We all know how a histogram has to be read, or at least we should know. It is a handy tool to check if the exposure of the image is correct, or as correct as possible. If the image is not exposed correctly we can read the luminance histogram on our camera LCD screen and know exactly how much the exposure needs to be corrected. Well, perhaps not exactly, but enough to prevent us from guessing.
But first a little reminder for those who have difficulties reading a luminance histogram. It shows the distribution of luminance values in the image. At the utmost left the amount of completely black pixels is shown, at the utmost right we see the amount of completely white pixels. In between are the 256 luminance values of the JPEG image. For each luminance value you can see how much pixels there are by looking at the height of the curve. Remember, even when you shoot RAW, the image on the LCD screen of the camera shows the histogram of a JPEG image. Always.
There is no correct histogram, no perfect shape. The shape, and thus distribution of the luminance values, depend not only on the exposure of the image, but also on the light situation and subject itself. If your subject has lots of darker tones, the histogram will be higher on the left side. If the subject has lots of lighter tones the histogram will be higher on the right side. A lot of contrast will show a peak at the left as well as the right side of the histogram, leaving not much in the way of midtones. Less contrast will show a peak in the middle of the histogram, with not much on the left and right side. Only midtones is also possible, with absolutely no bright and dark pixels nor completely white and black whatsoever. If you wonder when that is the case, just take a picture with fog or mist and look at the resulting histogram.
Learning to read a histogram is nice, but most of the times we see the histogram only when we look at it in a photo editing program. In the favorable case we looked at it right after we took the picture, and those who use live view a lot or have a mirrorless camera, can see the histogram before the picture is taken. In any case, when we check the histogram we can correct an exposure when the histogram is not looking the way we want.
If you look really carefully at a histogram on the LCD screen on the back of the camera, we can see a couple of vertical dividing lines. Each of these line correspond with one stop. So, if we see a lot of empty space on either side of the histogram, we can read how many stops we must correct to get the wanted exposure, just by counting the amount of intersections between the lines.
This can be very handy when you use the Exposure to the Right (ETTR) technique, for example. This way you know exactly how much you need to correct the exposure without guessing or trying. If you have a more traditional camera and without live view, you can make a test shot and correct the exposure when viewing the image of your LCD screen. When you use live view or a modern mirrorless camera it is possible to see the histogram before you take the picture, thus correcting it beforehand. This can be done even on the fly, because you see the histogram change when turning the dials.
The lines in the histogram can not only be used with normal exposure correction, but also with flash photography. As you may know, flash exposure on your subject and exposure of the ambient light are two different settings that can be changed almost independently of each other. When taking a picture with flashlight, you can use the histogram to read the flash exposure. If the flash exposure is too dark, you will see an empty space on the right side of the histogram. If the flash exposure is too bright, the empty space in the histogram is on the left side, of course. To correct this you need to change the flash exposure instead of the ambient exposure. By checking the histogram you have an easy way to set the flash exposure after only one test shot. You can use it for TTL flash with Flash Exposure Value (FEV) correction as well as manual flash setting.
By reading the histogram more carefully you can use it to your advantage in many ways. But always remember, it is the histogram of a JPEG image and should be treated as such. The dynamic range of nearly all cameras is larger than the histogram shows, but only when shooting raw. It also means it is not foolproof, but it gives a good idea how and how much to correct an image that has am incorrect exposure.
Let me know in the comments if you already use the histogram in this way to read exposure and if you use it in more ways to your advantage.