- The distance at which a converging mirror or lens will bring parallel light to a point, or the distance from which a parallel beam will appear to diverge after passing through a diverging lens or mirror For a simple mirror with a surface which is part of a sphere, the focal length is half of the radius of curvature. Since astronomical objects are in effect infinitely far away, a mirror or lens will focus light from them at its own focal length, near which an eyepiece can form a visible image.
- For a lens, the distance between its optical center and its principal focus.
- For a mirror, the distance between its surface and its principal focus.
- synonym focal distance
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