically detect these situations and disable the IS along the panning axis, or disable it completely if the camera is on a tripod. Sweep panoramic photography certainly use panning system. So, modern image stabilization system is not use 2 axis anymore, but up to 5 axis: horizontal axis, vertical axis and rotation of 3 axis.

Mounting a lens with optical image stabilization on a camera with in-body image stabilization does not provide improved results, since the combined effect of both systems will “overcorrect”. Users of image-stabilized lenses on bodies with sensor-shift IS should determine which system offers superior performance and turn the other off. But, in-lens image stabilization usually better than in-body image stabilization, if the lens is newer than the body.

Lens mounts
Main article: Lens mount
There is almost no commonality between different camera makers regarding lens mount systems. Each manufacturer has developed their own system, and build camera bodies and lenses that only work with their own lens mount, with the Four Thirds System being a partial exception. This was different before 1970 when most of the manufacturers use either M42 or M39 lenses, most of which can still be used depending on the particular adapter you can find.

This does not necessarily mean that one is limited to only mounting, for example, Pentax lenses on a Pentax camera body. There are independent optics companies that make lenses for the various otherwise proprietary mount systems, thus providing alternative sources for lenses that are often of equal quality and/or less expensive than the camera maker’s own lenses. Another possibility is the use of adaptors that allow mounting a lens for one system on a camera with a different lens mount. However, the use of an adaptor usually results in reduced functionality, typically requiring the manual setting of aperture and focus, or perhaps not being able to use any aperture other than “wide open”.

M42 lens mount
The M42 lens mount has been used by Leica, Nikon, Pentax, Canon, Zenit, Praktica, Fujica, Cosina,

The M42 lens mount is a screw thread mounting standard for attaching lenses to 35 mm cameras, primarily single-lens reflex models. It is more accurately known as the M42 × 1 mm standard, which means that it is a metric screw thread of 42 mm diameter and 1 mm thread pitch. (The M42 lens mount should not be confused with the T-mount, which shares the 42mm throat diameter, but differs by having a 0.75mm thread pitch.) It was first used in Zeiss’ Contax S of 1949; this East German branch of Zeiss also sold cameras under the Pentacon name; after merger with other East German photographic manufacturers, the name Praktica was used. M42 thread mount cameras first became well known under the Praktica brand, and thus the M42 mount is known as the Praktica thread mount.[1] Since there were no proprietary elements to the M42 mount, many other manufacturers used it; this has led to it being called the Universal thread mount or Universal screw mount by many. The M42 mount was also used by Pentax; thus, it is also known as the Pentax thread mount, despite the fact that Pentax did not originate it.

M39
Also known as LTM (Leica Thread Mount). Used by Leica and Contax and several Leica copies, like the Soviet era FEDs, and others.

Canon EF- and EF-S-mounts

The electronic contacts (gold-plated) of an EF mount lens.
Canon introduced the EF-lens mount in 1987 as part of the EOS system. It broke with the most common technique for implementing autofocus at that time by not having a mechanical connection to a motor in the camera body, having instead only electrical connections and requiring a motor to be part of each autofocus lens.

The EF-S-mount is a newer subset of the EF standard, introduced in 2003. EF-S lenses can only be used on Canon digital cameras that use the APS-C sensor, for example the 400D (EOS Digital Rebel XTi) and the 40D. EF-S lenses can be distinguished by a white dot on the mount ring, as opposed to the red dot seen on standard EF lenses. Note that while an EF-S lens cannot be mounted on a camera that uses the EF mount, EF lenses can be mounted on cameras designed for the EF-S standard: for this reason, EF-S cameras carry both a red dot and a white dot on the mount.

As noted above under focal length, Canon makes DSLRs with various sensor sizes, and all using the EF or EF-S lens mounts. This leads to the interesting phenomenon of the same EF lens providing different angles of view depending on which camera it is mounted on.

Third-party lenses compatible with Canon’s EF and EF-S mounts are manufactured by Sigma, Tamron, Tokina and Zeiss. The manufacturers of these lenses have reverse engineered the electronics of the EF lens mount. The use of these lenses is not supported by Canon. However, many users find these lenses to be cheaper (with the exception of Zeiss), and sometimes superior alternatives to Canon lenses.

Four Thirds mount
The Four Thirds System was created by Olympus and Kodak in 2001, and is designed exclusively for digital cameras.[10] It is a semi-open standard that may be licensed by third parties. Currently Olympus, Leica (in cooperation with Panasonic), and Sigma are making lenses under Four Thirds System consortium licensing.

The Four Thirds System sensor size (17.3 mm x 13 mm) is the smallest currently being used in DSLR cameras. This leads to both advantages (theoretically smaller, lighter and cheaper lenses and camera bodies) and disadvantages (slightly lower image quality, especially in low-light situations).

There are currently over 35 lenses available for Four Thirds System cameras. A complete list can be found on Andrzej Wrotniak’s web site.[11]

Micro Four Thirds mount
Micro Four Thirds is a variant on the standard Four Thirds system, developed by Olympus and Panasonic. While these cameras are technically not DSLRs, they are similar in operation, and use similar interchangeable lenses. The Micro Four Thirds lens mount is a slightly modified version of the standard Four Thirds mount, and a number of lenses have been built for it. Because the lack of a mirror, optical viewfinder, and a shorter flange-focal distance these lenses can be more compact than those for standard Four Thirds.

Standard Four Thirds lenses can be used on a Micro Four Thirds camera with full electronic communication using an adapter, but those that do not support contrast-detect autofocus will only autofocus slowly, if at all. Micro Four Thirds lenses can not be used on a standard Four Thirds camera.

Minolta/Konica Minolta/Sony A-mount

A Minolta Maxxum AF 50mm f/1.7 prime lens with type “A” bayonet mount with the aperture all the way closed at F22.

The same Minolta AF 50mm f/1.7 prime lens with the aperture all the way open at F1.7.

Rear view of Minolta Maxxum/Dynax lens showing the port for the external autofocus drive
The Minolta A-mount system was introduced with the Minolta Maxxum 7000 camera in 1985, along with 11 AF-mount lenses.

In North America, Minolta began using the name ‘Maxxum’ for the SLR autofocus cameras, lenses and flashes while in Europe they were called ‘AF’ (first series) and ‘Dynax’ (second and later generations), and in Asia the ‘α’ (Alpha) branding was used, though they were otherwise identical in appearance and function – all of the equipment is 100% interchangeable regardless which of the names it carries.

Most Minolta A-mount compatible lenses, whether built by Minolta or one of the aftermarket lens manufacturers, are focused externally by a shaft connecting the autofocus computer and motor inside the camera body that mechanically connects to the internal focusing gears inside of the lens body. A couple of later Minolta lenses do have a built-in ultrasonic focus motor (SSM lenses), like other SLR and DSLR systems (i.e. Canon and Nikon), where the AF computer is inside the camera body and there is a digital interface connecting body to an electric motor and the focusing gears built into the lens body.

This shaft driven autofocus design has several benefits such as allowing for smaller and lighter lenses and also keeps the cost of lenses down because there are no internal focusing motors or digital interfaces built into the lens. Keeping the autofocus motors inside the camera body and as far away from the lens glass as possible, reduces vibration, an additional benefit. This highly reliable shaft-driven autofocus system was extremely successful but is not seen in modern lenses.

Minolta (and later Konica Minolta) followed up by producing a large number of AF-mount lenses over the years up until 2004. Konica Minolta sold the rights to their Minolta AF lens mount to Sony in 2006.

Sony acquired Konica Minolta’s camera technologies in 2006, and chose the “α” brand name, already in use by Minolta in Asia, for their new “Sony α” digital SLR system. The company has since abandoned traditional SLR design, and now uses the “α” brand name for its current line of SLT and ILCA cameras with fixed semi-reflective mirrors and electronic viewfinders, as well as its current line of E-mount mirrorless cameras.

