The field of view of an imaging system, i.e., the angular range in which objects can be imaged, is always limited by some kind of optical aperture, which is called the field stop. Different apertures can play that role:
- In a digital photo camera, the limited size of the image sensor usually limits the field of view: light from more extreme angles might still get through the optical system, but would not hit the image sensor and therefore not contribute to the image.
- A field stop can also be a diaphragm which is located in an intermediate image plane.
- Sometimes, an optical aperture results from the construction of the instrument.
If an optical system contains multiple apertures, the field stop is that aperture which most severely limits the field of view.
Note that an optical aperture does not lead to sharp edges of the field of view, if it does not lie in a plane conjugate to the object plane. There is then some vignetting effect, i.e., a gradual decrease of image brightness at the edges. In a Keplerian telescope, for example, the field of view is limited by the ratio of the diameter of the ocular lens (essentially the barrel diameter) and the focal length of the objective. The field stop is then the barrel. As the barrel entrance is behind the intermediate image plane, there is some vignetting.