Stops

The limiting effect on the imaging process by the size of the unobstructed clear diameter in an optical system depends upon the location of that "stop", or limiting diameter. The limiting diameter which determines the amount of light which reaches the imaging area is called the aperture stop, typified by the adjustable diaphragm near the front of a compound camera lens. The limiting diameter which controls the size object which can be imaged is called the field stop.

Apertures and f-stops

Aperture Stops

An aperture stop is the opening which limits the amount of light which passes through an optical system. For example, the adjustable diaphragm near the front of a compound camera lens is the aperture stop for the lens. The amount of light admitted is controlled by the diameter of the diaphragm opening which is indicated on the camera by the "f number" or "f-stop number". Making the aperture smaller reduces the light, but increases the depth of focus.

Field Stops

Pupils

Entrance Pupil

The entrance pupil of a system is the image of the aperture stop as seen from a point on the optic axis in the object plane.

Exit Pupil

The exit pupil of a system is the image of the aperture stop as seen from a point in the image plane on the optic axis.