The Minolta A-mount was retained from the old cameras and was originally named the “Sony α mount system”.[12]

Sony has produced several new lenses for its A-mount, and the current list of Minolta and Sony A-mount lenses has over 60 entries.

Some of the newest A-mount lenses are designated “DT” for Digital Technology; these are for digital cameras with APS-C sensors, and will result in vignetting if used on a film SLR or a full frame DSLR or full frame SLT camera.

Third-party lenses for the AF lens mount are made by Zeiss, Sigma, Tamron, Tokina and Vivitar.

Further information: Minolta A-mount, Sony A-mount, and List of Sony A-mount lenses
Nikon F-mount
The Nikon F-mount was introduced by Nikon in 1959, and is thus one of the most venerable lens mounts still in existence. Another factor that makes the Nikon F-mount popular is that several other camera manufacturers, for example Fujifilm, have adopted it. F-mount photographic lenses are currently made by Nikon, Zeiss, Voigtländer, Schneider, Sigma, Tokina, Tamron, Hartblei, Kiev-Arsenal, Lensbaby, Vivitar, and others, and over 400 lenses are compatible with the system.

Most Nikon F-mount lenses cover the standard 36×24 mm area of 135 film, while “DX” designated lenses cover the 24×16 mm area of the Nikon DX format DSLR sensors, commonly referred to as APS-C format. While “DX” lenses can be physically mounted on any Nikon film or digital SLR that support the “AI” exposure indexing feature (produced from 1977), there will be some degree of vignetting when used on film cameras, depending on zoom setting. All Nikon full-frame “FX” DSLRs have a DX-compatible mode that, by default when a DX-format lens is attached, crops the captured frames to the DX format and adjusts the viewfinder to reflect the smaller capture area. This can be overridden by the user if desired.

There are basically three types of F mount Nikon lens:

MF = Manual focus lenses
AF & AF-D = Auto focus by camera body driven focus motor, the D version provides distance information
AF-I & AF-S = Auto focus by integrated/ultrasonic motor in lens; see also List of Nikon F-mount lenses with integrated autofocus motors
Industrial F-mount lenses have varying, often small, film/sensor coverage. Older F-mount lenses designed for film cameras will work on modern SLR or DSLR cameras with some limitations, typically not providing autofocus or automatic aperture setting. Entry level Nikon DSLRs such as Nikon D40, D40X, D60, D3000, D3100, D5000 and D5100 do not have an integrated focus motor, so they will not autofocus with AF & AF-D lenses. Similarly, some AF-I & AF-S lenses will not work on some older Nikon AF film SLRs.

Nikon D200 digital SLR
Nikon 1-mount
The Nikon 1-mount was announced on 21 September 2011 together with the Nikon 1 series high-speed mirrorless interchangeable-lens cameras.

Pentax K-mount

Pentax K10D “Crippled” KAF2 mount
The Pentax K-mount (or just “PK mount”) was created by Pentax in 1975, and has been used by all Pentax 35 mm and digital SLRs since. The mount has been developed over the years, resulting in a large number of designations such as KF mount, KA mount, KAF mount, KAF2 mount and KA2 mount, plus a couple of more recent versions that are not completely backward-compatible and are thus referred to as “crippled” versions. (“Crippled” in this context does not imply any lack of modern functionality, just a lack of compatibility with past lenses.) For more information see the Pentax K mount article or Bojidar Dimitrov’s web site.[13]

A number of other manufacturers have produced K-mount lenses, and several other manufacturers (such as Konica and Ricoh) have made K-mount cameras. In 2005 Pentax and Samsung entered into a cooperation resulting in the Samsung GX line of DSLRs, based largely on Pentax technology including the Pentax K mount.

Sigma SA-mount
Sigma Corporation, better known for manufacturing lenses for other cameras, has made some film SLR and DSLR cameras themselves. These cameras use the Sigma SA-mount, for which Sigma makes a line of lenses.

The Sigma DSLR cameras that use the SA mount are the Sigma SD9, SD10, SD14, SD15 and SD1 Merrill. These cameras are noteworthy for their use of the Foveon X3 sensor, an image sensor that works on quite different principles from the sensors used in all other digital cameras.

Mount compatibility across camera generations
The Nikon F-mount lens systems and the Pentax K-mount systems are the only 35 mm SLR camera systems (apart from the Leica M-mount rangefinder system) that allow a photographer to use a mechanical SLR camera body, a fully automatic SLR camera body, and a DSLR camera body, all utilizing the same lenses. The only aspects of these manufacturers’ lenses that have changed are the addition of electronic contacts, autofocus abilities and, in some cases, the elimination of the external aperture ring for electronic control (i.e., Nikon’s ‘G-type’ auto-Nikkors, which cannot be used on a mechanical SLR camera body).

Canon, Minolta (Sony), Olympus, and other manufacturers have changed lens mounts. Much older Canon film cameras used the FD lens mount, which was discontinued in 1987 in favor of the EF lens mount. Olympus discontinued the OM lens mount for the OM series cameras in favor of the Four Thirds System lens mount. However, due to the size of the Four Thirds mount it is possible to fit legacy SLR lenses from any manufacturer using an adapter, albeit with manual aperture and focus control. Minolta (Sony after 2006) phased out its bayonet-mount MC and MD Rokkor lenses for a modified bayonet mount (supporting autofocus) in 1985. All Minolta A-mount lenses work on compatible Minolta SLR film bodies and on Sony A-mount DSLR and SLT bodies.

The design of photographic lenses for use in still or cine cameras is intended to produce a lens that yields the most acceptable rendition of the subject being photographed within a range of constraints that include cost, weight and materials. For many other optical devices such as telescopes, microscopes and theodolites where the visual image is observed but often not recorded the design can often be significantly simpler than is the case in a camera where every image is captured on film or image sensor and can be subject to detailed scrutiny at a later stage. Photographic lenses also include those used in enlargers and projectors.

Contents
1 Design
1.1 Design requirements
1.1.1 Design constraints
1.2 Lens elements
1.2.1 Lens glass
1.3 Focus
1.4 Aperture control
1.5 Shutter mechanism
2 Types of lenses
2.1 Enlarger lenses
2.2 Projector lenses
3 History
4 See also
5 References
Design
Design requirements
Main article: Optical lens design
From the perspective of the photographer, the ability of a lens to capture sufficient light so that the camera can operate over a wide range of lighting conditions is important. Designing a lens that reproduces colour accurately is also important as is the production of an evenly lit and sharp image over the whole of the film or sensor plane.

For the lens designer, achieving these objectives will also involve ensuring that internal flare, optical aberrations and weight are all reduced to the minimum whilst zoom, focus and aperture functions all operate smoothly and predictably.

However, because photographic films and electronic sensors have a finite and measurable resolution, photographic lenses are not always designed for maximum possible resolution since the recording medium would not be able to record the level of detail that the lens could resolve. For this, and many other reasons, camera lenses are unsuited for use as projector or enlarger lenses.

The design of a fixed focal length lens (also known as prime lenses) presents fewer challenges than the design of a zoom lens. A high-quality prime lens whose focal length is about equal to the diameter of the film frame or sensor may be constructed from as few as four separate lens elements, often as pairs on either side of the aperture diaphragm. Good examples include the Zeiss Tessar or the Leitz Elmar.

Design constraints
To be useful in photography any lens must be able to fit the camera for which it is intended and this will physically limit the size where the bayonet mounting or screw mounting is to be located.

Photography is a highly competitive commercial business and both weight and cost constrain the production of lenses.

Refractive materials such as glass have physical limitations which limit the performance of lenses. In particular the range of refractive indices available in commercial glasses span a very narrow range. Since it is the refractive index that determines how much the rays of light are bent at each interface and since it is the differences in refractive indices in paired plus and minus lenses that constrains the ability to minimise chromatic aberrations, having only a narrow spectrum of indices is a major design constraint.

