What’s with that weird graph with all the peaks and valleys? You’ve seen it when you open Photoshop or go to edit a camera raw file. But what is that weird thing called a histogram, and what does it mean?

The histogram is one of the most important and powerful tools for the digital imagemaker. And with a few moments reading, you’ll understand a few simple rules can make you a much more powerful image editor, as well as helping you shoot better photographs in the first place. So what are you waiting for? Read on!

What Do I Need to Know About Histograms?

While it may be intimidating looking, histograms are nothing really all that complex. What they represent are the distribution of tones throughout the image—a simple algebraic graph, when it all comes down to it.

The horizontal line represents the various values in your image. The leftmost side stands for pure blacks and dark shadows. The right side are your highlights, and pure whites. The values between the two fall much the way you might imagine them, with dark tones transitioning to midtones, then on unto brighter and brighter highlights.

The vertical axis represents how much of any corresponding value, whether light or dark, appears in the image. Higher peaks represent high concentrations of that particular value. In our example, we can see that the image this histogram came from has a high concentration of brightest highlights, with the concentration falling sharply, as we look to the slightly dimmer highlights.

Digital images don’t have unlimited tones. They only have 256  (that’s 8-bits of information). On a Histogram, black is 0 and white is 255. The dark tones all have low values and the bright tones have high values.

Okay, But What Do I Use It For?

Histograms are great tools for photography because they allow you to do two key things. First of all, a histogram tool on a DSLR will allow you to see how balanced the composition you’re shooting is before you shoot it. Is it too heavy on the darks, or are the darks lost in the composition? Are the whites too bright—all the detail washed out of them? An in-camera histogram can give you a rough idea of how your image will take or has taken.

In addition to this, histograms can tell you what’s wrong with an image, as well. Sometimes, a potentially great shot gets exposed wrong, and you don’t have the time to bracket or recreate the moment. By looking at your photo’s histogram in an image editor after the fact, you can find out how to best bring your ruined exposures back from the brink, and get a decent, or possibly even a great image out what might have originally been a poor one.

Let’s take a minute to see a few of these badly exposed images, and how we can read a histogram to make them into better photographs. All of these images were shot in RAW by the author, and are processed and improved in Adobe Camera Raw. If you prefer not to use Adobe, there are usually free Raw Editing tools with DSLR cameras, as well as very good freeware programs like Raw Therapee. Adobe Lightroom is another program Adobe offers, this one a stand alone from Photoshop, often considered the new standard for Raw file editing and digital image developing.

For those of you shooting your images in JPG, and not Raw, you can definitely learn about histograms from this article, and pick up a few tips on how to improve your images, but you may learn more by learning how to adjust contrast like a pro specifically for non-Camera Raw files. All other readers, keep on going to pick up some simple tips on how to improve your photos.

The Shape Of Bad Histograms, and How to Improve Them

This shot is a complete disappointment. Obviously this was exposed to get detail in the sky, which it does, but it has ruined the shadows in nearly all of the image. Let’s take a look at the histogram to see what we should change to improve the image.

In this case, we see that our biggest spikes are in our leftmost (darkest) areas. These biggest spikes represent the majority of the tone in the image. There are some spikes in the midtone to highlight range, but they pale in comparison.

Some serious RAW file editing later, and our photograph has changed from unusable, to reasonably nice. Let’s see how our histogram has changed.

Because the exposure was bungled, our histogram is not quite a textbook perfect example, but it’s pretty decent for a completely botched shot. You can only push one image so far. There aren’t any incredibly obvious problems with the image at this point, anyway. We have succeeded in that we have a full tonal range from dark to light, and have managed to keep detail and color throughout most of the image. In case you were curious, we have achieved the majority of this dramatic change by adjusting the “Fill Light” slider to a dramatic, and extremely high setting. While many adjustments have been made to the image, that was the key to bringing out out detail in the shadows.

A second image, this one apparently exposed to grab the shadows has bleached this girl’s skin, ruining detail in highlights, and taking all the dark details down nearly to mid-tone range. Let’s look closer at the histogram.

Yikes. There are absolutely no darks (left side) and there are a large concentration of highlights (right side). The image also seems mostly flat. We should try and add a better range of value, and see if we can’t bring out the some beauty of this pic.

With some work on our RAW file, we’re able to bring out full, rich darks while keeping good detail in our highlights. The shadow from the umbrella feels more cool, and the light from the sun is still creating great highlights on her pale skin. The only difference is now she isn’t glowing!

A good first step with a camera raw file that is overexposed like this, lacking detail in highlights, is to first adjust the exposure slider. In this example, we first reduced by a whole stop (typing –1.0 into the exposure box). This begins shifting our entire value range toward the darker end of our histogram (toward the left side). From there, we can tool around with contrast (we’ve removed quite a bit of it here) and added lots and lots of black to the image to get a rich, dark color out of her hair without completely losing details.

We are, in this case, concentrating our tones in the darker areas for a reason. These darks really make the white highlights pop, and create great focal points along the face and neck. There’s a lot of room for personal choice with and artful decisions to be made.

Taking a Good Exposure One Step Further

While it may not have a perfect histogram with a wonderful range of light, dark, and midtones, this image is reasonably well exposed. But with a glance, we can improve the harsh qualities of the shadows and add detail quite simply, even though the image is more or less alright.

Adding a half stop to the exposure improves the somewhat underexposed shadows, and adds great highlights to the skin, providing the look of bright daylight. Adjustment to the “blacks” slider allows us to bring our shadows to the point of just barely touching the black on the left side of the histogram, while keeping the detail intact in all of our various shadow areas. With some minor artistic changes to “contrast” and “clarity,” our image is improved over what was already a reasonable, decent image.

Readers have asked me “how do I know what to change, when I’m editing a photo?” The short answer is almost always the histogram. Learning these techniques will show you the way to not only rescue horrible images, but also to make your good shots even better. Correctly reading a histogram will give you the tools you need to create a dynamic range of tones, with rich darks and bright whites, without losing detail in either. So, take some great shots, and keep your graphics questions coming to ericgoodnight@howtogeek.com!

Image Credit: Canon EOS by 아우크소(Auxo.co.kr), available under Creative Commons.