# Exposure value

Exposure value

In photography, exposure value (EV) denotes all combinations of
camera shutter speed and relative aperture that give the same
exposure. The concept was developed in Germany in the 1950s
(Ray 2000),in an attempt to simplify choosing among combinations of equivalent camera settings. Exposure value also is used toindicate an interval on the photographic exposure scale, with 1 EVcorresponding to a standard power-of-2 exposure step, commonly referred to asa &ldquo;stop.&rdquo; [In optics, the term &ldquo;stop&rdquo; properlyrefers to the aperture itself, while the term &ldquo;step&rdquo; refers to adivision of the exposure scale.Some authors, e.g., Davis (1999),prefer the term &ldquo;stop&rdquo; because they refer to steps (e.g., on a step tablet)that are other than powers of 2. ISO standards generally use&ldquo;step,&rdquo; while photographers normally use &ldquo;stop.&rdquo;]

Exposure value was originally indicated by the quantity symbol$E_v$; this symbol continues to be used in ISO standards, butthe acronym EV is now more common elsewhere.

Although all camera settings with the same exposure value nominally givethe same exposure, they do not necessarily give the same picture. Theexposure time (&ldquo;shutter speed&rdquo;) determines the amount of
motion blur, as illustrated bythe two images at the right, and the relative aperture determines the
depth of field.

Formal definition

Exposure value is a base-2 logarithmic scale defined by

:$mathrm \left\{EV\right\} = log_2 \left\{frac \left\{N^2\right\} \left\{t\right\} \right\}$

where

* $N$ is the relative aperture (f-number)

* $t$ is the exposure time (&ldquo;shutter speed&rdquo;)

EV 0 corresponds to an exposure time of 1
s and a relative aperture of $f$/1.0. If the EV isknown, it can be used to select combinations of exposure time andf-number, as shown in Table 1.

Each increment of 1 in exposure value corresponds to a change of one &ldquo;step&rdquo; (or, more commonly, one &ldquo;stop&rdquo;) in exposure, i.e., half as much exposure, either by halving the exposure time or halving the aperture area, or a combination of such changes. Greater exposure values are appropriate for photography in more brightly lit situations, or for higher film speeds.

EV as an indicator of camera settings

:

Notes

References

* Adams, Ansel. 1981. "The Negative." Boston: New York Graphic Society. ISBN 0-8212-1131-5

* ANSI PH2.7-1973. "American National Standard Photographic Exposure Guide". New York: American National Standards Institute. Superseded by ANSI PH2.7-1986

* ANSI PH2.7-1986. "American National Standard for Photography &mdash; Photographic Exposure Guide". New York: American National Standards Institute

* ASA PH2.5-1960. "American Standard Method for Determining Speed of photographic Negative Materials (Monochrome, Continuous Tone)". New York: United States of America Standards Institute

* Davis, Phil. 1999. " [http://books.elsevier.com/us/focalbooks/us/subindex.asp?isbn=0240803434 Beyond the Zone System] ", 4th ed. Boston: Focal Press. ISBN 0-240-80343-4

* Jones, Loyd A., and H. R. Condit. 1941. The Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure. "Journal of the Optical Society of America" 31:11, Nov. 1941, 651&ndash;678

* &mdash;&mdash;&mdash;. 1948. Sunlight and skylight as determinants of Photographic exposure. I. Luminous density as determined by solar altitude and atmospheric conditions. "Journal of the Optical Society of America" 38:2, Feb. 1948, 123&ndash;178

* &mdash;&mdash;&mdash;. 1949. Sunlight and skylight as determinants of Photographic exposure. II. Scene structure, directional index, photographic efficiency of daylight, safety factors, and evaluation of camera exposure. "Journal of the Optical Society of America" 39:2, Feb. 1949, 94&ndash;135

* Ray, Sidney F. 2000. Camera Exposure Determination. In " [http://books.elsevier.com/us/focalbooks/us/subindex.asp?isbn=0240515749 The Manual of Photography] ", 9th ed. Oxford: Focal Press. ISBN 0-240-51574-9

* Eastman Kodak Company. "Existing-Light Photography", 3rd ed. Rochester, NY : Silver Pixel Press, 1996. ISBN 0-87985-744-7

ee also

* APEX system
* Exposure compensation
* Exposure meter calibration
* High dynamic range imaging

* Doug Kerr's [http://doug.kerr.home.att.net/pumpkin/APEX.pdf The Additive System for Photographic Exposure] (PDF)
* Fred Parker's [http://www.fredparker.com/ultexp1.htm#Light%20Intensity%20Chart table of exposure values] for various lighting situations

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