Angular aperture

The angular aperture of a lens is the apparent angle of the lens aperture as seen from the focal point:

$a=2\arctan \left({\frac {A}{2f}}\right)$

where

$f$ is the focal length

$A$ is the diameter of the aperture.

The angular aperture is approximately equal to twice the numerical aperture of the lens.

See also

• f-number
• Numerical aperture