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