Definition: f number; f stop; Stop

Definition: f number; f stop; Stop | Glossary entry

f number; f stop; Stop

The term stop has multiple meanings:

1. F number or F stop;
2. Stop of light
3. Stops – a physical barrier to light (an object or opaque part of the optical elements)

1. F number or F stop

The ‘f number’ is used to indicate the size of the aperture. The f-numbers fall within a common range. Each of the following represents one ‘stop’:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22

Other f-numbers exist apart from the range given. In a DSLR there may also be f-numbers which lie between the above, but they each represent 1/3 of a stop.

The f number is given by a simple formula:

Aperture Ratio f/D

Aperture Ratio (focal length / aperture Diameter

The larger the f-number, the smaller the lens opening (f16 – narrow aperture). The opposite is also true. A wide aperture has a small f-number (f2.8 – wide aperture).

The f-number is the ratio of focal length to effective aperture diameter. Any lens has a set of marked “f-stops”. The aperture f-number can be set to any of these ‘stops’. In photography ‘one f-stop’ refers to a factor of √2 (square root 2 {approx. 1.41}) change in f-number. This corresponds to a factor of 2 change in light intensity. Either the light is doubled every stop the aperture is opened or halved every stop down.

The use of a ratio for the size of the aperture means that the physical size of the pupil (the aperture hole) is un-important. All aperture ratios can be reduced to an f-number. This means that different lenses can be compared to each other by the amount of light that is let in. A low f-number (wide aperture) means a lens that can use a very fast shutter speed when the aperture is wide open. A Lens with the advantage of wide apertures is referred to as a ‘fast lens’.

2. A stop of light

Stops are the unit used to quantify ratios of exposure related to the three components of an exposure (ISO, aperture, shutter speed). Each added stop doubles the light, or each subtracted stop halves the light. If ISO, aperture or shutter speed is changed so that the light is allowed to increase or decrease the overall impact will be to reduce or increase the total number of stops of light.

3. Stop (poss. light stop) – a physical barrier to light

The camera or optical system may have a physical barrier or opaque part of the system that is used modify or cut out light. If the stop is used outside the optical system it may be referred to as a field stop.

A more practical description of f-numbers can be found in the article on ‘aperture‘.


By Damon Guy (author and Photokonnexion editor)

Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

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