F-stop
The measure of the aperture setting on a lens. Confusingly, the f-stop number increases as the aperture gets smaller, letting in less light.
The f-stop number is actually a ratio of the size of the aperture to the "wide-open" size of the aperture. (A lens is said to be "wide open" when it's set on its smallest f-stop, or with the aperture opened as wide as possible.) Typically, the smallest f-stop will be something like 2 or 2.8 for a 35mm camera lens; from there, the progression is 4—5.6—8—11—16—22. Some lenses only go down to f/16, while other lenses (such as the larger lenses used on view cameras) may go down farther, to f/32, f/45 or even to f/64.
To make things a little easier, it turns out that each f-stop (the numbers shown above) lets in twice as much light as the next higher one, and half as much light as the next lower one. This makes it simple to adjust exposure by using shutter speed and f-stop combinations: for example, going from f/11 to f/8 has the same effect on exposure as going from 1/250 second to 1/125 second. All of the following combinations would result in the same exposure on film:
- f/11 at 1/125 sec.
- f/8 at 1/250 sec.
- f/5.6 at 1/500 sec.
The "faster" a lens, the smaller its smallest f-stop number; fast lenses for 35mm cameras may be f/1.8, f/1.4 or even f/1.2. (There is usually an inverse correspondence between the lense's smallest f-stop number and its price.)
The f-stop setting (the aperture size) affects depth of field; smaller apertures (larger f-stop numbers) increase the depth of field.
