The exposure value is a measurement of light intensity.

The "Exposure Value" of old (often abbreviated as 'EV') is a rather arcane way to measure the light density of an area. The definition is based upon an exposure value of 10 being 10 candles per square foot. Increasing this to 11 involves doubling of the candles. Going from a value of 10 to 9 is half as many candles.

The modern definition that I have found refers to a more modern system that defines EV 0 as the theoretical correct exposure of 1 second shutter speed at f/1.0. Each stop change in aperture or shutter speed darker increases the exposure value by 1. Thus f/1.4 at 1 second is EV 1 as is f/1.0 at 1/2 second.

Low light exposure values:

1. Security Lit Building
2. Bonfire or skyline after dark
3. Floodlit building
4. Fireworks and lit fountains
5. Christmas Trees and candles
6. School Auditoriums, Home Interiors, Traffic
7. Bright Interiors, Parks, Rides, Fairs
8. Bright Indoors, Indoor sports, stage shows
9. Neon Signs, Fires, Outdoor night sports
10. Sunrise and Sunset

The above table indicates that if you are taking a photograph of a Christmas tree, we are looking at an approximate exposure value of 5. This would indicate that at f/1.4 the shutter speed would be about 1/15th of a second. Likewise, at f/4 this would mean a shutter speed of 1/2 second.

EV = log2(f2/T)
"f" is the f-stop, and "T" is the time in seconds.

This newer form is used with SLR light metering that claim to be able to meter correctly over a certain range. For example: "EV 1-20 at ISO 100 and f/1.4". This statement means that with a f/1.4 lens and a film speed of 100, the camera will meter correctly for the level of light between EV 1 to EV 20. Using a lens three stops slower (f/4) will mean that the camera will meter correctly between EV 4 and EV 23.

## Why is this important?

The camera meter tries to produce a photograph that has an 'average' level of color. Most often, this average is defined as 18% gray (standard card used for metering). If the photograph averages to this value, then all is well and the photograph is metered correctly. However, if the photograph has a disproportionate amount of light or dark colors, the metering is off. If there is a significant amount of light colors in the photograph, the metering will darken the entire photograph to compensate. This may result in an underexposed photograph. Likewise, a large amount of dark colors will result in an overexposed photograph.

Increase the exposure value on the camera when photographing light subjects or subjects that are back lit. Decrease the exposure value when the subject is dominated by shadows or is dark in color.

http://www.chem.helsinki.fi/~toomas/photo/ev.html