The histogram is a useful tool for photographers. It can help you identify if your photograph is correctly exposed, and it can alert you if you are clipping or losing valuable information. This post will walk you through the basics of the histogram and how to use it to inform your photography.
Photos via Boost Your Photography
A histogram shows the frequency distribution of tones in a photograph. The more that particular tone is found in the photograph, the higher the bar at that value. The histogram graph has a range from 0 (total black) to 255 (total white) and all tones in between. (You can also find histograms that display the three RGB color channels separately, but the combined graph is easier to use if you are just starting out.)
Many cameras automatically generate and display histogram information when you are reviewing a photograph, or you can enable that option. Many photography programs, like Photoshop and Picasa, also allow you to see the histogram of an individual image.
The first thing that you want to look for in a histogram is the overall distribution. An ideal histogram contains values across the entire graph just up to, but not including, the end values. A high distribution of values at either end of the graph indicates that your photograph is clipped. Clipping means that all of those pixels will display as either black (0) or white (255) and that it will be difficult to restore any detail there, even in post-processing.
Clipping occurs most often if your photograph is incorrectly exposed. An overexposed photograph will have too many white tones, while an underexposed photograph will have too many black tones. You can see the difference by comparing the images above and below. The image above is underexposed by one stop: the sky and the hills are a little too dark, and the histogram values are clustered near the low end of the graph.
This image is overexposed by one stop. Many of the clouds are overly bright, as are the tops of the hills. The histogram shows a higher concentration of values on the high end of the graph. The photograph below shows the correctly exposed image. The histogram includes the full range of values, which are more evenly distributed.
Using the histogram while photographing
Many beginning photographers rely on the view screen of their camera to tell whether a given photograph is correctly exposed. It is much more reliable to use your histogram. Your view screen is only showing you a preview of the image, and its apparent brightness will be affected by the brightness of your screen and your surroundings. If you have ever tried to look at the back of your camera in the middle of bright sun glare, you will understand the difficulty.
Many cameras also have a feature that you can enable that will alert you if a photograph is overexposed and in danger of being clipped. (On my Canon, it is known as highlight alert.) Generally, any overexposed section of the image will flash or blink at you when you review the photograph.
Refer to your histogram while shooting to make sure that you are achieving the look that you want. If you are trying to capture a black silhouette, then you should see higher frequencies on the lower values of the graph. If you are trying to capture an evenly exposed image, then you should see a good distribution of tones across all values.
If you find that you are clipping values on either end of the graph, change your settings or use exposure compensation to adjust. You will find that you will be much happier with your photographs when you get home.