When you use your camera in some automatic modes like Program—or one of the semi-manual modes like Aperture Priority or Shutter Speed Priority—you don’t give up total control over everything: you can still control the exposure using exposure compensation. Here’s how that works.
What Your Camera’s Light Meter Sees
When your camera calculates what exposure settings to use, it makes one big assumption: that when you average everything out, what’s in front of it is pretty much gray. In other words, all the lights and darks sort of balance out to a middle gray.
This is the photo your camera is trying to take.
And, it’s a pretty good approximation. Here are some of my photos with the luminosity levels averaged out.
It’s not a perfect match, but your camera, just by assuming it’s trying to take a photo of a boring gray wall, is going to be in the right ballpark for a lot of scenes.
But not for every scene. Here are some more photos of mine averaged out.
This time, all the scenes are a good bit brighter than middle gray. When this happens, if you leave your camera in automatic mode, it’s going to underexpose the shots, so you’ll get something like the image on the right rather than the one on the left, which is correctly exposed.
Not ideal since you’re going to lose a considerable amount of image data in those dark black shadows. This is where exposure compensation comes in.
Using Exposure Compensation
Exposure compensation is a way of getting your camera to alter the exposure the light meter is suggesting if you don’t think it will give you the results you want. If the scene is brighter than middle gray, you need to overexpose the image a bit. If it’s darker, you have to underexpose the shot.
RELATED: What Is a "Stop" in Photography?
Like everything to do with exposure, exposure compensation is measured in stops. A single stop represents a doubling of the amount of light hitting the sensor—although that doesn’t necessarily mean that your photo will appear twice as bright.
Exposure compensation is available in Aperture Priority, Shutter Speed Priority, and Program modes on most cameras. You may also be able to use it in fully automatic mode, but it’s not guaranteed.
When you look through the viewfinder or on the back screen, you’ll see an exposure compensation graph.
0 is the metered value of the scene with no compensation. -1, -2, and -3 are one, two, or three stops underexposed while +1, +2, and +3 are one, two, or three stops overexposed. To adjust the exposure compensation, you normally hold the exposure compensation button—it’s the half-black half-white square in the image above—and turn the primary settings dial, although the process can vary between cameras. Check the manual if you’re unsure.
In the image above, I’m now underexposing by a stop. Here’s what the different compensation values look like in practice.
If things are really bright out, underexpose by a single stop; if things are really dark, overexpose by a stop. A stop or two of exposure compensation is normally more than enough to adapt for any scene. I’m not sure I’ve ever had to use exposure compensation of plus-or-minus three.