Noun
the distance from a lens to its focus
Source: WordNetA first order lens has the longest focal length, with the sixth being the shortest. Source: Internet
A newer 90mm Rodenstock lens for more distant subjects is $13,000 (and another telephoto lens with a longer focal length is on the way, too). Source: Internet
As stated above in the lead, the focal length of a parabola is the distance between its vertex and focus. Source: Internet
Conversely, people who cannot increase the focal length of their lens enough to allow for distant objects to be imaged on the retina suffer from myopia and have a far point that is considerably closer than infinity. Source: Internet
Because the magnification of a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece, smaller focal-length eyepieces cause greater magnification. Source: Internet
Close-up When the subject distance approaches the lens focal length, the focal length no longer is negligible, and the approximate formulas (11),(12) above cannot be used without introducing significant error. Source: Internet