How to Use the Histogram on Your Digital SLR Camera
Most digital SLRs have great metering systems, but even a great metering system can get it wrong when you’re shooting under difficult lighting conditions. That’s why your camera gives you the option to display 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. You study the histogram to decide whether the camera — or you, if you manually exposed the image — properly exposed the image. The histogram can tell you whether the image was underexposed or overexposed. A sharp spike on the right side of the histogram indicates that all detail has been lost in some of the highlights. Your camera can display a single histogram or display one histogram each for the red, green, and blue channels:
You see a peak in the histogram where there are a lot of pixels for a brightness level. A valley indicates fewer pixels at that brightness range. Where the graph hits the floor of the histogram, you have no data for that range of brightness.
When analyzing a histogram, you look for sharp peaks at either end of the scale. If you have a sharp peak on the shadow (left) side of the histogram, the image is underexposed. An image can also be underexposed if the graph is on the floor of the histogram in the highlight (right) side of histogram. If there’s a large spike that’s right up against the highlight side of the histogram, the image is overexposed, and a lot of the detail in the image highlights has been blown out to pure white. You can correct for overexposure and underexposure to a degree in your image-editing program, but it’s always best to get it right in the camera. If you analyze a histogram and notice that the image is overexposed or underexposed, you can use your camera’s exposure compensation feature to rectify the problem.
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 anyway. That’s when you’ll have to judge whether the image on the camera LCD monitor looks like the actual scene.