This is one the most important tools for providing us with an instant check on the exposure of the image.
Wiki describes a Histogram as “A graphical representation of the distribution of numerical data. To construct a histogram, the first step is to ‘bin’ the range of values, that is, divide the entire range of values into a series of intervals and then count how many values fall into each interval. The bins are usually specified as consecutive, non-overlapping intervals of a variable. The bins must be adjacent, and are usually equal size.” So our bins are shades of tones from pitch black to pure white. Cameras divide the full tonal range of the image into 256 consecutive chunks and count the density of pixels providing these amounts of tone.
Reading the histogram yields info:
- A graph with a curve that starts in the top LH corner and slopes down to the bottom RH corner will generally indicate the image has too many blacks, i.e. is underexposed.
- A graph with a curve that starts in the bottom LH corner and slopes up to the top RH corner will generally indicate the image has too many whites, i.e. is overexposed.
- A graph with a curve that starts in the bottom LH corner and rises up to the top somewhere in the middle of the graph, then slopes down to the bottom RH corner will generally indicate the image is well exposed.
Overexposure means lack of detail in the highlights; underexposure, loss of detail in the shadows. For over exposure, you can get around the problem increasing your shutter speed, close down aperture or lower your ISO; the opposite settings will serve to correct an underexposure.
You can usually access the graphical data by taking a test shot, pressing the ‘Play’ button then passing the ‘Info’. You may have to cycle through a series of options in order to see a smaller image but comes complete with the tonal graph.
Some cameras also come with the ability to view histograms covering the tonal range of the red, blue and green pixels within a scene as well as the full tonal range of the graph.