One of the things we talk about a lot in photography is exposure — that is, how light or dark your photo is. The camera’s shutter speed, aperture and ISO settings are all factors in how bright your photo will be. When you use your camera in manual mode, you can adjust all those settings yourself. That gives you a great deal of control, but it also takes time and practice to get really good at managing all of that, and so sometimes other modes of the camera are more convenient.
When you use your camera in program, shutter priority or aperture priority modes, or even in manual mode with auto-ISO, it typically does a pretty decent job of reading the light and metering accordingly to give you a properly exposed photo. One that is not too dark, not too light, but just right.
There are some lighting situations, though, where your artistic vision and your camera's directive to meter every scene to a happy medium might be at odds. Think about a snowy winter scene, for example -- or a lovely white-sand beach, if you aren't quite ready for wintery thoughts. Chances are, if that's what you're photographing, you want to capture the brilliance of the sun on the reflective foreground. Your camera, however, will read that light as too bright and meter down. And the resulting image will be disappointingly dark.
What can be done? The answer is Auto Exposure Compensation -- aka AEC. AEC allows you to push the exposure based on the camera's metering, making things brighter or darker. You can adjust the exposure in 1/3 f-stops up to +3 or down to -3.
Let's look at an example to give you an idea of when (and why) you might want to use AEC:
Here is an image from our Xtreme Wildlife Photography field trip to Africa a few years ago. I would call this overexposed. The grassland in the foreground looks OK, but the sky is blown out. Most of the detail in the clouds is lost.
Below we have the same scene metered appropriately -- at least according to the camera. By appropriately, I mean that the camera's light meter was at 0. The camera read the whole scene, decided which of those colors should be the median value, and metered accordingly.
Better! The grassland may look a little dark, but the sky isn't so blindingly white that you lose the details, and the drama of the dark clouds is more prominent. The viewer's eye is drawn into the scene, right to that gap in the clouds, which is probably the most interesting part of the photo, anyway. You could call it a day here and be pretty happy with the image you captured.
I don't think it's gone far enough, though, if what you really wanted to show was the gorgeous rays of sunshine pouring out of that gap in the threateningly dark clouds. So let's see what happens when you use AEC to force the camera to underexpose the scene:
Well! Now you've really got something. Lots of drama, lots more detail in the clouds, and the crepuscular rays of sun are visible all the way across the image. To me, this image truly captures the vastness of the plain, the oppressive heaviness of the storm clouds, and the awe of seeing the sun break through. And it would not have been captured if not for using AEC to override the camera's exposure setting.
Hopefully, you can begin to see how exposure setting can be an artistic choice and why you might want to use AEC sometimes to get the effect you’re hoping for in a photo.
If you'd like to learn more about Auto Exposure compensation -- among many other things -- we go into it in depth in the second class of our Photography 3 course.
