One of the most useful advantages of the digital evolution has been the histogram, but surprisingly few people know how to interpret them. Simply put, a histogram is a graph of the tonal values in an image - whatever they are, ranging from black to white. For convenience sake we speak of histograms as having 256 values from 0 on the left for pure black to 255 on the right for pure white. (These numbers were based on the number of tonal values available in each channel in an 8 bit image, which is 2 to the 8th. Even though 16 bit images have considerably more values, it’s easier to conceptualize and interpret a histogram while considering it to have 256 potential values.)
A spike on either end, particularly when the graph looks like a mountain that was abruptly sliced off, indicates there is clipping. This means that there is a group of pixels that are pure black or pure white without detail, as long as the histogram is a luminosity histogram.
The catch is that some cameras and software use luminosity histograms, some RGB histograms combined into a single set of data, and others RGB histograms that show each individual channel superimposed upon one another. In Aperture you have the choice of a multi channel RGB histogram, a luminosity histogram, and individual channel histograms.
Which is best? If you are outputting to print and your main concern is to ensure that there are no areas of your image that are pure white (where the paper itself would show through, showing whatever shade of white it is), then the luminosity histogram is the most efficient.
However if you are concerned with ensuring that you have detail in highly saturated areas, you may prefer the RGB view and occasionally check the individual channels. The reason is that since each color is comprised of a red channel, a green channel, and a blue channel value, there are many colors that contain 0 and/or 255 in one or more channels. (Pure red is 255, 0,0, pure blue is 0, 255, 0, and there are many colors that might have values such as 255, 12, 14 or 255, 23, 38, etc. These slight variations provide detail in the saturated colors and may be very important for the quality of your image. A luminosity histogram will not indicate any clipping since all three channels must be 0 or 255 for the tonal values to be white or black. A single channel RGB may be misleading since it will indicate clipping if even one channel of a color has a value of 0 or 255. A multichannel RGB will show you more clearly where there is clipping as you can see in this image.. You might then opt to reduce the saturation slightly in a particular color in order to preserve more detail or alter the exposure. Using the appropriate histogram can help you reveal the most detail possible in your image.
For more information on using histograms see my article, “Improving your photography through the histogram” on the Aperture Users site, www. Apertureprofessional.com or check out my online course, “What the histogram tells you About Exposure” at www.betterphoto.com.