|antique brass lens|
with f-stops from 8 to 44
The f-stop number is a ratio of the focal length of the lens to the diameter 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 normal marked 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/22, 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 selecting 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 lens's smallest f-stop number and its price.)
f-stop scale, U.S. diaphragm numbers, and Kodak's 1-2-3-4 scale
full 1/2 1/3
- To get both near and far objects in a scene in focus, use a small aperture (large f-stop number).
- To isolate a subject by letting the background go out of focus, focus on the subject and use a large aperture (small f-stop number).
The Unified System (U.S.) diaphragm numbers can be found on many U.S.-made leaf shutters made between 1890 and 1920, whilst the speed of U.S.-made lenses could have been documented as f-stop value. For example the popular Kodak Anastigmat f7.7 (f-stop) was sold together with shutters with max. aperture 4 (U.S. stop). Some Kodak cameras with simple lenses had a simplified aperture scale 1-2-3-4, meaning (estimated) U.S. stops 8-16-32-64.
- A Tedious Explanation of the f/stop by Matthew Cole
- explanation of U..S. diaphragm numbers and Kodak's simplified 1/2/3/4 stop scale by Rand Collins