In optics the exit pupil is the disc-shaped area from which all rays that exit the lens come from. Whenever you look at a piece of optics(photographic lens, binoculars, etc.) from the rear, the circle of light that you see is the exit pupil.
In practice (with photographic lenses) the exit pupil is defined the aperture mechanism. Depending on the design of the lens, the exit pupil can seem to be very far into the lens, or it can seem to be very near the rearmost lens’ surface.
An important concept in optics is the distance between the exit pupil and the imaging plane (sensor or film). This distance is not measured by taking the physical distance between the aperture mechanism and imaging plane, but is instead calculated based on the apparent ‘depth’ of how deep inside the lens the exit pupil seems to be.
Therefore, depending on the optical design a lens uses, the distance between exit pupil and imaging plane can vary significantly even with lenses of identical mainline specifications (focal length and maximum aperture)
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