In an optical system, the entrance pupil is the optical image of the physical aperture stop as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil. If there is no lens in front of it (such as a
pinhole camera), the physical aperture's location and size are identical to the entrance pupil's location and size. However, if there are refractive optics in front of the aperture (such as a more conventional camera lens or a human eye), the entrance pupil is a (usually) virtual image of the physical aperture. Due to the simple magnifying (or minifying) effect of the front lens(es), the entrance pupil's location and size are nearer and larger (or farther and smaller) than that of the physical aperture.
The geometric location of the entrance pupil is the vertex of the camera's angle of view and consequently its center of perspective, perspective point, view point, projection centre or no-parallax point. When the optical system is physically rotated about its entrance pupil, the perspective geometry of its image does not change. In
panoramic photography, for example, it is important to rotate or pivot the camera about its entrance pupil in order to avoid parallax errors in the final, stitched panorama. Depending on the lens design, the entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems.
In photography, the size of the entrance pupil (rather than the size of the physical aperture itself) is used to calibrate the opening and closing of the diaphragm aperture. The f-number ("relative aperture"), N, is defined by N = f/EN, where f is the focal length and EN is the diameter of the entrance pupil. Increasing the focal length of a lens (i.e. zooming in) will usually cause the f-number to increase, and the entrance pupil location to move further back along the optical axis.
The entrance pupil of the human eye, which is not quite the same as the physical pupil, is typically about 4 mm in diameter. It can range from 2 mm (f/8.3) in a very brightly lit place to 8 mm (f/2.1) in the dark.