One of the keys to a good photograph is to make sure that the final image makes full use of the available tonal range of the medium in question. In very simple terms, this means making sure that dark areas of the image print (or display) as black, and the brightest portions as white. This is an obvious over-simplification, but it is enough to bring us closer to brighter, snappier, more appealing digital images. As a bonus, it turns out that “stretching” the tonal range of an image also generally leads to brighter, cleaner colors as well.
To help understand this, consider what a “dull” image looks like: Everything is rendered in shades of gray, with nothing either fully black or white. it’s pretty easy to understand that stretching the available tonal differences across the full range from black to white will boost the contrast, making for a more appealing image. What’s less obvious is that this simple tonal stretch also boosts the color saturation, “cleaning up” dull, muddy colors. A full discussion of “contaminant colors” is beyond the scope of this article, but a little digital work with some dull, uninteresting photos will quickly reveal what I’m talking about. (A brief word of guidance: Rich, dark shadows usually lead to stronger, more saturated colors.)
Contrast And Brightness
Many applications provide “brightness” and “contrast” controls, similar to those on a TV set. The mechanics of these will vary somewhat from application to application, but the general concept is the same everywhere. The “brightness” control adjusts the overall brightness or lumination of the picture, and the “contrast” control adjusts the tonal range between the lightest and darkest parts of the image. Among the various “low end” image-editing applications, I like the tonal adjustments in Microsoft’s Picture It! and JASC’s Paint Shop Pro the best, although both are limited when they are compared to more capable (and more expensive) software.
Contrast and brightness controls are comfortable for most of us, given our familiarity with similar adjustments on television sets. They also relate directly to our perceptions of images, making feedback between our actions and changes in the image immediate and intuitive. Unfortunately, contrast and brightness adjustments have a disadvantage because they affect too much of the image at once, often making it difficult to achieve the results you want, even though the controls themselves are easy to understand.
Using Photoshop’s Levels
Many new digital users shy away from so-called “high-end” image-editing applications, either because of price, or because of an impression that they are too difficult to use. Price needn’t be an object, as “lite” editions of various professional products are often bundled with scanners or digital cameras. As for using the software, even a little understanding can make previously intimidating tools familiar and convenient.
In this column, I usually try to avoid discussion focused on a specific piece of software, but this time, I’m going to break my own rule. The reason is that Adobe Photoshop’s “Levels” control is so useful, and illustrates so well how tonal correction works, that I’d be remiss to ignore it. Also, a few other programs have similar “histogram-based” tonal corrections, so the Photoshop-specific discussion here will be more useful than would otherwise be the case.
At the heart of the “Levels” tool is a display of the tonal content of the image. This display takes the form of a graph with the number of pixels in the image assigned to a given brightness value, plotted for each possible brightness value within the computer’s range of 0 to 255. (See the accompanying screenshot for an example of what this looks like.) Underneath this graph are three sliders, in the form of small triangles, colored black, white, and gray. The white and black sliders set the “white point” and “black point” of the image, essentially telling the computer what parts of the tonal range you care about, and what parts you want to ignore. Working in combination, the white and black sliders allow you to decide exactly how you want to “stretch” the tonal range of the image. The white slider sets the “white point,” which is the brightness value that will be made pure white: Anything brighter than this level in the original image will also blow out to pure white. Likewise, the black slider sets the “black point,” the brightness (darkness?) level that will be forced to pure black. Anything darker than this level will also appear as pure black. In between these limits, the remaining tonal values will be distributed across the range from black to white proportionately, depending on how the gray slider is set.
The gray slider controls the “gamma” of the tone conversion. This is really just a fancy way of saying that it sets the point that the computer will force to a middle gray: Set it lower, and the midtones of the picture will get brighter; set it higher, and they’ll get darker.
If you play with the “Levels” control a bit, you’ll soon come to appreciate its power: By allowing you to adjust highlights, shadows, and midtones independently, it provides a great degree of control over how the final image looks. What’s more, it is actually very easy to use once you’ve gotten used to it. (Here’s a tip for Mac users: If you hold down the “Option” key while you move the sliders, Photoshop will show you exactly what portions of each image are being pushed to white or black. This is very handy in determining when you’re fully utilizing the available tonal range. Unfortunately there isn’t an equivalent function on the Windows platform.)