Once an image is imported into Photoshop, the histogram can be used to enhance the image. In many cases, digital files appear flat when compared to slides. This is because the dynamic range of digital is larger then the dynamic range of slide film. Where slides often have difficulty recording the entire tonal range of a scene, digital is often able to record the scene with latitude to spare. This can be seen in histograms which do not come close to either the right or left edges. These images do not contain the full range of tones between white and black. Instead they range from dark to light grey. Since we are accustomed to seeing a full range of tones, it is necessary to adjust the histogram on digital files so that white becomes white and black becomes black.

Adjustments to the histogram are most easily applied using levels (Layer → New adjustment layer → Levels). When levels opens, a histogram appears with three small triangles under it. From left to right, these triangles are black, grey, and white. For the moment, let's ignore the grey triangle and focus on the two outer triangles. The black triangle (bottom left of the histogram) controls pure black in the image. When it is all the way to the left, pixels with a value of 0 appear black. Similarly, the white triangle (bottom right of the histogram) sets the white point and starts with a value of 255. Moving either triangle towards the center of the histogram changes the black and white points. For example, say I move the black triangle in 10 points. This results in every pixel containing a value less then 10 being rendered as pure black with no detail, effectively redefining black. Moving the white triangle in 15 points causes all pixels with a value greater then 240 to be pure white; redefining white. By moving the outer triangles, I instruct Photoshop to stretch the histogram so that it utilizes all tones between black and white. The end result is that the darkest parts of the image get darker and the lighter parts of the image get lighter. This simple adjustment that takes less then 10 seconds can often transform a dull, lifeless image into a stunning keeper.

The next question, of course, is how far should the outer triangles be moved in? The crude answer is that white and black should be moved until they reach the edges of the histogram. Using the Alt key, however, lets me obtain maximum results without sacrificing detail. After opening the levels histogram, I hold down Alt as I move the black triangle towards the right. As the black setting changes, the image will be replaced by all white. As the black setting increases, black dots appear against the white background. Each black pixel indicates loss of detail within the image as these pixels are having their tonal value reset to 0, which is black without detail. The further to the right the black setting is moved, the more black appears because more pixels are rendered as pure black. For best results with minimum loss, the black setting should move to the right until just a few black pixels appear in the image. At this point, the adjusted image will contain pure black and all of the shades of grey leading to white.

The white point should be adjusted similarly. The only real difference is that instead of a white background, combining the Alt key with the white triangle will produce a black background. As the white setting moves toward the left, white pixels will appear, indicating pixels that will be rendered as pure white if the current settings are applied.

By altering the settings for the outer edges of the histogram, it is possible to stretch the histogram so that it covers the complete tonal range. This techniques of making black-black and white-white is one of the simplest techniques in Photoshop, but it produces amazing results. Its impact on image quality is so dramatic that it is the first technique I apply to every image in my digital workflow.

Once the white and black settings have been adjusted, the overall brightness of the image can be fine tuned using the center triangle in the levels window. This center adjustment is properly called gamma, but can accurately be thought of as the middle tone setting. At the default setting of 1.00, half of the available image tones are darker then the middle tone and half are lighter. Changing the middle tone setting alters the distribution of tones within the image. Moving the setting to the left causes Photoshop to darken the accepted middle tone of the image. The end result is an overall lightening of the image. This seems counterintuitive; after all, the middle tone is getting darker not lighter. Remember, that moving the middle tone towards the left of the histogram results in more pixels being lighter then the middle tone setting then before the adjustment was made. Similarly, moving the middle tone to the right darkens the overall image. Where changing the white and black points can reduce pixel quality if taken to the extremes, middle tone adjustments do not risk burning highlights or excessively darkening shadows because the changes do not alter the defining edges of the histogram.

Histograms provide detailed tonal information about digital images that can be used to retain detail and maximize proper exposure in the field. Once images are captured alternations to the histogram can be used to ensure a complete tonal range and to apply tonal adjustments to the entire image. Understanding how to read a histogram and the ways that histograms can be altered are two of the first steps needed to master digital photography.

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Chris Gamel has been an active wildlife photographer for the past 14 years. The combination of his biological research and photographic interests has taken him around the world. In 2001, Chris discovered the joys of digital photography and he hasn't looked back. Chris lives with his wife and daughter in Salt Lake City where he teaches courses at Weber State University and Salt Lake Community College. To see more of Chris's photography, visit his web site at www.Animal-Antics.net.