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.
The exposure value (EV) is usually defined as
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.