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.