Exit Pupil
The apparent size and position of the aperture as seen from behind the lens, which governs how light strikes the sensor.
Measured in
mm
In depth
An exit pupil far from the sensor sends light in nearly perpendicular, which digital sensors prefer; one close to the sensor sends corner rays in at steep angles, causing colour casts and smearing. The exit pupil's distance also determines the true effective aperture at close focus and affects how strongly a lens vignettes.
Examples in practice
A symmetrical rangefinder wide-angle has its exit pupil close to the film plane, producing magenta corners on a digital body.