Angular aperture

The angular aperture ${\displaystyle a}$ of a lens is the angular size of the lens aperture as seen from the focal point:

${\displaystyle a=2\arctan \left({\frac {D/2}{f}}\right)=2\arctan \left({\frac {D}{2f}}\right)}$

where

${\displaystyle f}$ is the focal length

${\displaystyle D}$ is the diameter of the aperture.