The lowly histogram is one of the least understood tools in digital photography, and quite possibly, one of the most useful. Virtually every digital camera, from the simplest point-and-shoot to the most sophisticated digital SLR has the ability to display a histogram. So let's start off explaining exactly what a histogram is.The simple definition is this: a histogram is a visual representation of the light and dark elements in your photograph. It helps you instantly determine whether your photos are being correctly exposed.
How to Read a Histogram
A histogram display is usually set up so that dark pixels are on the left and light pixels are on the right. Peaks in the histogram show you whether the photo is predominantly dark, light or somewhere in the middle. For example, if your photo includes a lot of shadows and dark areas, the peak of the histogram will be on the left side of the chart. If your photo includes a lot of bright areas, the peak will be toward the right side. If your photo is fairly evenly balanced (then the histogram will look like a bell curve, with the peak somewhere in the middle.
Using a Histogram to Determine a Properly Exposed Photo
This first example shows you an typical image that is correctly exposed. Since the image is correctly exposed, the matching histogram is a bell curve, with the peak toward the middle. The trick from an exposure standpoint is making sure that the bulk of the histogram falls in between the two endpoints. If the bulk of the histogram is over to the left your photo is under-exposed, and if it's too far over to the right your photo is probably over-exposed. Note: you must always weigh your particular scene. If your scene largely consists of black or shadows the histogram will be correctly exposed toward the left. There is no wrong histogram. You must judge each individual scene on its own.
Now that you know how to read a histogram, you can set up your camera to show you a histogram every time you take a photo. So if your camera by default isn't showing you a histogram when you preview a picture, it's time to get out your camera manual and lookup on how to turn on your histogram display. Histograms provide access to quick, easy to read valuable information with every photo you take.
Now that your able to read a histogram, you'll now be able to instantly tell when:
Your entire photo is over-exposed. Your entire photo is under-exposed. Your photo is clipping Highlights or Shadows.
You can use this information to make adjustments to your camera settings in the field, so that you get a perfect exposure every time you take a photo. After all, nobody wants to take that once in a lifetime photo, only to find out that it's underexposed and unusable when you get home on the computer.