What I Know Of: Expopsure Compensation
So we've discussed the various automated and semi-automated exposure modes. In all of these modes, the camera takes the leading role in determining the final exposure of the image by either taking over all the exposure parameters of the camera (Auto and P modes), or by giving you control of either the aperture (A mode) or the shutter speed (S mode) and then automatically setting the other parameter in response to yield a properly exposed photograph. The camera is making the final call on what the exposure is going to be. I also mentioned that sometimes the camera screws it up. Or it creates a perfectly fine exposure but you're looking for a different type of artistic effect such as a silhouette or a high key image. There are a variety of ways in which the camera can make a bad exposure decision, but that's not as important as what you do about it when it does. In Auto mode, you're pretty much stuck with the camera's opinion on the matter. But with P, S and A modes, you use exposure compensation to course-correct your camera.
Exposure compensation enables you to override the camera's exposure decision. You do so by instructing the camera to shift the exposure one direction or the other relative to what it thinks is the proper exposure. Sort of like saying, "A little brighter please."
Exposure compensation is set in units called "exposure value" or EV. There's a very detailed definition and explanation of exposure value in the Wiki article I just linked to, but the short explanation is that EV is simply the exposure level of an image. Exposure compensation adjustments are made relative to whatever the camera thinks is the correct exposure for the image, which is called "0EV". If it's too bright, you would dial in negative EV - say, -1EV - to darken the image; or if it's too dark, you might dial in +1EV of compensation. Exposure compensation of +1EV results in twice the amount of light hitting the sensor as the camera thinks is correct to "properly" expose the image, and -1EV is one half the light. The range of adjustment varies by camera but ranges of around -5EV to +5EV in 1/2 or 1/3 EV increments are common.
How much should you dial in? There are several methods but the ones I use are trial and error combined with eyeballing the resulting image on the LCD, and using the histogram and highlights displays on my camera. A histogram is a graph that shows the distribution of dark and light pixels in the image. One day I'll make a detailed post about histograms because they're essential tools in both shooting and post-production but how to use them isn't necessarily intuitive. A highlights display shows your image, with any areas that are at maximum brightness shown as blinking pixels. These areas are places where your camera ran out of headroom and over-exposed. If you have large areas of these, you could need some negative exposure compensation. There's a tried and true methodology from the film days of exposing an image for the highlights (i.e. making sure the brightest areas are properly exposed) and then lightening any dark areas in post-production. The logic is that it's easier/possible to lighten dark areas; but any areas that are "blown out" in over-exposure cannot be darkened. The highlights display greatly simplifies using this approach.
How does the camera actually accomplish exposure compensation? Actually there's no magic. It's using the same method you would use if you were manually controlling all the exposure parameters: It changes the aperture, shutter speed, or ISO settings. Exactly which settings it changes depends on the exposure mode being used and other camera settings. In general, the camera tries to honor your explicit settings and adjust only those settings that you've turned over to it. So in shutter priority mode, the camera will compensate exposure by adjusting the aperture and ISO, while in aperture priority, it will compensate using shutter speed and ISO. In program mode, it will adjust any combination of the three since that mode leaves everything up to the camera. On my camera, it will try to adjust all three parameters to compensate. In any of these modes, if the ISO is locked down (i.e. you've set ISO to a specific value and have turned Auto ISO off) then it won't use ISO to compensate exposure. The camera will limit its compensatory adjustments to aperture and/or shutter speed.
That leads nicely to a discussion of ISO which is where we'll go next...