Entrance pupil

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Glossary Terms

The Entrance pupil of a lens is the diameter of the clear opening which intercepts and focuses light, as seen from the outside. It varies in size according to the aperture opening; but it does not exactly equal the aperture's physical diameter. This is because the aperture's apparent diameter is distorted by the refractive lens elements in front of it.

The F/ratio of a particular aperture is the focal length of the lens divided by the entrance pupil at that aperture.