How can I tell if a lens has vignetting, or if a filter is causing vignetting?
Open the back and, if necessary, trick the camera into opening the shutter and stopping down. Imagine putting your eye right in the corner of the frame and looking at the diaphragm. Or course, you really can’t do this, so you have to move your head and sight through the corner of the frame, trying to imagine what you would see. If you “see” the entire opening in the diaphragm and through it to subject space, there is no vignetting. However, at wide apertures in most lenses the edge of the rear element or the edge of the front element or filter ring will obstruct your vision. This is vignetting. Try to guess the fraction of the area of the diaphragm is this obstructed. Log base two of this fraction is the falloff in f-stops at the corner. You can also do this from the front. With SLRs hold the camera a fair distance away with a fairly bright area behind the viewfinder hole. With non-SLRs open the back and arrange so a reasonably bright area is behind the camera. Look through the lens, a
