The Histogram is one of the most important tools in Photoshop (see Figure 1). Histograms help photographers understand the tonal or color structure of an image. For instance, a Histogram can show whether an image is composed primarily of dark tones, light tones, or a wide distribution of tones.
Histograms display tonal distributions (grayscale histograms) or color distributions (channel histograms). There are a total of 256 tones/colors in a Histogram. The darkest tone/color is assigned a value of zero. The lightest tone/color is assigned a value of 255. The horizontal axis displays these tones/colors from the darkest on the left to the lightest on the right. The vertical axis shows the number of pixels that have each tone/color. In Figure 1, for example, it can be seen that a large number of pixels are in the middle of the distribution. Therefore, the image from which this Histogram was created has a lot of midtones.
By viewing an image's Histogram, a photographer can understand the tonal/color distribution in an image and can analyze the tonal/color impacts of an edit.
People will sometimes try to evaluate a Histogram by looking at just the Histogram. For instance, they may look at a Histogram that has the tonal values shifted heavily to the left and make a statement such as, "This image is too dark" or "This image is underexposed". Such an evaluation would be invalid. Histograms can not be properly understood by themselves. In order to understand a histogram, it must be viewed in relation to the image it represents. In other words, both the image and the Histogram must be viewed at the same time.
Grayscale Histograms are used to evaluate the tonal distribution of an image. The information shown in an image's grayscale histogram can be used to determine what tonal edits need to be made or to evaluate the results of tonal edits that have already been performed.
Figure 6 shows an image of a canyon and the surrounding, snow dusted mountains. This image has a full range of tones. There are bright whites in the snow, a small amount of blacks in the foreground rock, and a large amount of mid tones in the mountains.
This tonal distribution is reflected in the image's histogram (see Figure 7). The Histogram shows the whites to the right, a small amount of the blacks on the left, and the large amount of midtones in the middle. One might suspect that a small amount of clipping occurred in the brightest areas because the right side of the histogram appears to cut off a very small amount of the highlights. This is acceptable in this image. This image was taken under a heavy overcast sky. The flat light insured that there would be little if any detail in the brightest areas of the snow. In addition, the snow is so far away that any minor loss of detail in the brightest areas of the snow will not be visible.
In short, the dynamic range of the scene matched very well with the dynamic range of the camera. In addition, the exposure was correct. As a result, this histogram shows that the tonal distribution of the image is very good and no major, tonal edits are needed.
This tonal distribution is reflected in the image's histogram (see Figure 9). The Histogram shows the light tones to the right and the dark tones to the left. There are very few midtones. On the right side of the image, the histogram shows that there are no whites or extremely light tones. This is verified by looking at the image. The left side of the histogram shows a very small amount of clipping of the darkest tones. This is probably not a major problem with this image as the amount of clipping is so small.
As with the prior image, this histogram shows that no major, tonal edits are needed.
This is demonstrated in the rock image's Histogram (see Figure 18). The Histogram shows a lot of midtones and no very dark or light tones. This is as it should be. Thus, this image shows that the image has the proper tonality and no major tonal edits are required.
This image points out a very important point that was mentioned above: Histograms can not be properly understood by themselves. In order to understand a histogram, it must be viewed in relation to the image it represents. If a photographer were to have viewed the Histogram in Figure 18 without looking at the image, the photographer might be tempted to conclude that the image needed to be adjusted to increase the contrast and bring in both dark and light tones (as in the prior image). This would have been a mistake and would have created an image that had way too much contrast.