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.

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2 Responses

  1. Benny says:

    Aperture is the physical opening of a lens. F-stop (located at the back of the lens) opens and closes to adjust the amount of light going into the camera. It finally makes sense. Thank You 

  1. June 22, 2012

    […] a lot of ‘em out there and they seem to be pretty consistent about various trade-offs with f-stop and autofocus (adding a teleconverter basically reduces f-stops and kills […]