f stop.

Also “f number.“ The relative aperture of a lens, or the numbers used to indicate lens apertures - the amount of light that a lens lets in. These numbers are a relative number and are equivalent to the focal length of the lens divided by the size of the lens aperture.

For example, if you were to take a 50mm lens with a 6.25mm diameter aperture you’d have a lens set to f/8. Generally each increase or decrease in f stop value either doubles or halves the aperture size. Since f stop values are relative to the focal length each camera lens should let the same amount of light through at the same f stop value regardless of focal length.

The usual f stop range on 35mm cameras is:

1.0  1.4  2  2.8  4  5.6  8  11  16  22

So going from f/2.8 to f/4, for example, involves a halving of the aperture size. Each number is approximately 1.4x more than its previous stop since 1.4 is the square root of 2 (to one decimal place), though since the specific numbers derive from tradition they are not always spot on. Lenses for larger camera systems such as large format cameras usually have even smaller apertures - going to f/64, for example.

This series of numbers may look difficult to work with, but in fact there’s a fairly simple way to remember the series. Just remember that the first two values are 1.0 and 1.4 respectively. Each following value then doubles by every other value. So 1.0 becomes 2, then 4, then 8 and then 16. 1.4 becomes 2.8, then 5.6, 11 and 22. (the only minor glitch, of course, to this handy mnemonic scheme is between 5.6 and 11)

The letter f is frequently italicized for good looks, and a slash is often placed between the letter f and the numerical f stop value to indicate that the f-stop value is a fraction of the focal length. eg: f/4 means that the aperture is a quarter of the focal length. The letter f stands for “focal,” “factor” or “focal length” depending on who you talk to, and the number is also often stated as a ratio. eg: 1:2.8. (see numeric ratio printed on lens.)

cf. aperture, exposure value (EV), focal length, f stop ratios.

Entry last updated 2002-04-29. Term 542 of 1487.

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