Have you ever taken a photo and thought it looked great when viewed on your digital camera’s LCD, only to discover that it was overexposed or underexposed when you viewed it on your computer screen? It can be difficult to discern if an image looks good or not when it’s on a small, bright screen, but one tool you can use to determine whether your image is too bright or too dark is your camera’s histogram. Not every digital camera offers this feature, but if yours does (usually a feature in advanced-level cameras), a little time spent learning it can help prevent surprises when you get to a big screen.
A mathematical histogram is a graphical representations of data distribution, displayed as bar graphs. An image histogram is similar in that it gives a visual representation of a digital picture’s tonal (brightness or darkness) distribution, but instead of bars, it’s depicted like an area chart that looks like spiky, mountainous peaks. You’ve probably encountered the histogram when you play back images you’ve taken on your digital camera’s LCD.
Novice photographers may skip over the feature because it’s not necessarily the easiest thing to read. On the most basic level, it’s not too difficult to understand: the horizontal axis shows the tonal distribution (light to dark), while the vertical axis shows the number of pixels in a particular tone – the higher the “peaks,” the most pixels there are. The far left of the horizontal axis shows the darkest areas of the image, then moving to medium grey in the middle and lightest in the far right. An image with a lot of light, for example, will see the graph’s data toward the middle and right of the histogram. A well-exposed photo will see pixels shaped like a mound. If you see a lot pixels situated mostly on the left, the histogram is telling you your photo has a lot of dark tones.
When your image is too bright or too dark, you lose details in those particular areas. If a photo is overexposed or underexposed, you will notice this in the histogram as missing pixel information on the far left or far right. This is when you should adjust your settings to compensate – try a different shutter speed, choosing a larger or smaller aperture, increasing or decreasing the ISO, adjust the exposure compensation.
Depending on the situation, sometimes an uneven distribution is OK. If you are shooting snow, a white object, or something against a white background, the histogram will display more data points on the right. The same goes for the opposite, if you’re shooting a dark scene or a black object.
Some cameras offer a live histogram. As you’re composing your shot, the live histogram on the LCD displays the information for that particular scene. This is useful in helping you adjust the settings before you shoot, which can save you some time from having to reshoot.
The histogram is a not a foolproof tool for shooting perfectly exposed photos, because that’s not always possible. You can’t push a button and make a picture better – not even the camera’s auto setting is smart enough to do that. But think of it as a useful tool that tells you quickly what your photo’s exposure levels are, instead of just guessing by looking at the image on the LCD. As your photography improves and you learn how to adjust the settings to compensate, you will learn to read the histogram better, as well. And, you can utilize the histogram in apps like Adobe Photoshop to improve image quality during post editing.
Still unsure? Check out this video from Creative Live that demonstrates how the histogram works.