The angular range from which an imaging system can receive light is always limited by some kind of optical aperture, which is called the aperture stop. The mentioned angular range is considered from the central point of the object plane, to which the instrument is focused. Different apertures can play the role of the aperture stop:
- There can be an optical element (e.g. a lens) of limited size.
- Alternatively, there may be an intentionally introduced optical aperture, e.g. an iris.
- In and optical element for direct viewing, for example a telescope or microscope, the effectively usable light may be limited by the pupil of the observing eye.
If an optical system contains multiple apertures, the aperture stop is that aperture which most severely limits the mentioned angular range.
The aperture stop sets a limit to the light gathering power of the instrument. It often needs to be reduced in order to avoid excessive optical aberrations which would occur for a larger opening angles.