focal ratio

Show Summary Details

Quick Reference

The ratio of the focal length of a lens or mirror to its aperture, usually expressed as a number, the f/number. The smaller the focal ratio, the smaller the image scale and the brighter the image for a given aperture. Small focal ratios, below about f/6, are termed fast, while those greater than about f/8 are slow, a terminology adopted from photography.

Subjects: Astronomy and Astrophysics.

Reference entries

Users without a subscription are not able to see the full content. Please, subscribe or login to access all content.