“Lens vignetting”, “Light Fall-off” or simply “Vignetting” is a problem that is associated with the darkening of the frame at its corners. This is typically most common with fast lenses such as f/2 or even faster when shot using its widest aperture. For those who have experience shooting with old film cameras and are now changing to digital SLRs, they may identify with this problem a little more as these older film SLRs had this typical problem more often. Today’s professional DSLR cameras often correct this problem internally, but technically vignetting is a problem that remains and it is very difficult and expensive to manufacture super fast lenses which has absolutely no vignetting at all.
Vignetting is a problem that affects more of the full frame cameras than the cropped cameras. Say if you are using something like an EOS 5D Mark III, you are bound to get affected by vignetting when shooting with a fast EF lens. Lenses which has a wide aperture such as f/2 or anything above that is bound to get affected with the problem of vignetting. However mount the same lens on to a cropped body say a 7000D and magically vignetting is gone! Why this happens? This is rather simple actually. The cropped sensor only uses the central part of the image when compared to the full frame bodies. As such the corners which mostly gets affected by vignetting are avoided in the frame..
Having said that vignetting can be worked around and for some users a slight bit of vignetting is not going to be a problem most of them times. Some people even like it to have in their picture. For professional photographers however, who want absolutely nothing but the best, vignetting is not at all tolerable. So, how to get rid of vignetting on a fast lens without having to necessarily shift to a more expensive piece of glass? Many photographers have used ingenious methods to counter vignetting on their full frame bodies. Circular polarizers have been one of the many accessories that have been used with some success. Circular polarizers (and not the linear ones) are a bit pricey but they do the job of allowing the light to enter through the lens in a particular angle. That in itself reduces the problem of vignetting to some extent.
Graduated neutral density filters can also suppress the problem of vignetting to some extent. Graduated neutral density filters (also known as Grad ND filters) are suitable when you want to suppress the brightness of the sky or the Sun against a foreground that is not so bright. They also serve the purpose of providing a good suppression for the problem of vignetting as they suppress the top or the bottom part of the frame (as per the direction you hold the filter over the lens). However Grad ND filters cannot compensate the problems in all four corners of the frame. A circular polarizer may be more suitable for that purpose. There are many colored Grad ND filters available that are used to put a bit of color to the otherwise bland frame, say if you want to warm up the sky in the final frame. These is also suitable for the purpose of warding off vignetting.