How to Read and Use Histograms

How to Read and Use Histograms

The histogram is a useful but often misunderstood tool that your camera provides to help you get the correct exposure in your images.

In this article, we’re going to look at how to read a histogram, and how to use it to your advantage. Getting the best exposure (there is no such thing as the “correct” exposure, as it’s all subjective) in-camera should be your goal every time you click the shutter.

Using these tips should help you increase your photographic success rate!

What is a histogram?

Here’s the dictionary definition: 

A histogram is a bar graph of a frequency distribution in which the widths of the bars are proportional to the classes into which the variable has been divided and the heights of the bars are proportional to the class frequencies.

Huh? Anyone else confused? So what does a histogram really do? And how do you read it?

Let’s have a look!

How to read the histogram

A histogram is a graphical representation of the pixels in your image. The left side of the graph represents the blacks or shadows, the right side represents the highlights or bright areas, and the middle section represents the midtones (middle or 18% gray). 

The heights of the peaks represent the number of pixels of a particular tone (with each peak corresponding to a different tone). Each tone from 0-255 (0 being black and 255 being white) is one pixel wide on the graph, so imagine the histogram as a bar graph all squished together with no spaces between each bar.

Have a look at the diagrams below:

how to use the histogram
how to use the histogram

What can we learn from a histogram?

There are many things we can learn about an image just by looking at its histogram.

We can tell that an image is well-exposed if it reaches fully from edge to edge without a gap on one side of the graph, and it isn’t heavily going up one side or the other. In an ideal world, the graph should just touch the left and right edges of the histogram, and not spill up the sides. The graph should also have a nice arch in the center. 

However, this “ideal histogram” doesn’t always apply in every situation for every scene. Here are a few examples:

an ideal histogram
This is how an ideal histogram might look, evenly distributed, edge to edge, not up the sides.
a darker histogram
This is a histogram for a dark subject. It is not wrong; it is just more shifted to the left to represent the tones of the subject. This might be a black cat on dark pavement.
a brighter histogram
This is a histogram for a light subject (e.g., a white cat) with mostly light tones in the scene and few dark areas. See how it is shifted to the right compared to the dark subject? This is what you want, assuming your scene is mostly light-toned. If you change your exposure to keep the graph centered, you will end up with a gray cat, not a white one.

When the histogram tells you to adjust your exposure

Gaps on either end indicate you are missing information and your exposure can be shifted safely without losing detail. When your graph is shifted too far in one direction or the other direction, so that it does not even touch the other edge, you can safely shift your exposure to cover more of the range of tones. Let’s look!

an overexposed histogram
This graph shows an overexposed image; notice the gap on the left side indicating a lack of any blacks in the image. It also means you will lose lots of detail in the white areas that may not be recoverable. In this case, shift to give your image less exposure and shoot the scene again.
an underexposed histogram
This histogram shows the opposite. Now we see a gap on the right side of the graph indicating there are no whites represented, so the image will be dark – too dark. You can safely give the image more exposure until you see the tones just touch the right edge of the histogram.

What do the spikes up the sides mean?

Spikes up the left or right edge of the histogram indicate “clipping” of that tone and a loss of detail in that area. Clipped areas are often unrecoverable, especially in the highlights.

It is generally advised to expose so that your graph just touches the right edge (which indicates that you’ve kept your highlight details). It is usually easier to recover some shadow detail and retain a decent image than to try and create highlight detail that isn’t in the file.

In some scenes, however, it may not be possible to keep the graph within an acceptable range. For example, you’ll struggle to get great results if you are photographing a scene with extreme contrast, such as:

  • Sunset
  • Bright sunlight and deep shadows
  • A building interior where you also show the area outside the windows

In all of those cases, you will not be able to keep from clipping either your blacks, your whites, or both.

a high-contrast histogram
High contrast graph

The graph above shows an image with extreme contrast, lots of blacks, a spike of white, and not much in the middle.

Is this wrong? Can you correct for it?

No, it’s not wrong.

And you can’t really “correct” for it, but you do have a decision to make when you see something like this. Do you shift the graph left and maintain highlight detail, or shift it right and keep shadow detail?

There is no right or wrong here. It’s all how you interpret the scene before you. If in doubt, shoot both and decide later. The graph above comes from the image below, so as you can see it is not the incorrect exposure at all. There are simply no midtones in the scene:

neon star sign

Here’s another example of a scene that will potentially go off the graph on both ends:

wide-angle cathedral with a blown-out ceiling and deep shadows
Notice the skylight at the top of the roof is blown out, and the deep shadows have little detail.
cathedral with better detail
Notice how, in this image, the details have been retained in both the highlights and the shadows.

Using advanced techniques like image merging and blending, HDR, or careful post-processing, you can compress the tonal range of a scene to fit within the histogram and therefore have details in all areas.

For the image above, I’ve used four bracketed images (taken two stops apart) and the HDR tone mapping process to bring the dynamic range of the scene down within a printable range.

One more handy thing on your camera: the “blinkies”

To help you establish how far you can go when exposing, most cameras have a setting called “highlight warnings.” It will make any overexposed highlights flash or blink when you preview your images on the camera LCD. Many people affectionately call these “the blinkies.”

histogram blinkies flashing
Notice the flashing areas; that means the highlights are being clipped in those parts of the image.

To do this on a Nikon camera, preview an image and press the Up or Down button (near the OK button) until you see the highlights flashing or outlined. This is the “highlight mode.” If you choose this setting, your camera will remember to use it for the next image you preview. You may need to activate the “highlight warnings” feature in your settings menu first, however.

To do this with a Canon camera, press the Display or Info button (depending on your model) until the blinking highlights show up on the screen when previewing images. You may also need to turn on this feature in the menu settings. Check your camera manual if you aren’t sure how to do this.

Summary

By using the tools your camera provides you, it’s easier to understand how to adjust your image exposure. There is a lot more to know about the histogram, and you can use it when you process your images in Photoshop or Lightroom, as well. 

Just keep in mind that, if you shoot in JPEG format, nailing the exposure in-camera is even more critical. If you shoot in RAW format, you have some leeway to make adjustments later, but it’s still a better idea to get it right in the first place.

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