# Focal length

The distance at which a telescope's lens or mirror brings light to a focus; with the eyepiece it sets the magnification.

A telescope's focal length divided by an eyepiece's gives the [magnification](https://astropilot.space/glossary/magnification): 1,200 mm with a 25 mm eyepiece makes 48×. Long-focus telescopes (SCTs, Maksutovs) give high power easily; short-focus ones (fast Newtonians, small refractors) give wide, low-power views.

[Barlow lenses](https://astropilot.space/glossary/barlow) and [coma correctors](https://astropilot.space/glossary/coma-corrector) lengthen the effective focal length; [focal reducers](https://astropilot.space/glossary/focal-reducer) shorten it.

**Related:** [Magnification](https://astropilot.space/glossary/magnification.md), [Focal ratio](https://astropilot.space/glossary/focal-ratio.md), [Barlow lens](https://astropilot.space/glossary/barlow.md), [Focal reducer](https://astropilot.space/glossary/focal-reducer.md)

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