entrancepupil

Entrance pupil (sometimes written as entrancepupil in some sources) is an optical term describing the image of the aperture stop of a lens system, as it is viewed from object space. It represents the effective aperture through which light enters the optical system and toward the image plane. The entrance pupil is formed by the aperture stop and the front lens elements; its size and position depend on focal length, focusing distance, and lens construction. Unlike the physical diaphragm opening, the entrance pupil can appear larger or smaller due to magnification by the front optics.

The entrance pupil is distinct from the exit pupil, which is the image of the aperture stop as seen from the image plane (the lens's pupil position for an observer looking into the camera). The entrance pupil governs how much light the lens can admit from a given angle and how light rays from the scene are sampled by the optics. It also influences vignetting and the entrance angle of rays at the image plane, affecting brightness uniformity across the frame.

In practice, photographers and optical designers refer to the entrance pupil when considering brightness and field coverage. It changes with focal length and focusing distance, so the apparent size and position of the pupil may move when zooming or focusing. Knowledge of the entrance pupil helps predict shading, edge performance, and the angular field of illumination, particularly in wide-angle and zoom lenses.