When more light reaches the center of the negative than reaches the periphery, the result is an image that is bright in the center but dark in the corners: a vignette. Vignetting is inherent to most lenses (especially lenses with a wide angle of view). It becomes most apparent when the aperture is wide open, but will disappear when it is set a stop or two down from maximum.
The effect of this light falloff will largely go unnoticed, but is often visible when the subject being photographed has large solid areas of even color and brightness.
In a very wide-angle lens, the falloff can leave the periphery two stops darker than the center. This can be corrected with a graduated neutral density filter, which is clear at the edges and darker at the center, ensuring the frame is evenly illuminated.
