Photographers and photgraphic equipment manufacturers sometimes use the term "Exposure Value" (EV). These are numbers which refer to certain combinations of lens aperture and shutter speed.
Lens apertures are usually measured as ratios of focal length to effective diameter of the lens, and the standard sequence of values is as follows (f = focal length of lens):
f/1.0 f/1.4 f/2.0 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64 ...
In practice the "f/" is usually omitted and just the "aperture values" 1.0, 1.4, etc are used instead. Two neighboring numbers in this series have the ratio of approximately the square root of two, 1.4142... A smaller aperture value corresponds to more light entering the camera.
Shutter speeds are usually measured in fractions of seconds, with the standard sequence having an approximately 1:2 ratio between the neighbors:
8 4 2 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 ...
In practice the "1/" part is usually omitted and just the denominator of the fraction is used, resulting in "shutter speed values" of 1, 2, 4, 8, etc. Again, a smaller shutter speed value corresponds to more light entering the camera. For shutter speeds longer than one second, extra information, such as an "s" is added, e.g. 8s, 4s, 2s, and now smaller values result in less light coming in.
The difference between two neighboring shutter speed or aperture values is often called a "stop", or "full stop". Many modern cameras have the shutter speed and aperture value inputs graduated at "half stops" of "third stops". Additional numbers are inserted between the above ones for this purpose.
Within certain limits, when the aperture is opened (aperture value decreased) by one stop, and the shutter speed shortened (shutter speed value increased) by one stop, the resulting exposure on the sensor (or film) remains the same. With most films, this is true up to exposure times between 1/4000 to 1 second, and digicam sensors behave linearly even in a wider range. Thus one gets a range of combinations of shutter speeds and aperture values which all correspond to the same final exposure. When such ranges are numbered, one gets the notion of Exposure Values or EVs.
Exposure Value zero (EV 0) corresponds to exposure time of 1 second and aperture of f/1.0 . EV 1 is either 1/2 seconds and f/1.0 or 1 second and f/1.4 , etc.
Other EV numbers can be found in the following table:
a p e r t u r e v a l u e 1.0 1.4 2.0 2.8 4.0 5.6 8.0 11 16 22 32 45 64 s 1 0 1 2 3 4 5 6 7 8 9 10 11 12 h 2 1 2 3 4 5 6 7 8 9 10 11 12 13 u 4 2 3 4 5 6 7 8 9 10 11 12 13 14 t 8 3 4 5 6 7 8 9 10 11 12 13 14 15 t 15 4 5 6 7 8 9 10 11 12 13 14 15 16 e 30 5 6 7 8 9 10 11 12 13 14 15 16 17 r 60 6 7 8 9 10 11 12 13 14 15 16 17 18 125 7 8 9 10 11 12 13 14 15 16 17 18 19 s 250 8 9 10 11 12 13 14 15 16 17 18 19 20 p 500 9 10 11 12 13 14 15 16 17 18 19 20 21 e 1000 10 11 12 13 14 15 16 17 18 19 20 21 22 e 2000 11 12 13 14 15 16 17 18 19 20 21 22 23 d 4000 12 13 14 15 16 17 18 19 20 21 22 23 24
It is not difficult to extend the table in any direction. With longer exposure times, the EV numbers become negative. Lens with apertures larger than f/1.0 are extremely uncommon.
When an exposure value is combined with a specific sensor sensitivity (ISO value) or film speed, the EV numbers can be used to indicate the brightness of an object (usually one which will be photographed). The importance of ISO value is sometimes overlooked, but without it, EV's indicate nothing more than certain camera settings. If a certain author gives no indication of ISO value, and it cannot be inferred from the context, ISO 100 may usually be assumed.
Camera manufacturers often indicate the sensitivity of auto exposure and auto focus equipment in EV's, . For example, a SLR's light meter may be capable of correct exposure with "EV 1-20 at ISO 100 and f/1.4". This means that if you mount a f/1.4 lens on the camera, and the sensitivity is set to ISO 100, then the manufacturer promises that exposure readings within EV 1 to EV 20 will be accurate. If you actually have a f/4.0 lens mounted, the light meter will see 3 stops less light than with the f/1.4 lens, and will be accurate between EV 4 and 23. If you use ISO 400 instead of the ISO 100 (2 stops difference), then with f/1.4 lens you can get accurate readings between EV 3 and EV 22. Please note that with different lens, the actual brightness range of the subject will differ, but with different sensor (film) sensitivity, the brightness range remains the same, since the sensor requires different expsoure with the same brightness of the subject. This once again demonstrates the importance of specifying ISO value when Exposure Values are related to subject brightness.