Focal length

Focal length is the distance, measured in millimeters, that light travels from a telescope's main lens or mirror to the point where it forms a sharp image. It is printed on almost every telescope tube and eyepiece box, and it is the second number, after aperture, that determines how a telescope behaves at the eyepiece. A longer focal length spreads the same view over more magnification for a given eyepiece, while a shorter one keeps views wider and brighter.

The practical use of focal length is calculating magnification: divide the telescope's focal length by the eyepiece's focal length. A 1000mm telescope with a 10mm eyepiece gives 100x, while the same telescope with a 25mm eyepiece gives only 40x. This relationship is why the same eyepiece produces very different results on different telescopes, and why buyers should not judge an eyepiece by magnification alone without knowing the telescope's focal length.

When choosing a telescope, longer focal lengths generally suit close-up planetary and lunar views, while shorter focal lengths suit wide fields for star clusters and large nebulae. A frequent point of confusion is assuming a telescope with a bigger tube automatically has a longer focal length, which is not always true for compact catadioptric designs that fold a long light path into a short body.