Angular aperture

The angular aperture of a lens is the angular size of the lens aperture as seen from the focal point: where In a medium with an index of refraction close to 1, such as air, the angular aperture is approximately equal to twice the numerical aperture of the lens.

[1] Formally, the numerical aperture in air is: In the paraxial approximation, with a small aperture,

< f

{\displaystyle D

:

The angular aperture of a thin lens with focal point at F and an aperture of diameter .