Lens elements

Elements of a cheap 28mm lens

Photographic lens cut for demonstration
Main article: Lens (optics)
Except for the most simple and inexpensive lenses, each complete lens is made up from a number of separate lens elements arranged along a common axis. The use of many lens elements serves to minimise aberrations and to provide a sharp image free from visible imperfections. To do this requires lens elements of different compositions and different shapes. To minimise chromatic aberrations, e. g., in which different wavelengths of light are refracted to different degrees, requires, at a minimum, a doublet of lens elements with a positive element having a high Abbe number matched with a negative element of lower Abbe number. With this design one can achieve a good degree of convergence of different wavelengths in the visible spectrum. Most lens designs do not attempt to bring infrared wavelengths to the same common focus and it is therefore necessary to manually alter the focus when photographing in infrared light. Other kinds of aberrations such as coma or astigmatism can also be minimized by careful choice of curvature of the lens faces for all the component elements. Complex photographic lenses can consist of more than 15 lens elements.

Most lens elements are made with curved surfaces with a spherical profile. That is, the curved shape would fit on the surface of a sphere. This is partly to do with the history of lens making but also because grinding and manufacturing of spherical surface lenses is relatively simple and cheap. However, spherical surfaces also give rise to lens aberrations and can lead to complicated lens designs of great size. Higher-quality lenses with fewer elements and lower size can be achieved by using aspheric lenses in which the curved surfaces are not spherical, giving more degrees of freedom to correct aberrations.

Lens glass
Main article: Refraction
The majority of photographic lenses have the lens elements made from glass although the use of high-quality plastics is becoming more common in high-quality lenses and has been common in inexpensive cameras for some time. The design of photographic lenses is very demanding as designers push the limits of existing materials to make more versatile, better-quality, and lighter lenses. As a consequence many exotic glasses have been used in modern lens manufacturing. Caesium[1] and lanthanum[2] glass lenses are now in use because of their high refractive index and very low dispersion properties. It is also likely that a number of other transition element glasses are in use but manufacturers often prefer to keep their material specification secret to retain a commercial or performance edge over their rivals.

Focus
Main article: Focus (optics)
Until recent years focusing of a camera lens to achieve a sharp image on the film plane was achieved by means of a very shallow helical thread in the lens mount through which the lens could be rotated moving it closer or further from the film plane. This arrangement, whilst simple to design and construct, has some limitations not least the rotation of the greater part of the lens assembly including the front element. This could be problematical if devices such as polarising filters were in use that require maintaining an accurate vertical orientation irrespective of focus distance.

Later developments adopted designs in which internal elements were moved to achieve focus without affecting the outer barrel of the lens or the orientation of the front element.

Many modern cameras now use automatic focusing mechanisms which use ultrasonic motors to move internal elements in the lens to achieve optimum focus.

Aperture control
Main article: Aperture
The aperture control, usually a multi-leaf diaphragm, is critical to the performance of a lens. The role of the aperture is to control the amount of light passing through the lens to the film or sensor plane. An aperture placed outside of the lens, as in the case of some Victorian cameras, risks vignetting of the image in which the corners of the image are darker than the centre. A diaphragm too close to the image plane risks the diaphragm itself being recorded as a circular shape or at the very least causing diffraction patterns at small apertures. In most lens designs the aperture is positioned about midway between the front surface of the objective and the image plane. In some zoom lenses it is placed some distance away from the ideal location in order to accommodate the movement of floating lens elements needed to perform the zoom function.

Most modern lenses for 35mm format rarely provide a stop smaller than f/22 because of the diffraction effects caused by light passing through a very small aperture. As diffraction is based on aperture width in absolute terms rather than the f-stop ratio, lenses for very small formats common in compact cameras rarely go above f/11 (1/1.8″) or f/8 (1/2.5″), while lenses for medium- and large-format provide f/64 or f/128.

Very-large-aperture lenses designed to be useful in very low light conditions with apertures ranging from f/1.2 to f/0.9 are generally restricted to lenses of standard focal length because of the size and weight problems that would be encountered in telephoto lenses and the difficulty of building a very wide aperture wide angle lens with the refractive materials currently available. Very-large-aperture lenses are commonly made for other types of optical instruments such as microscopes but in such cases the diameter of the lens is very small and weight is not an issue.

Many very early cameras had diaphragms external to the lens often consisting of a rotating circular plate with a number of holes of increasing size drilled through the plate.[3] Rotating the plate would bring an appropriate sized hole in front of the lens. All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed. Either a manual ring, or an electronic motor controls the angle of the diaphragm leaves and thus the size of the opening.

The placement of the diaphragm within the lens structure is constrained by the need to achieve even illumination over the whole film plane at all apertures and the requirement to not interfere with the movement of any movable lens element. Typically the diaphragm is situated at about the level of the optical centre of the lens.

Shutter mechanism
Main article: Shutter (photography)
A shutter controls the length of time light is allowed to pass through the lens onto the film plane. For any given light intensity, the more sensitive the film or detector or the wider the aperture the shorter the exposure time need to be to maintain the optimal exposure. In the earliest cameras, exposures were controlled by moving a rotating plate from in front of the lens and then replacing it. Such a mechanism only works effectively for exposures of several seconds or more and carries a considerable risk of inducing camera shake. By the end of the 19th century spring tensioned shutter mechanisms were in use operated by a lever or by a cable release. Some simple shutters continued to be placed in front of the lens but most were incorporated within the lens mount itself. Such lenses with integral shutter mechanisms developed in the current Compur shutter as used in many non-reflex cameras such as Linhof. These shutters have a number of metal leaves that spring open and then close after a pre-determined interval. The material and design constraints limit the shortest speed to about 0.002 second. Although such shutters cannot yield as short an exposure time as focal-plane shutter they are able to offer flash synchronisation at all speeds.

Incorporating a commercial made Compur type shutter required lens designers to accommodate the width of the shutter mechanism in the lens mount and provide for the means of triggering the shutter on the lens barrel or transferring this to the camera body by a series of levers as in the Minolta twin-lens cameras.

The need to accommodate the shutter mechanism within the lens barrel limited the design of wide-angle lenses and it was not until the widespread use of focal-plane shutters that extreme wide-angle lenses were developed.

Types of lenses

Example of a prime lens – Carl Zeiss Tessar.
The type of lens being designed is significant in setting the key parameters.

Prime lens – a photographic lens whose focal length is fixed, as opposed to a zoom lens, or that is the primary lens in a combination lens system.
Zoom lenses – variable focal length lenses. Zoom lenses cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly. In early varifocal lens lenses, the focus also shifted as the lens focal length was changed. Varifocal lenses are also used in many modern autofocus cameras as the lenses are cheaper and simpler to construct and the autofocus can take care of the re-focussing requirements. Many modern zoom lenses are now confocal, meaning that the focus is maintained throughout the zoom range. Because of the need to operate over a range of focal lengths and maintain confocality, zoom lenses typically have very many lens elements. More significantly, the front elements of the lens will always be a compromise in terms of its size, light-gathering capability and the angle of incidence of the incoming rays of light. For all these reasons, the optical performance of zoom lenses tends to be lower than fixed-focal-length lenses.
Normal lens – a lens with a focal length about equal to the diagonal size of the film or sensor format, or that reproduces perspective that generally looks “normal” to a human observer.

Cross-section of a typical short-focus wide-angle lens.
Wide angle lens – a lens that reproduces perspective that generally looks “wider” than a normal lens. The problem posed by the design of wide-angle lenses is to bring to an accurate focus light from a wide area without causing internal flare. Wide-angle lenses therefore tend to have more elements than a normal lens to help refract the light sufficiently and still minimise aberrations whilst adding light-trapping baffles between each lens element.

Cross-section of a typical retrofocus wide-angle lens.
Extreme or ultra-wide-angle lens – a wide-angle lens with an angle of view above 90 degrees.[4] Extreme-wide-angle lenses share the same issues as ordinary wide-angle lenses but the focal length of such lenses may be so short that there is insufficient physical space in front of the film or sensor plane to construct a lens. This problem is resolved by constructing the lens as an inverted telephoto, or retrofocus with the front element having a very short focal length, often with a highly exaggerated convex front surface and behind it a strongly negative lens grouping that extends the cone of focused rays so that they can be brought to focus at a reasonable distance.

