What is vignetting and light falloff?
Vignetting is a reduction in light falling on the film far from the center of the image that is caused by physical obstructions. Light falloff happens for two reasons. First an off-axis subject sees a forshortened apparent aperture (entrance pupil) so less light is collected. This results in a cos(theta) falloff, where theta is the angle off axis. Second, in a rectilinear lens the magnification increases with cos^3(theta), thus spreading the same amount of light over more film. As a result there is an overall cos^4(theta) falloff. The optical designer can compensate for these effects by making the entrance pupil enlarge and tip when viewed from off the optical axis. An alternative approach is to compensate by using a filter whose density varys appropriately with distance from the center.
