Editing Angular aperture (section)

[[Image:Angular aperture.svg|right|The angular aperture <math>a</math> of a [[thin lens]] with focal point at '''''F''''' and an aperture of diameter <math>D</math>.|thumb|250px]]

The '''angular aperture''' <math>a</math> of a [[lens (optics)|lens]] is the [[angular size]] of the lens [[aperture]] as seen from the [[Focus (optics)|focal point]]:

:<math>a = 2 \arctan \left( \frac {D/2} {f} \right) = 2 \arctan \left( \frac {D} {2f} \right)</math>

where 
:<math>f</math> is the [[focal length]]
:<math>D</math> is the [[diameter]] of the [[aperture]].