Definition of focal length in English:

focal length

Syllabification: fo·cal length


  • 1The distance between the center of a lens or curved mirror and its focus.
    More example sentences
    • Yet Stork determined the probable focal length of a concave mirror made by inverting and silvering the convex mirror shown in the painting.
    • This produces a rounded lens with a small focal length, so the drop can focus light only to a point near itself.
    • This will give a negative focal length for a convex lens of air inside water.
  • 1.1The equivalent distance in a compound lens or telescope.
    More example sentences
    • He developed a general rule for the focal length of lenses and described a reflecting telescope.
    • Huygens later went back to the study of optics and developed long focal length lenses used in ‘aerial telescopes.’
    • One evening he was observing with a rather crude refractor (a lens telescope), with a focal length of 24 feet.

More definitions of focal length

Definition of focal length in:

Get more from Oxford Dictionaries

Subscribe to remove ads and access premium resources

Word of the day skosh
Pronunciation: skōSH
a small amount; a little