The distance from a lens or mirror to its focal point. The focal length, f, is the defining parameter of most lenses and mirrors. See also concave, convex
Focal length is the distance between the point where incoming light meets the first optical element and the point where it passes through the eyepiece to create a magnified image. Focal length determines the telescope's magnification and the longer the focal length, the more powerful the telescope.
See Equivalent Focal Length.