Exposure values (EV)

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I need to introduce the concept of exposure values (EV) for some forthcoming posts. EV is an absolute measurement of the light quantity required to give a correct exposure at a specific ISO speed (it depends upon aperture and shutter speed) - though most charts only give these values at ISO 100 (these are then also known as light values). It's useful to be able to refer to the amount of light using just one variable rather than a combination of aperture and shutter speed. In the table above EVs run from 0 to 24 (they can be negative as well). The scale is based upon stops of light - ie doubling or halving of light. For a given EV there are many combinations of aperture and shutter speed which can be used to make a correct exposure.

Some examples of EVs to help you get a feel for the scale:

EV 17 white object in full sunlight

EV 15 noon daylight

EV 13 bright, cloudy days

EV7 indoors

EV -5 scene lit by the moon

Knowing the amount of light in a particular lighting scenario allows you to construct guidelines, such as the 'sunny 16 rule' - on a clear, sunny day set your camera at f/16 and the shutter speed to 1/ISO setting - this equates to a constant EV of 15.

You'll also see these values in the technical specifications in your camera manual. For example, my Canon EOS 5D autofocus system works in the EV range of -0.5 to 18 EV.

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