The Histogram

It is a graphic display of all the tones the camera has captured in the photograph.

When we use Levels in Photoshop, we are looking at the image's histogram. Black is on the left with a zero value, pure white is on the right with a value of 255.

The white triangle needs to be pulled in towards to right until it touches the histogram and the balck needs to be pulled in on the other side.

Now there will be a strong white and black in the image.

The histogram is just as important within a digital camera. It can tell us if the exposure is correct and if we have all the necessary detail in the image.
Certainly we can look at the preview image, but the preview is a rough jpeg processed within the camera from the real recorded image. Very often it is not a good representation of the image itself. Especially if one is shooting Raw, this preview guide can be misleading and wrong.

By looking at the histogram of the photograph just taken, it's possible to get much better information about the exposure.

Most cameras, even fairly basic digital cameras, can display a histogram on its LCD screen. It can be displayed over the image it applies to.

The histogram should show a good distribution of tones. Most cameras have a 8 stop dynamic range and we want to be sure our result is within this range.

The way a digital camera chip reacts to light is similar to the way colour transparency film works. Too much light will burn out highlights and result in a loss of detail. Far too little light will mean there is no effective shadow detail recorded.

If you are concerned with recording all the subtle tones on bright clouds or white surf, you would adjust exposure accordingly. Similarly, if photographing a very dark subject, exposure would be adjusted the other way.

Here the graph shows us that all the image detail is within our range.

Although the preview can tell us the image is on the light side, the histogram tells how bad it really is. Most of the information is squashed over to the right, which means it is seriously overexposed and a lot of the highlight detail will be burnt out.

There is always a limit as to how much can be recovered in Camera Raw and Photoshop. Another shot needs to be taken
of this scene with much less exposure.

A scene might have such a large tonal range that not everything can be included in one shot. Bright white clouds in the sun and shadow detail on the ground will be too wide a range for one digital capture. Two or more photographs will need to be taken and then merged later.

On some occasions, fill-in flash can be used on foreground material in shadow to reduce the contrast range.

The histogram should not be followed dogmatically, however; some human interpretation is required. With this image, a lot of the curve is over to the right, but in this case it is justified.

There is a lot of white in the scene and as the curve is not severely pushed over to the right, things look all right.

The same will happen on the dark side. A photograph with lots of dark will give a graph mainly over to the left. The graph shows a lot of dark but there are still enough midtones.
More exposure is required.

Whereas this histogram will indicate that the image is going to be underexposed and will not produce a viable digital file with a lack of detail in the shadows and midtones.

Tip of the Day: Make use of the histogram on the camera



Back to the top of the Histogram page

Home