J. Lipinski, Miniature and Precision Cameras, Iliffe & Sons, 1955

VIGNETTING

There is, however, one other factor which is very important. It is known as vignetting and represents the reduction of the effective aperture of a lens for even quite small angles off the lens axis. Fig. 3.19 shows how the lens aperture appears from the negative at the lens axis and then while progressing towards the corner. We see that not only does it vary with the angle but also with the size of the stop used. Generally, using a smaller stop improves matters.