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Well, let me explain a little bit about what a histogram is, and what exactly it can do to help you take a better photo.
If you've ever been out shooting on a bright sunny day, you know how difficult it is sometimes to review the photos you've taken on the LCD screen. The sun glare is just too bright for you to see any detail at all. The histogram is a great tool to use in these situations. It allows you to evaluate a photo's tone, without having to see the actual image in great detail.
So what is a histogram, exactly?
A histogram is a graph that shows you the tonal range of a photo (lights, darks, and grays). Knowing how to read the histogram on your camera could mean the difference between having a good exposure, or blown highlights, or very dark shadows. For example, look at the three graphs I have shown below:
You'll notice that in graph #1, the tone distribution is almost completely to the left. This may mean the photograph is underexposed, making the tones in the photo very dark.
Alternatively, in graph #2, the tone distribution is almost completely to the right of the graph. This may mean the photograph is overexposed, making the photo appear washed out. This is when blown highlights tend to creep up on you.
When the distribution of tones in your photograph is pretty much even, you'll see something similar to graph #3. This typically indicates good exposure, where there's even light throughout the photo.
How to determine Even Exposure
When shooting for even exposure, the histogram can help you achieve the results you want. Depending on what it tells you, the histogram can help you determine how to adjust your aperture. When the tonal distribution shifts to the left, you'll want to open up the aperture, letting more light in. If it's leaning more towards the right, you'll want to close down the aperture, letting in less light.
A little assignment
Look for information on the histogram in your camera's User's Manual. It may be listed under Photo Information or Shooting Data. Once you've found out where the histogram is located on your camera, I want you to get out there and shoot! Here's the challenge...don't evaluate the image itself on the LCD screen, look at the image's histogram and determine proper exposure from there.
Here are a few more examples of even exposure and their histograms. Notice that there may be spikes to the left or the right, but as long as they balance each other out, the photo has an even exposure as in the photo above.
But wait, there's more!
Like anything else, there are exceptions to the rule. You might have noticed in my previous explanations that I used the word, "may," when describing how the histogram may indicate overexposure or underexposure in a photo. The histogram itself doesn't determine if the exposure is right or wrong. You, the photographer, are in the driver's seat. There will be circumstances when you want to either underexpose or overexpose an image intentionally. For example, you might be out late at night shooting the moon in the night sky. Of course, your histogram is going to tell you that your photo is underexposed, represented by the tonal values shifting to the left.
Similarly, when photographing a sunset, you may choose to find the right exposure for the sky and ignore the foreground all together. This will allow the foreground to go completely dark, creating a beautiful silhouetted effect, such as the one shown below.
The more you practice and become familiar with how the histogram works, the more comfortable you'll be at determining good exposure for a photo.
It's important to remember that the histogram will never tell you if your photo is right or wrong. It will only indicate what the tonal range of your photo is, allowing you to adjust for over or underexposure. It's up to you to be as creative as you want with the exposure of your photograph. Now get out there and break some rules!
