Aperture Ratio

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Aperture Ratio

The ratio of the useful diameter of a lens to its focal length. It is the reciprocal of the f-number.

In application to an optical instrument, rather than to a lens, numerical aperture is more commonly used. The aperture ratio is then twice the tangent of the angle whose sine is the numerical aperture.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use