The NC 40.5 Neutral Color 40.5mm filter protects the front lens element of specific 1 NIKKOR lenses that have a 40.5mm filter thread. The filter does not affect the color hues (visible light spectrum) and multi-colored glass helps to reduce flare.
The Nikon NC 40.5mm Neutral Color Filter is a general protection filter that will guard against potential damage to the front element of your valuable lens. This filter has no affect on the color of your image but will guard against scratches, dust, dirt and fingerprints from contaminating the fine optics incorporated into the lens. The value of such a filter has been well-documented.
If you should accidentally break the filter, that’s one thing but if you break the front element of your lens, you’ll need a new lens. Some people refer to this kind of filter as relatively “cheap insurance”. Should be left on at all times to protect front element Guards against fingerprints and scratches damaging the front element of your lens