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An italic f, the distance from a lens to its focus.
Also known as: Focal distance, Focal length, Lens focal length
Focal length is how far behind a lens parallel light converges, and its sign carries the information: positive converges, negative diverges. The same italic f means frequency, the f-number and the Helmholtz free energy elsewhere.
Denotes the focal length of a lens or mirror: the distance from it to the point where parallel rays converge.
Contexts: Laboratory
Is signed: positive for a converging element and negative for a diverging one, under the usual sign convention.
Contexts: Laboratory
Is distinct from the f-number, written f/N, which is the focal length divided by the aperture diameter.
Contexts: Laboratory
An italic lowercase letter f, a vertical stroke with a hooked top and a crossbar.