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I have seen an "EV" (exposure value) scale that's sometimes used to express a camera's exposure settings, or scene brightness. How exactly does this scale work?

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Related: photo.stackexchange.com/questions/6769/… –  dpollitt Jan 23 '12 at 19:15

2 Answers 2

up vote 7 down vote accepted

We know that for any scene (really, any light meter measurement) of a particular brightness (and particular sensor sensitivity) there is usually more than one "correct" set of shutter speed and aperture settings. A scene that wants f5.6 and 1/125 will also be correctly exposed at f4.0 and 1/250 and so on.

EV numbers are a way to express the brightness of a scene in a scale that combines the shutter speed and aperture settings into one number -- letting the photographer choose what combination of shutter speed and aperture settings to use. Each EV number equals one stop of brightness, so a scene with an EV of 6 is one stop brighter than a scene with an EV of 5.

The EV values are used generally in the following ways:

  • To show the sensitivity of the light meter itself or of the autofocus system. Camera specs will often say that the metering system works from EV 0 to 20, or that the camera can autofocus down to an EV of 1.

  • Off camera light meters sometimes have a mode that reports in EV, often with a scale so that the photographer can quickly see what shutter speed/aperture combinations are available.

  • Especially with off camera spot meters -- to show the difference in the lightest and darkest part of the scene. The photographer would know if he needed fill light to balance the shadows. This is especially from the film days, where you couldn't judge from an LCD when shooting.

For all the technical details (including the formula), look at Wikipedia's "Exposure value" entry.

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EV 0 is a level of light at the camera that gives correct exposure at f1 at 1 second at ISO 100. See also photo.stackexchange.com/questions/20168/… –  Russell McMahon Feb 13 '12 at 15:41
1  
EV 0 is an exposure value, not a light level. It is agnostic of whether that exposure is correct for a given scene or not. –  Michael Clark Jan 1 at 21:00

The exposure value (EV) is usually defined as

where

  • N is the relative aperture (f-number)
  • t is the exposure time (“shutter speed”) in seconds

This means that EV 0 equals an aperture of f/1.0 and a shutter speed of 1 second. If you increase the aperture by the square root of 2 or decrease the shutter speed by a factor 2 you increase the exposure value to 1.

More generally an increase or decrease of the exposure value by 1 corresponds to what's called a "stop".

The exposure value is calculated from the camera's settings and all shots (of a given scene) taken with different settings that correspond to a certain exposure value will yield the same level of exposure. This is useful since you can alter the shutter speed and the aperture, while maintaining the same exposure level to accomplish different effects such as shallow/deep depth of field or freezing of movement/motion blur.

If you've metered the light you can calculate the different sets of settings that will give you correct exposure prior to taking the shot. This is what all modern cameras do automatically, but by knowing how this is done you can do a better job yourself and you'll also get a deeper understanding of photography and means to be more successful as a photographer.

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