pupilplane

The pupil plane is a plane in an optical instrument that is optically conjugate to the entrance pupil. It is the plane where the aperture stop is imaged, and it is often used to describe the distribution of light across the pupil, including its shape, transmission, and phase. The entrance pupil is the image, as seen from object space, of the aperture stop; the exit pupil is the image, as seen from image space, of the same stop. In many simple systems the entrance stop lies in a single physical plane and the pupil plane coincides with the stop plane; in more complex lenses there can be multiple conjugate pupil planes linked by the system’s optics.

The pupil function defined on the pupil plane specifies how amplitude and phase vary across the aperture. This function is central in Fourier optics: the image plane field is proportional to the Fourier transform of the pupil function (under appropriate approximations). Consequently, pupil-plane properties govern the optical transfer function, point spread function, and depth of field of the system. Pupil-plane analysis is used to model vignetting, aberrations, and to implement pupil apodization, phase masks, and adaptive optics corrections.

Practically, locating the pupil plane helps in placing diaphragms and stops, assessing illumination uniformity, and designing optical systems to control the angular distribution of light. In telescopes and microscopes, the pupil plane condition determines how off-axis rays are transmitted and how aberrations are corrected.