In astronomy, focal length is primarily used when referring to telescopes. It is the distance light travels starting from the lens of a telescope down to the point where it is focused. While in optics, focal length is simply defined as the strength by which an optical system (like a telescope) converges (focuses) or diverges (defocuses) light.

Depending on the needs of an astronomer, he might desire a telescope with a shorter focal length to achieve a wider angle of view (useful in searching the sky for fast moving objects); while a longer focal length will achieve larger magnification of more distant objects at the cost of having a smaller angle of view. Conventionally, telescopes with long focal lengths are used to view objects in the sky which have a predetermined path or movement, eliminating the need to have a wide angle of view for searching. It is also used in viewing especially very distant objects that cannot be seen clearly using a telescope with a much shorter focal length.

In optical systems with multiple mirrors or lenses, the focal length is usually called the EFL or the effective focal length; distinguishing it from the FFL (front focal length) and the BFL (back focal length).