Arnoud Quanjer Wildlife Photography

Every DSLR is capable of displaying the color histogram of a picture. A color histogram is a representation of the distribution of colors in an image, derived by counting the number of pixels of each of given set of color ranges in a color space. In layman's terms this means that a histogram is a graph counting the number of pixels at each level between black and white. In this article I'll try to explain why the color histogram is one of you best friends in the field.

How to read a histogram

As mentioned before a histogram is a graph counting the number of pixels at each level between black and white. In the histogram black is on the left of the histogram and white is on the right of the histogram. Generally speaking a correctly exposed picture will have a color spread all over the histogram. Using a a color histogram you can measure exposure more objectively then looking at the camera LCD. Although a trained eye can see if a picture is correctly exposed by looking at the picture a histogram can be very useful if you're shooting outdoors in bright and sunny conditions. In these circumstances it's often hard to judge the exposure using the LCD. In these cases the histogram can be helpful for judging the exposure.

The key thing to avoid with digital photography is blowing the highlights of your picture. By blowing the highlights I mean overexposing the brightest parts of the image in such a way that those areas are washed out to complete white. By using the histogram you can easily check if you're blowing the highlights. If you have washed-out areas of 100% white you'll see a tall vertical line at the far right of the histogram. This line indicates clipping. If you see clipping on the right side the reduce exposure by either selecting a faster shutter speed or a smaller aperture. Try to get the histogram as close to the right side as possible without touching it. Check the article 'Expose to the right' for an explanation why this is important. 
Like overexposure the same goes for underexposure. If you underexpose an image then the histogram will be biased to the left side of the histogram. To correct this you select either a longer exposure time or a smaller aperture. This all sounds like a hit and mis affair, but if you understand how a camera measures light then you'll probably get a correct exposure on your first shot most of the times. If you don't know how a camera measures light then the histogram can be a very good way to avoid over- and under exposed pictures.

What you should keep in mind is that the histogram is a useful tool, but it doesn't replace common sense. If for instance you're taking pictures of a buffalo standing in the shade of a tree then the histogram will be biased to the left side. This is normal as the buffalo is brown/black and the surroundings are also dark due to the shade of the tree. Don't try to overexpose to get the histogram to be more balanced to the middle as you'll only be overexposing the picture. Use your common sense and use the histogram as a tool.

Be aware....

Some Nikon DSLRs (D1X, D50, D100, D70s) only read a color histogram for the green channel. This means that these camera ignore the red and blue chanel which can lead to wrong interpretations of the exposure if you're not aware of this. If you base your exposure check on a color histogram that is based on the green channel only then the other colors might be overexposed withou you knowing it. You might even overexpose the picture if you base the exposure on the histogram alone. Make sure that your camera bases the color histogram on all the channels (red, green and blue) before using the color histogram for checking the exposure. It's easy to check this, take a picture transfer it to Photoshop or a similar program and compare the color histogram that Photoshop shows with the histogram shown on your camera. If they are similar then your camera bases the color histogram on all channels.