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Do you ever wonder how, no matter what the coloring or lighting situations are in your photo, your camera will almost always get the exposure right? Understanding how auto-exposure works will help you compensate for when your camera doesn't get it right.

In the vast majority of photos, if you took all of the colors in the photo (when properly exposed) and swirled them all together, it would come out as a medium gray. This is called 18% gray. The light meter in your camera is designed to expose your photos to match 18% gray.

Most of the time this works with fantastic results. It is amazing that such a basic idea works out 80% to 90% of the time. However, there are a few instances when 18% gray won't work.

For example, if you have an overly bright photo, like kids playing in the snow, your shot will likely have gray-colored snow. If the photo is overly dark, like a close-up of three Oakland Raiders in their black uniforms, your shot will likely have gray uniforms and overexposed faces. The 18% gray is the culprit in both situations.

In the shot with the kids, the auto-exposure tries to make the snow to be 18% gray. In the shot with the Raiders, what does it try to make the uniforms? Yep, 18% gray.

You can get past this problem by using the exposure compensation setting on your camera. Most cameras have it, even point-and-shoots. It allows you to add or subtract exposure from your shot before you take it.

By adding in one to two stops (or levels) of extra exposure in your shot of snow, everything should turn out looking bright and white. Similarly, by subtracting one to two stops of exposure in the close-up of the Raiders players, the uniforms should be brought down to their usual black.

For digital cameras, there is tool called the histogram that can greatly aid in getting the proper exposure.

The histogram is a graph that shows the total amount of lightness values in the photo, from blackest black on the left side of the graph to whitest white on the right. Understanding how to read your histogram can quickly tell you if your photo is over- or under-exposed.

Explaining how to read a histogram without illustrations and examples, however, is practically impossible. For a good description, see Digital Outback Photo's explanation at:

http://www.outbackphoto.com /workflow/wf_41/essay.html

Avoi ding over- and under-exposed photos is much easier when you know what situations to look for. When you find yourself needing to override your camera's auto-exposure, experiment a little with what works best.

Just don't forget to dial that exposure compensation back to zero when you are done, or you will likely regret it the next time you pull out your camera!

Learn more about this author, Chris d'Aquin.
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