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From Wikipedia, the free encyclopedia
The angular aperture of a lens is the angular size of the lens aperture as seen from the focal point:
where
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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