Angular aperture

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 {D/2} {f} ight)

where :f is the focal length:D is the diameter of the aperture.

Relation to numerical aperture

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. [citebook|title=Studies in Optics |author=Albert Abraham Michelson|year= 1995|publisher=Courier Dover|pages=32|id=ISBN 0486687007|url=]

Formally, the numerical aperture in air is:

:mathrm{NA} = sin a/2 = sin arctan left( frac {D} {2 f} ight)

In the paraxial approximation, with a small aperture, D:

:mathrm{NA} approx a/2


ee also

*Numerical aperture

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