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Telescopes Optics Focal length

This is the distance between the objective lens or mirror and the focal point.

Compared to the aperture, the focal length of a telescope is not such an important parameter. The following rule applies: the longer the focal length, the less the image needs to be enlarged with an eyepiece. Higher magnification is therefore possible with a telescope with a long focal length. But the field of view is however smaller.

Telescopes with a long focal length are generally more stable and less susceptible to optical aberration, since the light is less refracted and needs to be reflected through a smaller angle in order to reach the focal point.

In contrast, telescopes with a short focal length are faster. They display a brighter image and, for astrophotography, require a shorter exposure time for the same result. However the decisive characteristic for the "speed" of an optics, is not just the focal length, but also the aperture ratio, that is the ratio of aperture to the focal length. There is a separate glossary entry relating to aperture ratios.