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F-stops

Written by Gail Bjork

F-stop refers to a quantity taking into account aperture and focal length. This articles describes how to calculate the effect of f-stops.

The aperture is the physical opening of a lens that lets light pass through to reach the sensor or film. Measuring the aperture by itself is not very informative, however.

The amount of light reaching a camera’s sensor is dependent on aperture and focal length. Because of this, photographers do not refer to the actual aperture of a lens, but rather to a quantity taking into account aperture and focal length: the f-stop, sometimes called f-number.

The following simplified figure is an example to illustrate this:

f-stop1

On this image, we see that even though both lenses have a different diameter (aperture) and distance to the sensor (focal length), both illuminate the sensor in the same way. Thus, both lenses would have the same f-number. This schematic is oversimplified, but serves the purpose of explaining why different lenses can have similar f-numbers.

How F-stops are calculated

F-stops are calculated by dividing the aperture by the focal length. The amount of light passing through a lens with a focal length of 100mm and an aperture size of 50mm will be the same as for a lens with a focal length of 200 mm and an aperture of 100mm. In both cases, the ratio will be 1/2:

50 / 100  =  100 / 200  =  1 / 2

Such an f-stop will be noted f/2, f2, 1:2 or f-2 (all are equivalent).

f-stop2

 

When reducing the aperture area by a factor of 2 (half the light will pass through), the aperture diameter will be reduced b a factor of 1,4 (square root of 2). This is why f-stops increments (each halving the amount of light passing through) seem to have unusual values (note that those values are rounded):

Increasing numbers = decreasing aperture size

Remember that increasing numbers actually mean decreasing apertures, because an f-stop is a ratio. Each increment representing a halving of the aperture is called a “stop”, so reducing an f/2 aperture by two stops would require a setting of f/4.

About the author

Gail Bjork

Gail Bjork, who is passionate about digital photography, is the owner and editor of Digicamhelp.Gail is the author of three illustrated ebooks about digital photography. A number of her photos and digital photography related articles appear at other websites.In 2006, a series of her photos, People in the Louvre, were exhibited at the Underground Photo Gallery
in Iisalmi Finland. Eight of her photos taken in the Florida scrub are on permanent exhibition at Parrish Medical Center in Titusville, Florida.Gail served twelve years as an elected member of The School Board of Palm Beach County, Florida, one of the largest school districts in the U.S. She has also been the editor of a small town newspaper and a free-lance writer. Gail and her husband owned and ran several small businesses.

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