Understanding Histograms

One of the best things about digital SLRs is the ability to review your captured images on the back of the camera, in order to assess whether or not you’ve got your shot. Composition and colour are relatively easy to evaluate by glancing at the LCD screen, and sharpness can be determined by zooming in on your photos. Judging the exposure,
however, is a little harder, as it’s not always easy to tell if your images are too bright or dark on the LCD screen. The same picture will appear bright and vibrant viewed in the dark but much duller if you look at it in direct light. A more reliable way to check your exposures is to use the histogram.

What’s a histogram?

It’s not always easy to tell if your images are too bright or too dark on the LCD screen. A more reliable way to check your exposures is to use the histogram.

A histogram is a simple bar chart showing the distribution of pixels in an image, based on brightness. The left-hand side represents the darkest pixels, the middle corresponds to the midtones and the right-hand side the white pixels. The higher the graph is at a given point, the more pixels of that brightness are present in the image. For an average image with no strong highlights or shadows you can expect to see a peak in the graph in the middle, showing an average distribution of tones. With a darker image, such as a lowlight scene, you can expect to see a peak to the left. A peak to the right would denote a high-key image. That said, there is no such thing as a perfect histogram. You can only judge it to see if the graph looks how you’d expect it to and adjust the exposure to correct it if things don’t tally up.