Angular aperture

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

where

Formally, the numerical aperture in air is:

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