Angular aperture

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The angular aperture of a thin lens with focal point at F and an aperture of diameter .

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


is the focal length
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.[1]

Formally, the numerical aperture in air is:

In the paraxial approximation, with a small aperture, :


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See also