Cross-section – typical telephoto lens.
L1 – Tele positive lens group
L2 – Tele negative lens group
D – Diaphragm
Fisheye lens – an extreme wide-angle lens with a strongly convex front element. Spherical aberration is usually pronounced and sometimes enhanced for special effect. Optically designed as a reverse telephoto to enable the lens to fit into a standard mount as the focal length can be less than the distance from lens mount to focal plane.
Long-focus lens – a lens with a focal length greater than the diagonal of the film frame or sensor. Long focus lenses are relatively simple to design, the challenges being comparable to the design of a prime lens. However, as the focal length increases the length of the lens and the size of the objective increase in size and length and weight quickly become significant design issues in retaining utility and practicality for the lens in use. In addition because the light path through the lens is long and glancing, the importance of baffles to control flare increases in importance.
Telephoto lens – an optically compressed version of the long-focus lens. The design of telephoto lenses reduces some of the problems encountered by designers of long-focus lenses. In particular, telephoto lenses are typically much shorter and may be lighter for equivalent focal length and aperture. However telephoto designs increase the number of lens elements and can introduce flare and exacerbate some optical aberrations.
Catadioptric lens – catadioptric lenses are a form of telephoto lens but with a light path that doubles back on itself and with an objective that is a mirror combined with some form aberration correcting lens (a catadioptric system) rather than just a lens. A centrally-placed secondary mirror and usually an additional small lens group bring the light to focus. Such lenses are very lightweight and can easily deliver very long focal lengths but they can only deliver a fixed aperture and have none of the benefits of being able to stop down the aperture to increase depth of field.
Anamorphic lenses are used principally in cinematography to produce wide-screen films where the projected image has a substantially different ratio of height to width than the image recorded on the film plane. This is achieved by the use of a specialised lens design which compresses the image laterally at the recording stage and the film is then projected through a similar lens in the cinema to recreate the wide-screen effect. Although in some cases the anamorphic effect is achieved by using an anamorphising attachment as a supplementary element on the front of a normal lens, most films shot in anamorphic formats use specially designed anamorphic lenses, such as the Hawk lenses made by Vantage Film or Panavision’s anamorphic lenses. These lenses incorporate one or more aspheric elements in their design.
Enlarger lenses
Lenses used in photographic enlargers are required to focus light passing through a relatively small film area on a larger area of photographic paper or film. Requirements for such lenses include

the ability to record even illumination over the whole field
to record fine detail present in the film being enlarged
to withstand frequent cycles of heating and cooling as the illumination lamp is turned on and off
to be able to be operated in the dark – usually by means of click stops and some luminous controls
The design of the lens is required to work effectively with light passing from near focus to far focus – exactly the reverse of a camera lens. This demands that internal light baffling within the lens is designed differently and that the individual lens elements are designed to maximize performance for this change of direction of incident light.

Projector lenses
Projector lenses share many of the design constraints as enlarger lenses but with some critical differences. Projector lenses are always used at full aperture and must produce an acceptably illuminated and acceptably sharp image at full aperture.

However, because projected images are almost always viewed at some distance, lack of very fine focus and slight unevenness of illumination is often acceptable. Projector lenses have to be very tolerant of prolonged high temperatures from the projector lamp and frequently have a focal length much longer than the taking lens. This allows the lens to be positioned at a greater distance from the illuminated film and allows an acceptable sized image with the projector some distance from the screen. It also permits the lens to be mounted in a relatively coarsely threaded focusing mount so that the projectionist can quickly correct any focusing errors.

History
Main article: History of photographic lens design

Diagram of Petzval’s 1841 portrait lens – crown glass shaded pink, flint glass shaded blue
The lenses of the very earliest cameras were simple meniscus or simple bi convex lenses. It was not until 1840 that Chevalier in France introduced the achromatic lens formed by cementing a crown glass bi-convex lens to a flint glass plano-concave lens. By 1841 Voigtländer using the design of Joseph Petzval manufactured the first commercially successful two element lens.

Carl Zeiss was an entrepreneur who needed a competent designer to take his firm beyond just another optical workshop. In 1866, the service of Dr Ernst Abbe was enlisted. From then on novel products appeared in rapid succession which brought the Zeiss company to the forefront of optical technology.

Abbe was instrumental in the development of the famous Jena optical glass. When he was trying to eliminate astigmatism from microscopes, he realised that the range of optical glasses available was insufficient. After some calculations, he realised that performance of optical instruments would dramatically improve, if optical glasses of appropriate properties were available. His challenge to glass manufacturers was finally answered by Dr Otto Schott, who established the famous glassworks at Jena from which new types of optical glass began to appear from 1888, and employed by Zeiss and other makers.

The new Jena optical glass also opened up the possibility of increased performance of photographic lenses. The first use of Jena glass in a photographic lens was by Voigtländer, but as the lens was an old design its performance was not greatly improved. Subsequently, the new glasses would demonstrate their value in correcting astigmatism, and in the production of achromatic and apochromatic lenses. Abbé started the design of a photographic lens of symmetrical design with five elements, but went no further.

Zeiss’ innovative photographic lens design was due to Dr Paul Rudolph. In 1890, Rudolph designed an asymmetrical lens with a cemented group at each side of the diaphragm, and appropriately named “Anastigmat”. This lens was made in three series: Series III, IV and V, with maximum apertures of f/7.2, f/12.5, and f/18 respectively. In 1891, Series I, II and IIIa appeared with respective maximum apertures of f/4.5, f/6.3, and f/9 and in 1893 came Series IIa of f/8 maximum aperture. These lenses are now better known by the trademark “Protar” which was first used in 1900.

At the time, single combination lenses, which occupy one side of the diaphragm only, were still popular. Rudolph designed one with three cemented elements in 1893, with the option of fitting two of them together in a lens barrel as a compound lens, but it was found to be the same as the Dagor by C.P. Goerz, designed by Emil von Höegh. Rudolph then came up with a single combination with four cemented elements, which can be considered as having all the elements of the Protar stuck together in one piece. Marketed in 1894, it was called the Protarlinse Series VII, the most highly corrected single combination lens with maximum apertures between f/11 and f/12.5, depending on its focal length.

But the important thing about this Protarlinse is that two of these lens units can be mounted in the same lens barrel to form a compound lens of even greater performance and larger aperture, between f/6.3 and f/7.7. In this configuration it was called the Double Protar Series VIIa. An immense range of focal lengths can thus be obtained by the various combination of Protarlinse units.

Rudolph also investigated the Double-Gauss concept of a symmetrical design with thin positive menisci enclosing negative elements. The result was the Planar Series Ia of 1896, with maximum apertures up to f/3.5, one of the fastest lenses of its time. Whilst it was very sharp, it suffered from coma which limited its popularity. However, further developments of this configuration made it the design of choice for high-speed lenses of standard coverage.

Probably inspired by the Stigmatic lenses designed by Hugh Aldis for Dallmeyer of London, Rudolph designed a new asymmetrical lens with four thin elements, the Unar Series Ib, with apertures up to f/4.5. Due to its high speed it was used extensively on hand cameras.

The most important Zeiss lens by Rudolph was the Tessar, first sold in 1902 in its Series IIb f/6.3 form. It can be said as a combination of the front half of the Unar with the rear half of the Protar. This proved to be a most valuable and flexible design, with tremendous development potential. Its maximum aperture was increased to f/4.7 in 1917, and reached f/2.7 in 1930. It is probable that every lens manufacturer has produced lenses of the Tessar configuration.

Rudolph left Zeiss after the First World War, but many other competent designers such as Merté, Wandersleb, etc. kept the firm at the leading edge of photographic lens innovations. One of the most significant designer was the ex-Ernemann man Dr Ludwig Bertele, famed for his Ernostar high-speed lens.

