a feature of many cameras, chiefly small and medium format whereby the photographer is assisted, in different degrees and manners in choosing a proper exposure for a picture. Often abbreviated AE.
I always wanted to use the word whereby.

There are three primary types of automatic exposure:

  1. time priority: long a specialty of Canon cameras, in this type the photographer chooses an exposure time, e.g. 1/2s, and the camera chooses (according to some form of light metering) the diaphragm.
    This type of AE is well suited to situation where exposure time is chritical, like sports photography and wildlife photography.

  2. aperture priority, in which the photographer chooses an aperture, e.g. f 8 and the camera chooses an exposure time. Useful for situations where you absolutely want a certain depth of field, like portraits and landscape photography.

  3. program; in this mode the camera chooses, according to its own lights, exposure. The photographer does nothing.
    Modern program cameras usually have several "programs", some privileging depth of field, others rapid shutter speeds.

Once upon a time, when baffo had no moustache because it would not grow (yet), the above three types of AE were a source of much flamage. Endless discussions and trade rag articles about what was the right AE abunded.
Actually, the very notion of automatic exposure was not very respectable. This is a bit of a moot point, now that many SLRs offer diaphragm priority, time priority, full manual and program, with a choice of spot metering, center weighted metering, and matrix metering.

It is a fact that, once you deliver yourself to the mercy of your camera's tiny brain, you are no longer master of your own pictures.
The current wisdom is that, for difficult lighting situation (like backlight or contrasty scenes) or material that is very easily pissed off, like slides, no camera exposes better than a spot meter connected to a well trained brain.
Nonetheless, there are situations where the photographer is in a hurry. Modern automatic exposure systems, especially coupled to matrix metering are quite smart. For negative film, in fact, they are almost always acceptably right.

what I do: I don't shoot much action photography. When I do, I try to pre-set a good exposure, and recur to AE only if the lighting situation changes.
In quiet situations, I always do manual exposure.
My personal beef with AE is that, to get a good result you often have to second guess the tiny ASIC's brain, or dial in exposure compensation. If I have to do all that effort, I might as well have complete control - as far as natural light is the issue.

OTOH, a good AE system, is priceless when it comes to controlling flashes as fill or main light in photojournalism type situations. Both Canon's and Nikon's are very good nowadays,

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