- the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens
WordReference Random House Unabridged Dictionary of American English © 2026
fo′cal length′, [Optics.]
- Opticsthe distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f
- Opticsthe distance between an object lens and its corresponding focal plane in a telescope. Also called fo′cal dis′tance.
- 1745–55
Collins Concise English Dictionary © HarperCollins Publishers::
focal length, focal distance n
'focal length' also found in these entries (note: many are not synonyms or translations):
amplifier
- dioptre
- f-number
- fisheye lens
- focus
- power
- relative aperture
- telephoto lens
- wide-angle lens
- zoom lens
- achromatic lens
- astronomical telescope
- bull's-eye
- chromatic aberration
- closeup
- compound microscope
- converter
- diverging lens
- F, f
- fixed-focus camera
- focometer
- Galilean telescope
- portrait lens
- Ramsden eyepiece
- retrofocus lens
- wide-angle
- spherical aberration