With the advent of the Contax by Zeiss-Ikon, the first serious challenge to the Leica in the field of professional 35 mm cameras, both Zeiss-Ikon and Carl Zeiss decided to beat the Leica in every possible way. Bertele’s Sonnar series of lenses designed for the Contax were the match in every respect for the Leica for at least two decades. Other lenses for the Contax included the Biotar, Biogon, Orthometar, and various Tessars and Triotars.

The last important Zeiss innovation before the Second World War was the technique of applying anti-reflective coating to lens surfaces invented by Olexander Smakula in 1935.[5] A lens so treated was marked with a red “T”, short for “Transparent”. The technique of applying multiple layers of coating was also described in the original patent writings in 1935.[6]

After the partitioning of Germany, a new Carl Zeiss optical company was established in Oberkochen, while the original Zeiss firm in Jena continued to operate. At first both firms produced very similar lines of products, and extensively cooperated in product-sharing, but they drifted apart as time progressed. Jena’s new direction was to concentrate on developing lenses for the 35 mm single-lens reflex camera, and many achievements were made, especially in ultra-wide angle designs. In addition to that, Oberkochen also worked on designing lenses for large format cameras, interchangeable front element lenses such as for the 35 mm single-lens reflex Contaflex, and other types of cameras.

Since the beginning of Zeiss as a photographic lens manufacturer, it has had a licensing programme which allows other manufacturers to produce its lenses. Over the years its licensees included Voigtländer, Bausch & Lomb, Ross, Koristka, Krauss, Kodak. etc. In the 1970s, the western operation of Zeiss-Ikon got together with Yashica to produce the new Contax cameras, and many of the Zeiss lenses for this camera, among others, were produced by Yashica’s optical arm, Tomioka. Yashica’s owner Kyocera ended camera production in 2006. Yashica lenses were then made by Cosina, who also manufactured most of the new Zeiss designs for the new Zeiss Ikon coupled rangefinder camera. Another licensee active today is Sony who uses the Zeiss name on lenses on its video and digital still cameras.

The design of photographic lenses for use in still or cine cameras is intended to produce a lens that yields the most acceptable rendition of the subject being photographed within a range of constraints that include cost, weight and materials. For many other optical devices such as telescopes, microscopes and theodolites where the visual image is observed but often not recorded the design can often be significantly simpler than is the case in a camera where every image is captured on film or image sensor and can be subject to detailed scrutiny at a later stage. Photographic lenses also include those used in enlargers and projectors.

Contents
1 Design
1.1 Design requirements
1.1.1 Design constraints
1.2 Lens elements
1.2.1 Lens glass
1.3 Focus
1.4 Aperture control
1.5 Shutter mechanism
2 Types of lenses
2.1 Enlarger lenses
2.2 Projector lenses
3 History
4 See also
5 References
Design
Design requirements
Main article: Optical lens design
From the perspective of the photographer, the ability of a lens to capture sufficient light so that the camera can operate over a wide range of lighting conditions is important. Designing a lens that reproduces colour accurately is also important as is the production of an evenly lit and sharp image over the whole of the film or sensor plane.

For the lens designer, achieving these objectives will also involve ensuring that internal flare, optical aberrations and weight are all reduced to the minimum whilst zoom, focus and aperture functions all operate smoothly and predictably.

However, because photographic films and electronic sensors have a finite and measurable resolution, photographic lenses are not always designed for maximum possible resolution since the recording medium would not be able to record the level of detail that the lens could resolve. For this, and many other reasons, camera lenses are unsuited for use as projector or enlarger lenses.

The design of a fixed focal length lens (also known as prime lenses) presents fewer challenges than the design of a zoom lens. A high-quality prime lens whose focal length is about equal to the diameter of the film frame or sensor may be constructed from as few as four separate lens elements, often as pairs on either side of the aperture diaphragm. Good examples include the Zeiss Tessar or the Leitz Elmar.

Design constraints
To be useful in photography any lens must be able to fit the camera for which it is intended and this will physically limit the size where the bayonet mounting or screw mounting is to be located.

Photography is a highly competitive commercial business and both weight and cost constrain the production of lenses.

Refractive materials such as glass have physical limitations which limit the performance of lenses. In particular the range of refractive indices available in commercial glasses span a very narrow range. Since it is the refractive index that determines how much the rays of light are bent at each interface and since it is the differences in refractive indices in paired plus and minus lenses that constrains the ability to minimise chromatic aberrations, having only a narrow spectrum of indices is a major design constraint.

Lens elements

Elements of a cheap 28mm lens

Photographic lens cut for demonstration
Main article: Lens (optics)
Except for the most simple and inexpensive lenses, each complete lens is made up from a number of separate lens elements arranged along a common axis. The use of many lens elements serves to minimise aberrations and to provide a sharp image free from visible imperfections. To do this requires lens elements of different compositions and different shapes. To minimise chromatic aberrations, e. g., in which different wavelengths of light are refracted to different degrees, requires, at a minimum, a doublet of lens elements with a positive element having a high Abbe number matched with a negative element of lower Abbe number. With this design one can achieve a good degree of convergence of different wavelengths in the visible spectrum. Most lens designs do not attempt to bring infrared wavelengths to the same common focus and it is therefore necessary to manually alter the focus when photographing in infrared light. Other kinds of aberrations such as coma or astigmatism can also be minimized by careful choice of curvature of the lens faces for all the component elements. Complex photographic lenses can consist of more than 15 lens elements.

Most lens elements are made with curved surfaces with a spherical profile. That is, the curved shape would fit on the surface of a sphere. This is partly to do with the history of lens making but also because grinding and manufacturing of spherical surface lenses is relatively simple and cheap. However, spherical surfaces also give rise to lens aberrations and can lead to complicated lens designs of great size. Higher-quality lenses with fewer elements and lower size can be achieved by using aspheric lenses in which the curved surfaces are not spherical, giving more degrees of freedom to correct aberrations.

Lens glass
Main article: Refraction
The majority of photographic lenses have the lens elements made from glass although the use of high-quality plastics is becoming more common in high-quality lenses and has been common in inexpensive cameras for some time. The design of photographic lenses is very demanding as designers push the limits of existing materials to make more versatile, better-quality, and lighter lenses. As a consequence many exotic glasses have been used in modern lens manufacturing. Caesium[1] and lanthanum[2] glass lenses are now in use because of their high refractive index and very low dispersion properties. It is also likely that a number of other transition element glasses are in use but manufacturers often prefer to keep their material specification secret to retain a commercial or performance edge over their rivals.

Focus
Main article: Focus (optics)
Until recent years focusing of a camera lens to achieve a sharp image on the film plane was achieved by means of a very shallow helical thread in the lens mount through which the lens could be rotated moving it closer or further from the film plane. This arrangement, whilst simple to design and construct, has some limitations not least the rotation of the greater part of the lens assembly including the front element. This could be problematical if devices such as polarising filters were in use that require maintaining an accurate vertical orientation irrespective of focus distance.

Later developments adopted designs in which internal elements were moved to achieve focus without affecting the outer barrel of the lens or the orientation of the front element.

Many modern cameras now use automatic focusing mechanisms which use ultrasonic motors to move internal elements in the lens to achieve optimum focus.

Aperture control
Main article: Aperture
The aperture control, usually a multi-leaf diaphragm, is critical to the performance of a lens. The role of the aperture is to control the amount of light passing through the lens to the film or sensor plane. An aperture placed outside of the lens, as in the case of some Victorian cameras, risks vignetting of the image in which the corners of the image are darker than the centre. A diaphragm too close to the image plane risks the diaphragm itself being recorded as a circular shape or at the very least causing diffraction patterns at small apertures. In most lens designs the aperture is positioned about midway between the front surface of the objective and the image plane. In some zoom lenses it is placed some distance away from the ideal location in order to accommodate the movement of floating lens elements needed to perform the zoom function.

