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The entrance pupil is the image of the aperture stop viewed from the front of the optical system and here it is a virtual image. Chief rays and marginal rays determine the location and the size of the entrance pupil, respectively. A camera lens adjusted for large and small aperture. The visible opening is the entrance pupil of the lens.
The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
The entrance pupil is located at infinity, and chief rays before the objective are parallel to the optical axis. An object-space telecentric lens has the entrance pupil (the image of the lens's aperture stop, formed by optics before it) at infinity and provides an orthographic projection instead of the perspective projection in an entocentric lens.
The entrance pupil is an image of the aperture stop formed by the optics in the front of it, and the location and size of the pupil are determined by chief rays and marginal rays. Date 18 July 2024
The image of the pupil as seen from outside the eye is the entrance pupil, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea. On the inner edge lies a prominent structure, the collarette , marking the junction of the embryonic pupillary membrane covering the embryonic pupil.
Refraction in the cornea causes the effective aperture (the entrance pupil in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from as narrow as 2 mm (f /8.3) in diameter in a brightly lit place to 8 mm (f /2.1) in the dark as part of adaptation.
From the source point of view, etendue is the product of the area of the source and the solid angle that the system's entrance pupil subtends as seen from the source. Equivalently, from the system point of view, the etendue equals the area of the entrance pupil times the solid angle the source subtends as seen from the pupil.
A better choice of the point about which to pivot a camera for panoramic photography can be shown to be the centre of the system's entrance pupil. [9] [10] [11] On the other hand, swing-lens cameras with fixed film position rotate the lens about the rear nodal point to stabilize the image on the film. [11] [12]