Histograms are a valuable tool for digital photographers, yet they can seem a bit intimidating. There is all sorts of information about this or that regarding histograms that you must do or understand. While all of that might be correct, you really can learn to use a histogram rather simply. It can be a quick and easy way of checking exposure with a visual scale if you look for just a few things.
Histograms have gotten overly complex when they show RGB (red, green, blue) information. You really only need the main “luminance” histogram (this just shows a visual representation of brightness values in your photograph starting with black at the left and going to white at the right, and the amount of these values showing up as how high the hills are on the histogram). There is no ideal shape for a histogram as it will change depending on exactly what is bright and dark in your scene.
When you look at the histogram, a key place to check is the right side. This tells you what is happening to the bright parts of your photo, and as a consequence, to everything else. There should be no large gap there between the last histogram hill and the end of the graph. There also should be no “cliff” where the histogram is cut off at the right end of the graph either.
If there is a large gap, this means you have significantly underexposed your photo. That puts your bright areas into middle tones, which can be corrected, but that also means your middle tones get shoved into dark areas and dark areas lose detail altogether. So even if you correct the bright tones, you don’t have the normal range of middle and dark tones for the photo, so those areas can look harsh and have poor color.
This also is underusing the sensor, which was a big part of the price of your camera, because the sensor handles color and tonality best in the mid-range of exposure, so when colors and tonalities are shoved into the dark, the sensor does not capture them as well. Finally, underexposure can increase the appearance of noise as you brighten too dark parts of the photo.
If you have a cliff, a sharply cut off right edge to the histogram, you have overexposure. This means you have lost detail and color in the brightest parts of your photo. That can be a big problem if those areas are important to the scene because no amount of processing will get them back.
A qualification to all of this. If you are shooting in conditions that are foggy or hazy, you might not have bright tones. Then a gap at the right would be normal (otherwise, the photo might be too bright).
Sometimes you will hear that you don’t have to pay attention to the histogram if you shoot Raw. I can tell you from a lot of experience shooting many thousands of RAW photos and from working with thousands of students over the years that that is not true. It is true that the histogram is based on the JPEG (whether you shoot it or not) for the file - so it doesn’t match the Raw exactly. Implying you cannot then use the histogram is like saying you cannot use a thermometer because it only reads full degrees rather than tenths of a degree.
Editor's Note: Outdoor Photographer editor at large Rob Sheppard teaches many excellent courses at BetterPhoto's digital photography school online, including Creating Storytelling Photos and Composition Boot Camp.