|antique brass lens|
with f-stops from 8 to 44
image by Mario Groleau (Image rights)
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 numerically smallest f-stop, i.e. with the aperture opened as wide as possible. Typically, the smallest f-stop will be something like 1.8 or 2 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.
|f-stops from 3.5 to 22|
image by Geoff Harrisson (Image rights)
The "faster" a lens, the smaller its widest 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.)
Effect of different F-stops
The f-stop setting (the aperture size) affects depth of field; smaller apertures (numerically larger f-stops) increase the depth of field. This can be used by the photographer at both ends of the scale:
- 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).
Uniform System stop numbers
The Uniform System (scales are sometimes marked U.S. on lenses) is an alternative system for expressing the aperture size, used by many manufacturers for a rather short period: U.S. diaphragm numbers can be found on many leaf shutters made between 1890 and 1920. The U.S. system never displaced the f-stop notation; it may sometimes be found on the aperture scale even when the speed is engraved on the lens as an f-stop value (the lens and shutter are often from different makers, and the camera may be by another). For example a popular Kodak Anastigmat with max. aperture f/7.7 was sold together with shutters with an iris scaled to aperture 4 (U.S. stop). Whereas f-stop numbers are proportional to the reciprocal of the aperture diameter, the Uniform System stop numbers are proportional to the reciprocal of its area.
full 1/2 1/3
Manufacturer's proprietary aperture scales
Some Kodak cameras with simple lenses had a simplified aperture scale 1-2-3-4, equivalent to f/11-f/32. The Argus Match-Matic C3 had a scale derived from EV numbering where e.g. "5" meant f/5.6 and "8" meant f/16. Many other cameras dispensed with numbering entirely, and simply showed icons representing bright sunny or overcast conditions.
"Stops" in other usage
Colloquially, photographers may use "stops" to mean any change in aperture, shutter speed, or film sensitivity equivalent to one f-stop. That is , an ISO 400 film might be described as two stops faster than an ISO 100 type. This extends even to push and pull processing of film to raise or lower its effective sensitivity (and change its color response, contrast, et cetera); to "push" film one stop is to double its box ISO by increasing the development time.
- Strictly speaking, to the entrance pupil diameter, the apparent diameter of the aperture as seen through the front of the lens
- 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