Entrance pupil
A size of the beam of light which, entering the elements of a compound lens that are in front of the aperture, completely fills the iris diaphragm.
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More Photographic Terms ...
Exit Pupil The exit pupil of a lens system is an image of the entrance pupil (hence conjugate to it) and normally should be the image of the limiting diaphram.
where f is the focal length, and D is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value.
A more accurate term is the entrance pupil, but even this refers to a small area and not an individual point.
Panoramas - Finding the entrance pupil of a lens
Do your panoramas right, by finding the entrance pupil with this simple method
by Nikonian Bill Claff (bclaff) ...
a ratio of the equivalent focal length of a lens to the diameter of its entrance pupil. This indication is called "f" number, express in the ways like f/8 F8 1:8.
The "f-number" defines the relation between the focal length of a lens (e.g. 50mm) and the diameter of the opening of a lens (roughly the size of the entrance pupil = front element) ...
f-number = (effective focal length) / (clear aperture) ...
The degree of background blur is mainly a function of the lens's angle of view and the diameter of its entrance pupil, and this means that the X100's 23mm F2 lens offers broadly similar capability to the Panasonic 20mm F1.
 See also: Lens, Pupil, Subject, Focus, Aperture
  
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