Aperture stops and field of view

Every lens rim, iris, or barrel can obstruct rays. The aperture stop is the element that limits the angular cone admitted from the axial object point for the chosen conjugates. It need not be the physically smallest opening because the intervening optics change apparent sizes and angles. One way to identify it is to trace the axial bundle and determine which clear aperture is reached first as the launch angle grows. The selected object position and focus setting matter.

The entrance pupil is the image of that stop through the optics preceding it, as seen from object space. The exit pupil is its image through the following optics, as seen from image space. A pupil can be real or virtual and can differ substantially from the physical iris diameter. In a simple thin lens with an iris at its plane and no other powered elements, the iris itself serves as both pupils, which is why elementary camera examples often hide the distinction.

A field stop limits the range of object points represented in the image. A detector edge or an opening near an intermediate image often supplies this role because different field points are spatially separated there. For a distant scene and a thin lens, the chief ray from field angle beta passes through the pupil center and reaches height approximately f times beta. An image-plane opening therefore translates directly into an angular field limit under this simple model.

The chief ray connects an off-axis object point to the center of the aperture stop, while the axial marginal ray runs from the on-axis object point through the stop edge. They describe different aspects of the accepted beam. Imaging the field stop through preceding or following optics gives entrance or exit windows, whereas imaging the aperture stop gives pupils. Calling a field stop a pupil confuses a boundary on object position with a boundary on ray direction.

Closing an iris reduces the admitted cone, generally reducing collected power and changing diffraction and aberration tradeoffs. If the detector and image scale are unchanged, it does not by itself narrow the geometric field boundaries. Cropping the detector narrows field while leaving the axial pupil diameter alone. In a real instrument, another aperture may begin clipping off-axis cones before the intended field boundary, producing gradual vignetting rather than a perfectly sharp field cutoff.

Pupil placement also matters for eye position in a telescope and for telecentric imaging. An eye outside the exit pupil can lose part of the bundle even when all glass diameters seem adequate. These effects require ray heights at several physical planes, so a single f-number is insufficient to describe the whole instrument. Whenever a scene corner darkens, distinguish reduced bundle area from loss of focus or a change in source radiance; those mechanisms demand different remedies.