Photo Tip of the Week: Get to Know the Histogram

Most cameras give you the option to check the histogram for each image you shoot. That funny looking pixel mountain contains a wealth of information about exposure that you simply can’t get from looking at an image on an LCD.

A histogram is a graph, which allows you to quickly assess the tonality of an image. The vertical axis represents the number of pixels, while the horizontal axis represents the different tones, starting with black on the far left, moving through progressively lighter shades of grey towards white on the far right. If your image is quite dark, the histogram will show more activity on the left side of the graph. If it is quite bright it will show more pixels on the on the right.

Chances are it wasn’t your love of graphs that got you interested in photography. Even so, there’s a strong case for getting to know how this one works. You see, the histogram is by far the best way to tell if an image is underexposed or overexposed. Viewing an image on a camera’s LCD certainly offers some clues, but the difference between dark grey (which is ok) and black (which is not) can be hard to pick – even on the best cameras.

If the histogram mountain is bunched up on the right side of the graph (see below) it’s likely your image is overexposed. Conversely, if the mountain is pushed up on the left side it’s probably underexposed (see below).

If you check the histogram and it tells you that there’s something wrong with your image, it’s usually a relatively easy matter to tweak the exposure and reshoot. That’s not the case if you find the problem later on when you go to open the image in a computer.

With a few important exceptions, the ideal histogram looks something like a bell shape (see below). Of course, if you are shooting something that is actually black and white, like a shot of the moon in the night sky, it would be normal for the histogram to be weighted at both ends.

So, how do you bring up the histogram display on your camera? In most cases it’s as simple as entering your camera’s review or playback mode and then pressing the “info” button several times until the histogram appears on screen. If that doesn’t work for you, check the manual.

Images in this story are courtesy of HDR Soft (www.hdrsoft.com). The final image was created with HDRSoft's Photomatix Pro.


In this image, the histogram is “falling off” the left side of the graph indicating the picture is underexposed and detail has been lost from the shadows of the picture. It's also butting up against the right side of the graph, indicating the photo is slightly overexposed. You can see this outside the windows where the lighter skt tones have turned to white. The dynamic range of the scene is larger than the dynamic range of the camera. (Image courtesy HDRSoft, www.hdrsoft.com)




In this image, the histogram is bunched up on the right side of the graph indicating the photo is overexposed and detail has been lost in the highlights of the image. (Image courtesy HDRSoft, www.hdrsoft.com)





In this image, the bell-shaped histogram shows a good spread of tones with no significant clipping of the highlights or shadows. (HDR image created with Photomatix Pro. Image courtesy HDRSoft, www.hdrsoft.com)