Histogram - Explained

The histogram shows the tonal range and to what extent the different tones are represented in the picture.

If you look at the "mountain" in the histogram that is composed of 256 columns (one column for each tonal value), you may think that the height of the columns represent the number of pixels of each tone value in the image. High bar indicates the number of pixels of the tone value, low column indicates few pixels of the tone value. In image processing it is the height of each column of little interest (Various programs also operate with different sizing, so when a column hits the upper edge of the frame in an application, it may have some length left to go in another program.).

What is of great importance, is whether the "mountain" ends on the baseline or if it touches the side walls of the frame.

In the examples below we are talking about a histogram composed of all three channels of a file with 8 bit depth per channel. It is also easy to imagine that the columns represent shades of gray.

	In example 1 the "mountain" is located right up against the right wall, while it ends on the baseline on the left. This means that the image contains many bright tones, highlights. On the left side, the mountain ends in the upper part of the mid-tones, which means that the darkest tones in the image will appear as relatively bright where most of the tones will turn almost completely white. The image will appear as overexposed and we lose the ability to show detail in the light parts. The image contains burnt-out areas. On the basis of the histogram, we can also see that the image does not contain any black, black is represented on the far left in the frame, while the white is represented on the far right.
	In example 2 the "mountain" ends on the baseline within the frame limits at both ends, here we have an image with reduced tonal range in both highlights and shadows. The image will appear as flat with no content of black or white. If we here imagine a picture in shades of gray, the image will be uniformly gray, with a greater content of the brighter mid-tones. In the example you see that it lacks black, but it is nevertheless represented some very dark tones.
	In example 3 the "mountain" climb on the left side. That is, there are relatively large content of black, but no content of white (because the mountain ends well within the frame delineation).

The histograms you see in the image processing program, says the something about the need to adjust the picture. We can redistribute the tonal range by adjusting the command (Levels) or (Curves). If you have Photoshop CS 3, you can also use the Brightness / Contrast. Do you have a previous version of Photoshop you should stay far away from this command. In earlier versions, it is the same throughout the tone scale, and therefore often leads to the highlights being burnt out (no detail) or that shadow parties are overgrown (no details).You should always check the histogram in the camera when shooting. If you get a histogram like in example 1, the picture is overexposed. Then you can reduce the aperture, or use a shorter shutter speed. If you use the program (P), you can use exposure control to put this in a negative value.

In example 3 you should do the opposite. Increase the aperture or shutter speed advantage. Alternatively, set the exposure compensation to a positive value.

It is advantageous to expose to the right of the histogram, but it is of course a balance. If you fall too far to the right, the highlights will be burned out.

One should also have in mind that the histogram is indicative. If you're shooting in RAW format, you have to some extent a lot going on both ways. You should, in other words learn to recognize the histogram on your camera and be able to assess the exposure using both histogram and preview.

Keep in mind that you should not follow any rules slavishly. In some cases, take the pictures where the camera is unable to capture the entire tonal range and you do your own reviews.

Gaps in the histogram indicates the lack of tonal values and arise when you adjust the picture. The image can withstand a little gap, but if they are large it indicates that you are trying to adjust too much. It is often said that the histogram indicates posterizing.