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The **angular aperture** of a lens is the angular size of the lens aperture as seen from the focal point:

where

- is the focal length
- is the diameter of the 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, :

- f-number
- Numerical aperture
- Acceptance angle, half the angular aperture
- Field of view