Copyright Michael Karbo, Denmark, Europe.
Chapter 20. The histogram
When it comes to taking well-exposed images, one of the most important things is to have the right colour tone distribution. It is important that there are both light and dark tones in the image and that they are well distributed. A histogram is a function, which can be of great help in controlling the colour tone distribution and by this, the imageís exposure. A histogram is a graphic representation, which tells us if there is anything wrong with the distribution of the colour tones.
The histogram is displayed on the LCD screen in many cameras and it is a part of most image programs. It is a brilliant aid in finding the right toning of an image. And it isnít at all difficult to understand a histogram!
Looking at the histogram
Many cameras have a little button called Display:
This button works both while you are taking an exposure and when you are looking through the images in the camera. It changes the display so that more or less information is shown on the LCD screen. In most cameras it activates a histogram, which can be seen somewhere in the screen image.
A histogram is really a bar chart. There are 256 small thin vertical bars, each one illustrating the number of pixels there are within a particular colour tone area. This sounds rather technical, but is, in fact, very simple. The horizontal axis being a colour scale, which goes from black to white. By looking at the histogram you can see whether there is an excess of dark tones (the graph is highest on the left side), or if there is an excess of light tones (on the right side).
Figur 78. Histogram display on an LCD screen in two different cameras. A histogram can have different sizes and positions but the function is the same every time. (Pentax 450 og Pentax *ist)
A histogram can be helpful during an exposure, if the display is switched on.
Most people will, however, probably prefer to check the histogram when the image has been taken. It can be difficult to evaluate the colour tones precisely on the cameraís little LCD screen. A histogram can give some extra information, which you can use to evaluate the image. If the histogram doesnít live up to what you expect, then it should be taken once more.
Figur 79. This photo is, in fact, correctly exposed. The light from the window is very dominant, which makes the main subject a bit too dark.
The histogram shows an even graph with distribution of pixels in most of the colour tone area:
Here, the exposure has been defined with a spot metering of the person in the middle, which gives a visible overexposure:
The histogram clearly shows that the image has been overexposed. The whole of the graph is pushed towards the right. This is because there are more pixels in the light colour tones:
Figur 80. The over-exposure is signifacant.