Most modern lenses for 35mm format rarely provide a stop smaller than f/22 because of the diffraction effects caused by light passing through a very small aperture. As diffraction is based on aperture width in absolute terms rather than the f-stop ratio, lenses for very small formats common in compact cameras rarely go above f/11 (1/1.8″) or f/8 (1/2.5″), while lenses for medium- and large-format provide f/64 or f/128.

Very-large-aperture lenses designed to be useful in very low light conditions with apertures ranging from f/1.2 to f/0.9 are generally restricted to lenses of standard focal length because of the size and weight problems that would be encountered in telephoto lenses and the difficulty of building a very wide aperture wide angle lens with the refractive materials currently available. Very-large-aperture lenses are commonly made for other types of optical instruments such as microscopes but in such cases the diameter of the lens is very small and weight is not an issue.

Many very early cameras had diaphragms external to the lens often consisting of a rotating circular plate with a number of holes of increasing size drilled through the plate.[3] Rotating the plate would bring an appropriate sized hole in front of the lens. All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed. Either a manual ring, or an electronic motor controls the angle of the diaphragm leaves and thus the size of the opening.

The placement of the diaphragm within the lens structure is constrained by the need to achieve even illumination over the whole film plane at all apertures and the requirement to not interfere with the movement of any movable lens element. Typically the diaphragm is situated at about the level of the optical centre of the lens.

Shutter mechanism
Main article: Shutter (photography)
A shutter controls the length of time light is allowed to pass through the lens onto the film plane. For any given light intensity, the more sensitive the film or detector or the wider the aperture the shorter the exposure time need to be to maintain the optimal exposure. In the earliest cameras, exposures were controlled by moving a rotating plate from in front of the lens and then replacing it. Such a mechanism only works effectively for exposures of several seconds or more and carries a considerable risk of inducing camera shake. By the end of the 19th century spring tensioned shutter mechanisms were in use operated by a lever or by a cable release. Some simple shutters continued to be placed in front of the lens but most were incorporated within the lens mount itself. Such lenses with integral shutter mechanisms developed in the current Compur shutter as used in many non-reflex cameras such as Linhof. These shutters have a number of metal leaves that spring open and then close after a pre-determined interval. The material and design constraints limit the shortest speed to about 0.002 second. Although such shutters cannot yield as short an exposure time as focal-plane shutter they are able to offer flash synchronisation at all speeds.

Incorporating a commercial made Compur type shutter required lens designers to accommodate the width of the shutter mechanism in the lens mount and provide for the means of triggering the shutter on the lens barrel or transferring this to the camera body by a series of levers as in the Minolta twin-lens cameras.

The need to accommodate the shutter mechanism within the lens barrel limited the design of wide-angle lenses and it was not until the widespread use of focal-plane shutters that extreme wide-angle lenses were developed.

Types of lenses

Example of a prime lens – Carl Zeiss Tessar.
The type of lens being designed is significant in setting the key parameters.

Prime lens – a photographic lens whose focal length is fixed, as opposed to a zoom lens, or that is the primary lens in a combination lens system.
Zoom lenses – variable focal length lenses. Zoom lenses cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly. In early varifocal lens lenses, the focus also shifted as the lens focal length was changed. Varifocal lenses are also used in many modern autofocus cameras as the lenses are cheaper and simpler to construct and the autofocus can take care of the re-focussing requirements. Many modern zoom lenses are now confocal, meaning that the focus is maintained throughout the zoom range. Because of the need to operate over a range of focal lengths and maintain confocality, zoom lenses typically have very many lens elements. More significantly, the front elements of the lens will always be a compromise in terms of its size, light-gathering capability and the angle of incidence of the incoming rays of light. For all these reasons, the optical performance of zoom lenses tends to be lower than fixed-focal-length lenses.
Normal lens – a lens with a focal length about equal to the diagonal size of the film or sensor format, or that reproduces perspective that generally looks “normal” to a human observer.

Cross-section of a typical short-focus wide-angle lens.
Wide angle lens – a lens that reproduces perspective that generally looks “wider” than a normal lens. The problem posed by the design of wide-angle lenses is to bring to an accurate focus light from a wide area without causing internal flare. Wide-angle lenses therefore tend to have more elements than a normal lens to help refract the light sufficiently and still minimise aberrations whilst adding light-trapping baffles between each lens element.

Cross-section of a typical retrofocus wide-angle lens.
Extreme or ultra-wide-angle lens – a wide-angle lens with an angle of view above 90 degrees.[4] Extreme-wide-angle lenses share the same issues as ordinary wide-angle lenses but the focal length of such lenses may be so short that there is insufficient physical space in front of the film or sensor plane to construct a lens. This problem is resolved by constructing the lens as an inverted telephoto, or retrofocus with the front element having a very short focal length, often with a highly exaggerated convex front surface and behind it a strongly negative lens grouping that extends the cone of focused rays so that they can be brought to focus at a reasonable distance.

Cross-section – typical telephoto lens.
L1 – Tele positive lens group
L2 – Tele negative lens group
D – Diaphragm
Fisheye lens – an extreme wide-angle lens with a strongly convex front element. Spherical aberration is usually pronounced and sometimes enhanced for special effect. Optically designed as a reverse telephoto to enable the lens to fit into a standard mount as the focal length can be less than the distance from lens mount to focal plane.
Long-focus lens – a lens with a focal length greater than the diagonal of the film frame or sensor. Long focus lenses are relatively simple to design, the challenges being comparable to the design of a prime lens. However, as the focal length increases the length of the lens and the size of the objective increase in size and length and weight quickly become significant design issues in retaining utility and practicality for the lens in use. In addition because the light path through the lens is long and glancing, the importance of baffles to control flare increases in importance.
Telephoto lens – an optically compressed version of the long-focus lens. The design of telephoto lenses reduces some of the problems encountered by designers of long-focus lenses. In particular, telephoto lenses are typically much shorter and may be lighter for equivalent focal length and aperture. However telephoto designs increase the number of lens elements and can introduce flare and exacerbate some optical aberrations.
Catadioptric lens – catadioptric lenses are a form of telephoto lens but with a light path that doubles back on itself and with an objective that is a mirror combined with some form aberration correcting lens (a catadioptric system) rather than just a lens. A centrally-placed secondary mirror and usually an additional small lens group bring the light to focus. Such lenses are very lightweight and can easily deliver very long focal lengths but they can only deliver a fixed aperture and have none of the benefits of being able to stop down the aperture to increase depth of field.
Anamorphic lenses are used principally in cinematography to produce wide-screen films where the projected image has a substantially different ratio of height to width than the image recorded on the film plane. This is achieved by the use of a specialised lens design which compresses the image laterally at the recording stage and the film is then projected through a similar lens in the cinema to recreate the wide-screen effect. Although in some cases the anamorphic effect is achieved by using an anamorphising attachment as a supplementary element on the front of a normal lens, most films shot in anamorphic formats use specially designed anamorphic lenses, such as the Hawk lenses made by Vantage Film or Panavision’s anamorphic lenses. These lenses incorporate one or more aspheric elements in their design.
Enlarger lenses
Lenses used in photographic enlargers are required to focus light passing through a relatively small film area on a larger area of photographic paper or film. Requirements for such lenses include

the ability to record even illumination over the whole field
to record fine detail present in the film being enlarged
to withstand frequent cycles of heating and cooling as the illumination lamp is turned on and off
to be able to be operated in the dark – usually by means of click stops and some luminous controls
The design of the lens is required to work effectively with light passing from near focus to far focus – exactly the reverse of a camera lens. This demands that internal light baffling within the lens is designed differently and that the individual lens elements are designed to maximize performance for this change of direction of incident light.

Projector lenses
Projector lenses share many of the design constraints as enlarger lenses but with some critical differences. Projector lenses are always used at full aperture and must produce an acceptably illuminated and acceptably sharp image at full aperture.

