As those of us who used to shoot film can attest, one of the best things about shooting digitally is that you can evaluate your images right after you shoot them. Instead of having to wait until your film is processed to see whether you nailed your exposures, you can do test shots and review the histogram on your camera’s LCD panel to spot any exposure issues that you may have with your scene.
If you’re off, you can adjust accordingly, do more test shots and fine tune your exposures until you’re right on. When everything looks right, you can take finished shots of your scene with confidence and know that you nailed your exposure.
For those photographers who haven’t spent much using the histogram, it’s essentially the graph that represents the brightness values of all of the pixels in your image. From left to right, it represents the darkest pixels to the brightest white pixels, while the height of the graph shows how much of the image falls at any given brightness level. If any portion of the histogram is pushed all the way to the edge at either end, it means that your image is “clipped,” which means it is either under or over exposed to the point of total detail loss. A well exposed image will produce a histogram where the graph gradually falls off in a smooth curve at both ends. Where each side falls off depends on the overall brightness of your image.
To see how that data is arranged, let’s assume that an average digital image has 32 different levels of brightness across five stops. If we remember that in photography, one “stop” of light doubles the amount of light as the previous stop, we can understand that the number of different brightness values in an image doubles from darkest to lightest. Thus, the five different stops of brightness across the tonal range of our image respectively contain, 1, 2, 4, 8, and 16 available levels of data.
Note that the highest stop, which contains the brightest tonal values, contains 16 different levels of possible brightness, which is equal to half of the 32 total levels. Put simply, half of the color and brightness data in a digital image is contained in the far right side of the histogram. Since a 12-bit image actually contains 4,096 levels of brightness, not 32, that last fifth on the right side of the histogram contains a tremendous amount of information. Whereas with film, it was ok to underexpose your images, with digital, if you’re not right on, it’s better to overexpose a little bit than go too far under.
To put this into practice, when you’re making your test shots and fine tuning your exposures, adjust so that your histogram fills up with as much information as possible towards the right side of the graph without clipping. This will ensure that you preserve as much color data as possible and get the best signal to noise ratio and the least amount of pixelation, or color distortion in your image. You can also apply this method when you’re adjusting the exposure slider in your image processing software if you’re shooting in RAW.
In the image above, I’ve simulated what you might see in your histogram when slightly underexposing a scene and then exposing properly.