exit pupil

Definition: Image of hole or aperture formed by lens (which may be limited by the iris diaphragm) as seen at the focal plane i.e. from the image side of lens. * The effective diameter of the exit pupil is one factor determining the effective f/number of lens.
Accuracies: Go forth and mutliply
The 'effective diameter' is so qualified because few apertures are truly circular. It's important because the size of the exit pupil as seen by a through-the-lens metering system obviously affects the meter reading: as different focal length lenses may present different sized exit pupils to the metering system, the lens must signal a correction in order for metering to be accurate.

Related Terms: entrance pupil

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