# Focal ratio

Also called: f-number, f-ratio, f/, fast, slow.

Focal length divided by aperture (f/5, f/10…). “Fast” (low) ratios give wide fields; “slow” (high) ones give easy high power.

An f/5 telescope has a focal length five times its aperture. For visual use, the focal ratio sets how wide a field your eyepieces can give and how short an eyepiece you need for high power — exit pupil in mm is the eyepiece's focal length divided by the focal ratio.

Fast Newtonians (below about f/5.5) suffer from [coma](https://astropilot.space/glossary/coma) at the edge of the field and need precise [collimation](https://astropilot.space/glossary/collimation); slow ones are forgiving.

**Related:** [Focal length](https://astropilot.space/glossary/focal-length.md), [Aperture](https://astropilot.space/glossary/aperture.md), [Coma](https://astropilot.space/glossary/coma.md), [Exit pupil](https://astropilot.space/glossary/exit-pupil.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/focal-ratio