The Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens review is a reasonably-priced lens that comes in a small package and packs a really nice focal length range.
The wide focal length range covered by this lens is definitely its strongest asset. Before I go any farther, I need to specify that this is a Sigma “DC” or Digital Camera-only lens meaning it is only compatible with Canon 1.6x FOVCF DSLR bodies. From 18mm on the wide end to 125mm on the long end, few lens changes are needed to get the desired framing for a wide range of scenes.
This lens has a relatively narrow max aperture (which helps keep the weight, size and price down), but Sigma’s OS (Optical Stabilization) comes to the rescue in low light situations. This OS implementation is rated at 3-4 stops. In a not-terribly-stable shooting position, I can achieve sharp images (at a decent hit rate) handholding this lens at shutter speeds of 1/5 second at 18mm and 1/20 second at 125mm. This is about 3- and 3+ stops of improvement respectively for me personally.
The OS settings for the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens are simply On and Off. Even when the OS switch is off, the light whirring sound indicating OS is functioning can be heard while the camera meter is active. Sigma stated to me that OS remains active to hold the lens in an optically centered position – and you see the viewfinder framing jump to a new position when the meter is activated or deactivated. Keep this in mind when framing your shot – especially when setting up a tripod-based shot (keep the meter active).