The Exposure Value (EV) system, which originated in Germany in the 1950s, was created to be a simple-to-use substitute for the shutter speed/aperture combination, using a single number instead of two.
As an example, a shutter speed/aperture combination of 1/125 sec at ƒ-11 and all exposure equivalents of this combination have an EV of 14. So, 1/250 sec at ƒ-8 or 1/500 sec at ƒ-5.6 and so on, also have an EV of 14.
Note that a change of one EV from EV14 to EV13 is equal to a one stop decrease in either shutter speed or aperture - for example, a change from 1/250 sec at ƒ-8 to either 1/125 sec at ƒ-8 or to 1/250 sec at ƒ-5.6.
Aperture & shutter speed combinations and their equivalent exposure values.
An increment of 1 in the EV scale indicates a factor of 2 in exposure. This may sound confusing. It shouldn't, though, when you consider that it means that you double the amount of light entering the camera through the lens when you open the aperture by one stop OR when you decrease shutter speed by one stop. So a change of 1EV means either a doubling or halving of the amount of light entering the camera.
A number of cameras from the 1950s and 1960s were made with shutter speed and aperture combinations linked together and tied to an EV scale. Some modern cameras also employ the EV scale, and EV plus and minus scales are commonly used on the exposure compensation and auto-bracketing controls of automatic cameras, and also to indicate filter factors.
The EV system, however, is not widely employed by photographers in setting exposure. It does find frequent usage by photographers, though, when setting exposure compensation or when bracketing exposures.
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