Entrance pupil

The entrance pupil is a useful concept for determining the size of the cone of rays that an optical system will accept.

Once the size and the location of the entrance pupil of an optical system is determined, the maximum cone of rays that the system will accept from a given object plane is determined solely by the size of the entrance pupil and its distance from the object plane, without any need to consider ray refraction by the optics.

[5] This point is important in panoramic photography without digital image processing, because the camera must be rotated around the center of the entrance pupil to avoid parallax errors in the final, stitched panorama.

[6][7] Panoramic photographers often incorrectly refer to the entrance pupil as a nodal point, which is a different concept.

[1] Similarly, vignetting can cause different lateral locations at a given object plane to have different aperture stops, and therefore different entrance pupils.

The entrance pupil made by a single lens with an aperture (aperture stop) behind it. 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 apparent location of the anatomical pupil of a human eye (black circle) is the eye's entrance pupil location. The outside world appears to be seen from the point at the center of the entrance pupil. The anatomical pupil itself is slightly different from the entrance pupil because the image is magnified by the cornea .