Focal length is the distance from a telescope’s objective element (lens or primary mirror) to the point where rays of light from the objective converge to a focus. It’s measured in inches or millimeters.
Longer focal lengths will have more capacity for high magnification but narrower fields of view than shorter focal lengths. For example, a telescope with a focal length of 2000 mm has twice the power and half the field of view of a 1000 mm telescope when using the same eyepiece.
Most manufacturers specify the focal length of their scopes. If it‘s unknown and you know the focal ratio, you can use the following formula to calculate focal length. Focal length is the aperture (in mm) times the focal ratio. Example, the focal length of an 8 in (203.2 mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032 mm.