However, because projected images are almost always viewed at some distance, lack of very fine focus and slight unevenness of illumination is often acceptable. Projector lenses have to be very tolerant of prolonged high temperatures from the projector lamp and frequently have a focal length much longer than the taking lens. This allows the lens to be positioned at a greater distance from the illuminated film and allows an acceptable sized image with the projector some distance from the screen. It also permits the lens to be mounted in a relatively coarsely threaded focusing mount so that the projectionist can quickly correct any focusing errors.

History
Main article: History of photographic lens design

Diagram of Petzval’s 1841 portrait lens – crown glass shaded pink, flint glass shaded blue
The lenses of the very earliest cameras were simple meniscus or simple bi convex lenses. It was not until 1840 that Chevalier in France introduced the achromatic lens formed by cementing a crown glass bi-convex lens to a flint glass plano-concave lens. By 1841 Voigtländer using the design of Joseph Petzval manufactured the first commercially successful two element lens.

Carl Zeiss was an entrepreneur who needed a competent designer to take his firm beyond just another optical workshop. In 1866, the service of Dr Ernst Abbe was enlisted. From then on novel products appeared in rapid succession which brought the Zeiss company to the forefront of optical technology.

Abbe was instrumental in the development of the famous Jena optical glass. When he was trying to eliminate astigmatism from microscopes, he realised that the range of optical glasses available was insufficient. After some calculations, he realised that performance of optical instruments would dramatically improve, if optical glasses of appropriate properties were available. His challenge to glass manufacturers was finally answered by Dr Otto Schott, who established the famous glassworks at Jena from which new types of optical glass began to appear from 1888, and employed by Zeiss and other makers.

The new Jena optical glass also opened up the possibility of increased performance of photographic lenses. The first use of Jena glass in a photographic lens was by Voigtländer, but as the lens was an old design its performance was not greatly improved. Subsequently, the new glasses would demonstrate their value in correcting astigmatism, and in the production of achromatic and apochromatic lenses. Abbé started the design of a photographic lens of symmetrical design with five elements, but went no further.

Zeiss’ innovative photographic lens design was due to Dr Paul Rudolph. In 1890, Rudolph designed an asymmetrical lens with a cemented group at each side of the diaphragm, and appropriately named “Anastigmat”. This lens was made in three series: Series III, IV and V, with maximum apertures of f/7.2, f/12.5, and f/18 respectively. In 1891, Series I, II and IIIa appeared with respective maximum apertures of f/4.5, f/6.3, and f/9 and in 1893 came Series IIa of f/8 maximum aperture. These lenses are now better known by the trademark “Protar” which was first used in 1900.

At the time, single combination lenses, which occupy one side of the diaphragm only, were still popular. Rudolph designed one with three cemented elements in 1893, with the option of fitting two of them together in a lens barrel as a compound lens, but it was found to be the same as the Dagor by C.P. Goerz, designed by Emil von Höegh. Rudolph then came up with a single combination with four cemented elements, which can be considered as having all the elements of the Protar stuck together in one piece. Marketed in 1894, it was called the Protarlinse Series VII, the most highly corrected single combination lens with maximum apertures between f/11 and f/12.5, depending on its focal length.

But the important thing about this Protarlinse is that two of these lens units can be mounted in the same lens barrel to form a compound lens of even greater performance and larger aperture, between f/6.3 and f/7.7. In this configuration it was called the Double Protar Series VIIa. An immense range of focal lengths can thus be obtained by the various combination of Protarlinse units.

Rudolph also investigated the Double-Gauss concept of a symmetrical design with thin positive menisci enclosing negative elements. The result was the Planar Series Ia of 1896, with maximum apertures up to f/3.5, one of the fastest lenses of its time. Whilst it was very sharp, it suffered from coma which limited its popularity. However, further developments of this configuration made it the design of choice for high-speed lenses of standard coverage.

Probably inspired by the Stigmatic lenses designed by Hugh Aldis for Dallmeyer of London, Rudolph designed a new asymmetrical lens with four thin elements, the Unar Series Ib, with apertures up to f/4.5. Due to its high speed it was used extensively on hand cameras.

The most important Zeiss lens by Rudolph was the Tessar, first sold in 1902 in its Series IIb f/6.3 form. It can be said as a combination of the front half of the Unar with the rear half of the Protar. This proved to be a most valuable and flexible design, with tremendous development potential. Its maximum aperture was increased to f/4.7 in 1917, and reached f/2.7 in 1930. It is probable that every lens manufacturer has produced lenses of the Tessar configuration.

Rudolph left Zeiss after the First World War, but many other competent designers such as Merté, Wandersleb, etc. kept the firm at the leading edge of photographic lens innovations. One of the most significant designer was the ex-Ernemann man Dr Ludwig Bertele, famed for his Ernostar high-speed lens.

With the advent of the Contax by Zeiss-Ikon, the first serious challenge to the Leica in the field of professional 35 mm cameras, both Zeiss-Ikon and Carl Zeiss decided to beat the Leica in every possible way. Bertele’s Sonnar series of lenses designed for the Contax were the match in every respect for the Leica for at least two decades. Other lenses for the Contax included the Biotar, Biogon, Orthometar, and various Tessars and Triotars.

The last important Zeiss innovation before the Second World War was the technique of applying anti-reflective coating to lens surfaces invented by Olexander Smakula in 1935.[5] A lens so treated was marked with a red “T”, short for “Transparent”. The technique of applying multiple layers of coating was also described in the original patent writings in 1935.[6]

After the partitioning of Germany, a new Carl Zeiss optical company was established in Oberkochen, while the original Zeiss firm in Jena continued to operate. At first both firms produced very similar lines of products, and extensively cooperated in product-sharing, but they drifted apart as time progressed. Jena’s new direction was to concentrate on developing lenses for the 35 mm single-lens reflex camera, and many achievements were made, especially in ultra-wide angle designs. In addition to that, Oberkochen also worked on designing lenses for large format cameras, interchangeable front element lenses such as for the 35 mm single-lens reflex Contaflex, and other types of cameras.

Since the beginning of Zeiss as a photographic lens manufacturer, it has had a licensing programme which allows other manufacturers to produce its lenses. Over the years its licensees included Voigtländer, Bausch & Lomb, Ross, Koristka, Krauss, Kodak. etc. In the 1970s, the western operation of Zeiss-Ikon got together with Yashica to produce the new Contax cameras, and many of the Zeiss lenses for this camera, among others, were produced by Yashica’s optical arm, Tomioka. Yashica’s owner Kyocera ended camera production in 2006. Yashica lenses were then made by Cosina, who also manufactured most of the new Zeiss designs for the new Zeiss Ikon coupled rangefinder camera. Another licensee active today is Sony who uses the Zeiss name on lenses on its video and digital still cameras.

A zoom lens is a mechanical assembly of lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed focal length (FFL) lens (see prime lens).

A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes.[1] Most consumer zoom lenses do not maintain perfect focus, but are still parfocal designs.

The convenience of variable focal length comes at the cost of complexity – and some compromises on image quality, weight, dimensions, aperture, autofocus performance, and cost. For example, all zoom lenses suffer from at least slight, if not considerable, loss of image resolution at their maximum aperture, especially at the extremes of their focal length range. This effect is evident in the corners of the image, when displayed in a large format or high resolution. The greater the range of focal length a zoom lens offers, the more exaggerated these compromises must become.[2]

Contents
1 Characteristics
2 History
3 Design
4 Varifocal lens
5 See also
6 References
6.1 Citations
6.2 Sources
Characteristics

A photograph taken with a zoom lens, whose focal length was varied during the course of the exposure
Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or “4×” zoom. The term superzoom or hyperzoom is used to describe photographic zoom lenses with very large focal length factors, typically more than 5× and ranging up to 19× in SLR camera lenses and 22× in amateur digital cameras. This ratio can be as high as 300× in professional television camera lenses.[3] As of 2009, photographic zoom lenses beyond about 3× cannot generally produce imaging quality on par with prime lenses. Constant fast aperture zooms (usually f/2.8 or f/2.0) are typically restricted to this zoom range. Quality degradation is less perceptible when recording moving images at low resolution, which is why professional video and TV lenses are able to feature high zoom ratios. High zoom ratio TV lenses are complex, with dozens of optical elements, often weighing more than 25 kg (55 lb).[4] Digital photography can also accommodate algorithms that compensate for optical flaws, both within in-camera processors and post-production software.

