The theoretical limit of useful magnification (power) for a telescope is 50 or 60 times the telescope's diameter (aperture) in inches, or two times the aperture in millimeters. So, for a 60mm refractor, the maximum useful power is 120x (despite what some manufacturers claim!). Higher magnification is pointless, because the image seen in the eyepiece will be too dim and fuzzy.

Powers lower than the theoretical limit are usually preferred, because sky conditions are rarely ideal. Also, many deep-sky objects need low magnification because of their low surface brightness. For larger telescopes, the highest useful power is about 300x in typical atmospheric conditions. You can vary the magnification by using eyepieces of different focal lengths. The magnification is the telescope focal length divided by the eyepiece focal length, in millimeters.

Telescope F.L. ÷ Eyepiece F.L. = Magnification

For example, a 40mm eyepiece used with a Celestron 8" telescope (focal length 2000mm) produces 50x magnification: 2000mm ÷ 40mm = 50x. Replace the 40mm with a 15mm eyepiece and you get 133x.

Powers of 10x to 15x per inch of aperture are used most frequently. Doubling the power gives you one-fourth the image brightness and reduces the sharpness by one half.