Whether we are using film or digital, cameras are designed to capture the visual world in a range of gray tones from black to white. Our camera’s meters are measuring the amount of light reflected from the objects seen in our view finder. Whether we are shooting in black and white or color, objects have varying degrees of reflexivity. Black absorbs light and reflects relatively less light then lighter tones. A good exposure captures these tones, preserving the detail we want in the highlights and shadows.
A histogram is a graphical display of the tones captured in an exposure. It shows us the tonal distribution captured. We can see if the file has a full or partial range of tones that spans from black to white. Does an image have a lot of tones in the darker or lighter range? Will there be areas that are pure white or black with no detail captured (called clipping)? A quick glance at the histogram allows us assess whether an exposure captured enough information to make a image that will display or print with good color, acceptable amount of noise (or none) and a good tonal range. Read your digital camera’s manual to find out if it has the ability to show a histogram (all DSLRs have one) and how to view it. If you don’t have a manual for your camera try Oldtimercameras.com.
Put simply, there are 255 tonal levels from pure black to pure white, aka , units of brightness or tonal steps. A histogram gives us a visual way to see how an image’s tones are falling. The left side of a histogram is the darker tones, starting with 0 equaling black with no detail and the right side ending with 255 equaling pure white with no detail. If the wave is bunched up all on the left side, you know you have a file with a lot of dark tones, a lot of the image will be pure black and you will have no tones representing white. If your histogram is all bunched up on the right side, you will have a lot of clipped, pure white areas and no darker tones. Usually the average scene, if exposed properly, will yield a histogram that has a spread through out most of the range of tones. Typically, a well exposed subject with an average tonal range, will have a histogram with a hump in the middle or right third. For example a subject that doesn’t have a lot of dark shadowy areas or a lot of blown out white.
The above image is a pretty typical subject, with a broad tonal range form black to white.
Photoshop gives us a tool called levels that allows us to view an image’s histogram and adjust tones with three sliders. This image has a lot of grays and darker tones and no tones that are pure white. Notice that the curve is the highest in the darker areas where there are a lot of tones and trails all the way off about 3/4 of the way across, showing us that there aren’t any tones that are pure white.
Here’s an image with a lot of white area, as shown in the curve spiking up on the right. There is some clipping in the high end where the reflections in the tuba’s go to white without detail. The white wall is responsible for the surge up, otherwise, the tones are pretty evenly distributed through out as seen by the even line across most of the histogram.
Here’s an image where all the tonal levels are in the mid to low end of the tonal scale. Much of the image is pure black, as represented by the spike all the way up the left end. Notice that the curve flattens about 2/3 of the way across, indicating there are no tones in the upper third of the tonal range.
Here we have a file where the peaks occur in the upper third of the tonal range, representing the flesh tones that dominate this image. The hair and shadow areas are seen in the curve moving all the way to the left of the histogram.