Some photographic zoom lenses are long-focus lenses, with focal lengths longer than a normal lens, some are wide-angle lenses (wider than normal), and others cover a range from wide-angle to long-focus. Lenses in the latter group of zoom lenses, sometimes referred to as “normal” zooms[citation needed], have displaced the fixed focal length lens as the popular one-lens selection on many contemporary cameras. The markings on these lenses usually say W and T for “Wide” and “Telephoto”. Telephoto is designated because the longer focal length supplied by the negative diverging lens is longer than the overall lens assembly (the negative diverging lens acting as the “telephoto group”).[5]

Unusual trailed-zoom view of a VLT telescope building[6]
Some digital cameras allow cropping and enlarging of a captured image, in order to emulate the effect of a longer focal length zoom lens (narrower angle of view). This is commonly known as digital zoom and produces an image of lower optical resolution than optical zoom. Exactly the same effect can be obtained by using digital image processing software on a computer to crop the digital image and enlarge the cropped area. Many digital cameras have both, combining them by first using the optical, then the digital zoom.

Zoom and superzoom lenses are commonly used with still, video, motion picture cameras, projectors, some binoculars, microscopes, telescopes, telescopic sights, and other optical instruments. In addition, the afocal part of a zoom lens can be used as a telescope of variable magnification to make an adjustable beam expander. This can be used, for example, to change the size of a laser beam so that the irradiance of the beam can be varied.

History

The Voigtländer Zoomar, 36–82 mm f/2.8
Early forms of zoom lenses were used in optical telescopes to provide continuous variation of the magnification of the image, and this was first reported in the proceedings of the Royal Society in 1834. Early patents for telephoto lenses also included movable lens elements which could be adjusted to change the overall focal length of the lens. Lenses of this kind are now called varifocal lenses, since when the focal length is changed, the position of the focal plane also moves, requiring refocusing of the lens after each change.

The first true zoom lens, which retained near-sharp focus while the effective focal length of the lens assembly was changed, was patented in 1902 by Clile C. Allen (U.S. Patent 696,788). An early use of the zoom lens in cinema can be seen in the opening shot of the movie “It” starring Clara Bow, from 1927. The first industrial production was the Bell and Howell Cooke “Varo” 40–120 mm lens for 35mm movie cameras introduced in 1932. The most impressive early TV Zoom lens was the VAROTAL III, from Rank Taylor Hobson from UK built in 1953. The Kilfitt 36–82 mm/2.8 Zoomar introduced in 1959 was the first varifocal lens in regular production for still 35mm photography.[7] The first modern film zoom lens, the Pan-Cinor, was designed around 1950 by Roger Cuvillier, a French engineer working for SOM-Berthiot. It had an optical compensation zoom system. In 1956, Pierre Angénieux introduced the mechanical compensation system, enabling precise focus while zooming, in his 17-68mm lens for 16mm released in 1958. The same year a prototype of the 35mm version of the Angénieux 4x zoom, the 35-140mm was first used by cinematographer Roger Fellous for the production of Julie La Rousse. Angénieux received a 1964 technical award from the academy of motion pictures for the design of the 10 to 1 zoom lenses, including the 12-120mm for 16mm film cameras and the 25-250mm for 35mm film cameras.

Since then advances in optical design, particularly the use of computers for optical ray tracing, has made the design and construction of zoom lenses much easier, and they are now used widely in professional and amateur photography.

Canon AE-1, a 35mm camera with a zoom lens. The advantage of a zoom lens is the flexibility, but the disadvantage is the optical quality. Prime lenses have a greater image quality in comparison.
Design

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A simple zoom lens system. The three lenses of the afocal system are L1, L2, L3 (from left). L1 and L2 can move to the left and right, changing the overall focal length of the system (see image below).
There are many possible designs for zoom lenses, the most complex ones having upwards of thirty individual lens elements and multiple moving parts. Most, however, follow the same basic design. Generally they consist of a number of individual lenses that may be either fixed or slide axially along the body of the lens. While the magnification of a zoom lens changes, it is necessary to compensate for any movement of the focal plane to keep the focused image sharp. This compensation may be done by mechanical means (moving the complete lens assembly while the magnification of the lens changes) or optically (arranging the position of the focal plane to vary as little as possible while the lens is zoomed).

A simple scheme for a zoom lens divides the assembly into two parts: a focusing lens similar to a standard, fixed-focal-length photographic lens, preceded by an afocal zoom system, an arrangement of fixed and movable lens elements that does not focus the light, but alters the size of a beam of light travelling through it, and thus the overall magnification of the lens system.

Movement of lenses in an afocal zoom system
In this simple optically compensated zoom lens, the afocal system consists of two positive (converging) lenses of equal focal length (lenses L1 and L3) with a negative (diverging) lens (L2) between them, with an absolute focal length less than half that of the positive lenses. Lens L3 is fixed, but lenses L1 and L2 can be moved axially in a particular non-linear relationship. This movement is usually performed by a complex arrangement of gears and cams in the lens housing, although some modern zoom lenses use computer-controlled servos to perform this positioning.

While the negative lens L2 moves from the front to the back of the lens, the lens L1 moves forward and then backward in a parabolic arc. In doing so, the overall angular magnification of the system varies, changing the effective focal length of the complete zoom lens. At each of the three points shown, the three-lens system is afocal (neither diverging or converging the light), and hence does not alter the position of the focal plane of the lens. Between these points, the system is not exactly afocal, but the variation in focal plane position can be small enough (about ±0.01 mm in a well-designed lens) not to make a significant change to the sharpness of the image.

An important issue in zoom lens design is the correction of optical aberrations (such as chromatic aberration and, in particular, field curvature) across the whole operating range of the lens; this is considerably harder in a zoom lens than a fixed lens, which needs only to correct the aberrations for one focal length. This problem was a major reason for the slow uptake of zoom lenses, with early designs being considerably inferior to contemporary fixed lenses and usable only with a narrow range of f-numbers. Modern optical design techniques have enabled the construction of zoom lenses with good aberration correction over widely variable focal lengths and apertures.

Whereas lenses used in cinematography and video applications are required to maintain focus while the focal length is changed, there is no such requirement for still photography and for zoom lenses used as projection lenses. Since it is harder to construct a lens that does not change focus with the same image quality as one that does, the latter applications often use lenses that require refocusing once the focal length has changed (and thus strictly speaking are varifocal lenses, not zoom lenses). As most modern still cameras are autofocusing, this is not a problem.

Designers of zoom lenses with large zoom ratios often trade one or more aberrations for higher image sharpness. For example, a greater degree of barrel and pincushion distortion is tolerated in lenses that span the focal length range from wide angle to telephoto with a focal ratio of 10× or more than would be acceptable in a fixed focal length lens or a zoom lens with a lower ratio. Although modern design methods have been continually reducing this problem, barrel distortion of greater than one percent is common in these large-ratio lenses. Another price paid is that at the extreme telephoto setting of the lens the effective focal length changes significantly while the lens is focused on closer objects. The apparent focal length can more than halve while the lens is focused from infinity to medium close-up. To a lesser degree, this effect is also seen in fixed focal length lenses that move internal lens elements, rather than the entire lens, to effect changes in magnification.

Varifocal lens
Many so-called “zoom” lenses, particularly in the case of fixed-lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximal aperture, size, weight, cost) than true parfocal zoom, and which is practical because of autofocus, and because the camera processor can move the lens to compensate for the change in the position of the focal plane while changing magnification (“zooming”), making operation essentially the same as a true parfocal zoom.