thumb|fast shutter speed, short exposure of a water wave. thumb|slow shutter speed, long exposure of the wave. in photography, exposure value (ev) is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same ev value (for any fixed scene luminance). exposure value is also used to indicate an interval on the photographic exposure scale, with 1 ev corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.in optics, the term "stop" properly refers to the aperture itself, while the term "step" refers to a division of the exposure scale. some authors, e.g., davis (1999, 13), prefer the term "stop" because they refer to steps (e.g., on a step tablet) that are other than powers of 2. iso standards generally use "step", while photographers normally use "stop". the ev concept was developed in an attempt to simplify choosing among combinations of equivalent camera settings, by the german shutter manufacturer  in the 1950s (ray 2000, 318). exposure value was originally indicated by the quantity symbol e_v; this symbol continues to be used in iso standards, but the acronym ev is more common elsewhere. although all camera settings with the same ev nominally give the same exposure, they do not necessarily give the same picture. the f-number (relative aperture) determines the depth of field, and the shutter speed (exposure time) determines the amount of motion blur, as illustrated by the two images at the right (and at long exposure times, as a second-order effect, the light-sensitive medium may exhibit reciprocity failure, which is a change of light sensitivity dependent on the irradiance at the film). ==formal definition== thumb|extended exposure time of 26 seconds. exposure value is a base-2 logarithmic scale defined by ray (2000, p. 318): :\mathrm {ev} = \log_2 {\frac {n^2} {t} } \,, where * n is the relative aperture (f-number) * t is the exposure time (âshutter speedâ) in secondsin a mathematical expression involving physical quantities, it is common practice to require that the argument to a transcendental function (such as the logarithm) be dimensionless. the definition of ev ignores the units in the denominator and uses only the numerical value of the exposure time in seconds; ev is not the expression of a physical law, but simply a number for encoding combinations of camera settings. ev&nbsp;0 corresponds to an exposure time of 1&nbsp;s and a relative aperture of f/1.0. if the 
Photometry Science_of_photography Units_of_illuminance