Review Digital Images with the Histogram
Most digital cameras give you the option of displaying a histogram alongside the image on your camera LCD monitor. A histogram is a wonderful thing: It’s a graph — well, actually it looks more like a mountain — that shows the distribution of pixels from shadows to highlights.
Digital camera metering systems are wonderful, but sometimes they don’t quite get it right when you’re shooting under difficult lighting conditions, such as late afternoon or early dusk when cameras actually make a scene brighter than it should be. Another difficult lighting scenario is when you’ve got the sun in your image and the camera exposes the scene correctly but the sun is overexposed.
After you take a picture, study the histogram to determine whether the image is properly exposed. The histogram can tell you whether the image was underexposed or overexposed. Notice the spike on the right side of the histogram in the figure. This indicates that all detail has been lost in some of the highlights.
In the case of this image, the setting sun was too bright for the camera settings, even though exposure compensation was used to decrease the exposure by one stop. Your camera may have the option to display a single histogram or a histogram for the red, green, and blue channels.
This figure shows the same scene after adding additional exposure compensation and using a graduated neutral density filter.
The histogram is a tool. Use it wisely. When you’re analyzing a scene that doesn’t have any bright highlights, you may end up with a histogram that's relatively flat on the right side. When that happens, judge whether the image on the camera LCD monitor looks like the actual scene.
If you rely on the histogram when you see a flat area in the highlights, and add exposure compensation, you may make the image brighter than the scene actually was.
If your camera has the option to display a blinking warning on any part of the image that contains blown-out highlights, enable it. This feature tells you in an instant if your image is overexposed. If the image is overexposed, you use a feature known as exposure compensation to reduce the exposure of the image.
You also use exposure compensation if the image you see on your LCD monitor is too dark or if the histogram has a spike at the far left side of the scale. In this case, you would increase the exposure of the image. Exposure compensation is usually available in one-third EV increments.