Paul Jolly Photography Chapter 24.1

Physical vignetting

This is caused by an obstruction in the light path, for a photographer this is often the lens hood (if you have chosen badly), or a lens that can only produce an image over part of the sensor area; the latter may be by design.
The second physical effect is from the lens construction itself: As we approach the edges of the image light entering the lens optics is prevented by the physical edges of the lens, thus reducing the level detected at the sensor.
The vignetting is a gradual change rather than a sharp cutoff because the image is not focused on the obstruction, we see the edge as an out of focus blur - rather than having to explain the effect each time we have given it a name - vignetting.