How to read a histogram

I’ve been called more than once a very “technical” photographer. Although I do often use blur and other creative techniques in my imagery, I have spent a lot of years perfecting exposure and color in my work. One way I like to “double check” my exposures is by using the histogram when I edit.

The histogram mystifies a lot of people. It’s too “math-y” or it just doesn’t make sense…all those lines and maybe colors in a graph. Most people just say “no thank you” and move on. Lucikly, the histogram is actually not that difficult to read once you understand what it represents. The good news (or maybe it’s bad news) is that there is no perfect histogram; each is unique to its coordinating photo, and once you realize that, it all starts to fall into place.

A histogram is nothing more than a chart showing the relative distribution of dark colors vs light colors in your image. A darker, more shadowy image will have a lot of information on the left side. A light and airy or high key image will have a histogram that favors the right side. Having one side or the other with more information is not inherently bad, and a good exposure for specific images does not require information perfectly spread across the histogram.

In many cases, however, a histogram is weighted relatively evenly across the frame. Depending on what program you use, you may see the histogram represented as a single color, indicating solely brightness levels, or you may see color channels as well; in Lightroom, you will see both, assuming a color image, with the gray portion representing general brightness information, and each color coordinating with its color channel. The image below has a good distribution of information from dark to light throughout the frame. You’ll see the histogram has a bit of a spike in the blue channel, which is to be expected with such a large sky portion, as well as yellow and red for the steps. But all in all, this histogram shows a perfect exposure; there are no crushed blacks and no bright highlights. As an aside, I personally try to make sure I have no very bright whites that crawl to the right as areas that are blown won’t print well. Occasionally you’ll have an image like where you might be backlighting and the sky goes white, and that is acceptable. But this image has no true white in it, and as such should have no data to the extreme right.

Now, lets look at images that aren’t balanced between light and dark. Examine the two histograms below. One might think that the first one represents a very underexposed image. The opposite might be considered for the second image.

Yet when you look at the photos that go with each histogram, you can see why each histogram is weighted to different sides. The low light image is very low light, with a large portion of the image taken up by full shadow. There is no true white in the image, and the only bit of light streaming in is very low. There is a mood set to this image that is enhanced by the low light vibe, and increasing exposure would negate that mood. So, while the histogram by itself looks wrong, in conjunction with the photo, you can see that it is a perfect representation of darks and lights throughout the image.

In contrast, this flower image has no real dark areas in it. The shadows are merely a darker gray, and they do not take up much visual space in the image. The histogram for this image should favor the highlight side. Notice again, there is no data on the extreme right, indicating that there is nothing blown in this image.

By knowing how to read a histogram with its image, you can address potential exposure issues. If you look at the following image, you’ll see that the bottom of the image is fully blocked with no details in the shadow, and the sky looks like a twilight sky, not a shade you’d see with a rainbow. Looking at the histogram, this confirms that the image is underexposed as there is a small spike on the left from the blocked shadows and no detail at all in the brights and highlights range where an illuminated sky would show (refer back to the rainbow stair image for that histogram with a section of sky).

With a global exposure fix, we have now lifted the blocked shadows and brought in detail and the sky is back in the normal range of brightness. There is a bit of blown white from the railings on the house, but this is actually a film image and that little bit of information was lost on film.

I hope this helps you a bit with understanding histograms more and how to use them as part of your editing workflow. I’d love to hear from you in the comments if you have any questions.