# Angular aperture

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

where

- 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, :

## References

- ↑ {{#invoke:citation/CS1|citation |CitationClass=book }}

## See also

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