Many modern cameras have the ability to display a graph, known as a histogram, on the rear screen. Perhaps something like this:
But what use is this to us as photographers? What is it telling us?
Well, a histogram, is a graph showing the distribution of something and in the case of a digital camera, it shows the distribution of pixels of varying tonal values in the image. Dark pixels are shown to the left, lighter pixels to the right. So completely black pixels are on the far left and completely white pixels to the right.
Generally, an image should have a wide range of distribution of tonal values, too many dark pixels and the image is likely to be underexposed, too many light pixels and the chances are the image is overexposed.
So is there an ideal histogram? Well, not really, as it depends on the image that you’re aiming for. Take a look at these examples:
Dark image – pixels bunched to the left
Light image – pixels bunched to the right
A low key image is one that may have a dark or even black background, with high key being generally the opposite, perhaps even a completely white background and bright subject, popular in portrait photography.
But the histogram can tell us other things too, for example, how much contrast there is in an image:
Low contrast – pixels tightly grouped
High contrast – pixels widely distributed
The more contrast in an image, the fatter or wider the histogram will appear, compared to the left hand example above where the pixel distribution is bunched tightly together.
These types of histograms only convey the light/dark distribution, many cameras can also display the distribution of the primary colours: red, green and blue or RGB:
Using the histogram can provide instant feedback on the back of you camera. You’ll be able to see if there are too many pixels on the far right, indicating blown highlights or an over exposed image, too many on the far left, and detail will be lost in the dark or shadow areas, indicating an underexposed image. You’ll also be able to tell if an image has good contrast, without contrast an image can appear flat and uninteresting.
We cover histograms in our Next Steps course in more detail.