Description
This article is from the
Photographic Lenses FAQ, by David Jacobson with numerous
contributions by others.
03. What is meant by f-stop?
The focal length of the lens divided by the diameter of the
aperture (as seen from the front). It is also called an f-number, and
is written like f/8, which means the aperture diameter is 1/8th the
focal length.
The term is used both in regard to the maximum aperture of a lens and
in regard to the aperture selected in a specific situation.
The brightness of the image on the film is inversely proportional to
the f-number squared. The depth of field increases but diffraction is
worsened when using a large f-number. The effective f-number for all
3 effects changes if the lens is focused extremely close. See Q7.
The term "stops" purportedly comes from old technology in which the
aperture was selected by turning a wheel with various sized holes in
it, each one of which let in twice the light of the preceding one.
Thus the phrase "open up a N stops" means to change to an aperture
allowing in 2^N times as much light, and conversely with "stop down N
stops".
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photography, lenses, focal length, bellows, focus, pivot
