She even remembers the aperture stop (lens opening) she used to obtain a particular effect more than 50 years ago.

The aperture stop can be adjusted to control the amount of light reaching the film or image sensor.

Note that the aperture stop is not necessarily the smallest stop in the system.

Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system.

The aperture stop is an important element in most optical designs.

In both cases, the size of the aperture stop is constrained by things other than the amount of light admitted; however:

A larger aperture stop requires larger diameter optics, which are heavier and more expensive.

In addition to an aperture stop, a photographic lens may have one or more field stops, which limit the system's field of view.

It was known in the 1500s that an aperture stop would improve lens image quality.

The exit pupil is the image of the aperture stop in the optics that follow it.