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Glossary > Photography >Focal Length
Focal Length
The focal length is the length of the path through a telescope which incoming light follows. In a Newtonian this is the distance from the primary mirror to the eyepiece. In a refractor it is the distance from the objective lens to the eyepiece. Schmidt-Cassegrains (SCTs) are a special example in which the effective focal length is actually longer than the path the light follows. The reason for this is that the secondary mirror in a Schmidt-Cassegrain has the effect of multiplying the focal length of the primary mirror by a factor of about 5. Therefore a Schmidt-Cassegrain can be much shorter than its effective focal length, which is part of the reason SCTs are so popular. The magnification of a telescope is determined in part by the focal length of the scope.
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