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03. What is meant by f-stop?




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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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previous page: 02. What is the meaning of focal length? In other words, what about a 50mm lens is 50mm?
  
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next page: 04. What is the basic formula for the conditions under which an image is in focus?