Difference between revisions of "F-stop"
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| − | + | An '''F-stop''' or '''f/stop''' is a number representing the [[aperture]] setting on a [[lens]]. Confusingly, the f-stop number increases as the aperture gets smaller, letting in less light. | |
{{Flickr image | {{Flickr image | ||
| image_source=http://www.flickr.com/photos/mario_groleau/2296321283/in/pool-camerapedia | | image_source=http://www.flickr.com/photos/mario_groleau/2296321283/in/pool-camerapedia | ||
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| image_text=antique brass lens<br/>with f-stops from 8 to 44 | | image_text=antique brass lens<br/>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, | + | |
| + | ==Standard F-numbers== | ||
| + | |||
| + | The f-stop number is a ratio of the [[Lens#Focal Length|focal length]] of the lens to the diameter of the aperture<ref>Strictly speaking, to the [[entrance pupil]] diameter, the apparent diameter of the aperture as seen through the front of the lens</ref>. 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 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 camera]]s) 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: | 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: | ||
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* f/5.6 at 1/500 sec. | * f/5.6 at 1/500 sec. | ||
| − | The "faster" a lens, the smaller its | + | 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). | ||
| − | |||
| − | == | + | ==Whole and fractional F-stops; US Diaphragm numbers== |
| + | The U.S. diaphragm numbers can be found on many U.S.-made [[leaf shutter]]s made between 1890 and 1920, even when the lens speed was expressed in f-stop values. For example a popular [[Kodak lenses|Kodak Anastigmat]] with max. aperture f/7.7 was sold together with shutters opening to aperture 4 (U.S. stop). | ||
| + | {{br}} | ||
<span style="background:#A0FFA0;">full</span> <span style="background:#F0F020;">1/2</span> <span style="background:#CCFFFF;">1/3</span> | <span style="background:#A0FFA0;">full</span> <span style="background:#F0F020;">1/2</span> <span style="background:#CCFFFF;">1/3</span> | ||
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|style="background:#A0FFA0;"| 2.8 | |style="background:#A0FFA0;"| 2.8 | ||
|| 3.2 | || 3.2 | ||
| − | |style="background:#F0F020;"| 3. | + | |style="background:#F0F020;"| 3.4 |
| − | || 3. | + | || 3.6 |
|style="background:#A0FFA0;"| 4 | |style="background:#A0FFA0;"| 4 | ||
|| 4.5 | || 4.5 | ||
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|} | |} | ||
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| − | |||
| − | |||
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| − | == | + | ==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. | ||
| + | |||
| + | ==Notes== | ||
| + | <references/> | ||
| + | |||
| + | ==Links== | ||
* [http://www.uscoles.com/fstop.htm A Tedious Explanation of the f/stop] by Matthew Cole | * [http://www.uscoles.com/fstop.htm A Tedious Explanation of the f/stop] by Matthew Cole | ||
* [http://randcollins.wordpress.com/2010/01/05/the-u-s-f-stop-system/ explanation of U..S. diaphragm numbers and Kodak's simplified 1/2/3/4 stop scale] by Rand Collins | * [http://randcollins.wordpress.com/2010/01/05/the-u-s-f-stop-system/ explanation of U..S. diaphragm numbers and Kodak's simplified 1/2/3/4 stop scale] by Rand Collins | ||
{{glossary}} | {{glossary}} | ||
Revision as of 16:23, 1 March 2011
An F-stop or f/stop is a number representing the aperture setting on a lens. Confusingly, the f-stop number increases as the aperture gets smaller, letting in less light.
|
| antique brass lens with f-stops from 8 to 44 |
Contents
Standard F-numbers
The f-stop number is a ratio of the focal length of the lens to the diameter of the aperture[1]. 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 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 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).
Whole and fractional F-stops; US Diaphragm numbers
The U.S. diaphragm numbers can be found on many U.S.-made leaf shutters made between 1890 and 1920, even when the lens speed was expressed in f-stop values. For example a popular Kodak Anastigmat with max. aperture f/7.7 was sold together with shutters opening to aperture 4 (U.S. stop).
full 1/2 1/3
| f-stop | 1 | 1.1 | 1.2 | 1.3 | 1.4 | 1.6 | 1.7 | 1.8 | 2 | 2.2 | 2.4 | 2.5 | 2.8 | 3.2 | 3.4 | 3.6 | 4 | 4.5 | 4.8 | 5 | 5.6 | 6.3 | 6.7 | 7.1 | 8 | 9 | 9.5 | 10 | 11 | 12 | 13 | 14 | 16 | 18 | 19 | 20 | 22 | 25 | 27 | 28 | 32 | 39 | 45 | 55 | 64 |
| U.S. stop | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 | 256 |
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.
Notes
- ↑ Strictly speaking, to the entrance pupil diameter, the apparent diameter of the aperture as seen through the front of the lens
Links
- 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
