Every tonal edit you make causes some data loss. The purpose of this bald statement isn't to scare you, but simply to make you aware that like most things in digital imaging, editing tone and color involves a series of trade-offs. Photoshop's tools let you make the proverbial silk purse from a sow's ear, but they also let you do reverse!
Losing data is a natural and normal component of image editing. The trick is to throw away what you don't need, and to keep what you do need, and teaching you that valuable trick is one of the goals of this book.
When you work with images in Photoshop, they're often made up of one or more 8-bit channels, in which each pixel is represented by a value from 0 (black) to 255 (white). Grayscale images have one such channel, while color images have three (RGB or Lab) or four (CMYK). If you're more adventurous, you may work with high-bit images, where each channel uses 16 bits per pixel to represent a value from 0 (black) to 32,768 (white).
The key point is that when you use Photoshop's tonal controls, you're stretching and squeezing various parts of the tonal range, and in doing so, you inevitably lose some information. You lose a great deal more information in 8-bit-per-channel files than you do in high-bit ones, simply because you have much less data to start with. Here's a worst-case scenario that you can try yourself.
What happened here? With the first midtone adjustment, you lightened the midtonesstretching the shadows and compressing the highlights. With the second midtone adjustment, you darkened the midtonesstretching the highlights and compressing the shadows.
But with all that stretching and squeezing, you lost some levels. Instead of a smooth blend, with pixels occupying every value from 0 to 255, some of those levels became unpopulatedin fact, if we're counting right, some 76 levels are no longer being used.
If you repeat the pair of midtone adjustments, you'll see that each time you make an adjustment, the banding becomes more obvious as you lose more and more tonal information. Repeating the midtone adjustments half a dozen times produces a file that contains only 55 gray levels instead of 256. And once you've lost that information, there's no way to bring it back.
Now repeat the experiment, but create a 16-bit-per-channel file instead of an 8-bit one (in the New dialog box, choose 16-bit from the menu to the right of the Color Mode popup menu). The difference in the result is dramatic, as shown in Figure 6-3.
Figure 6-3. Data loss due to tonal correction
This set of adjustments represents, as we noted earlier, a worst-case scenario. No one with any significant Photoshop experience would make edits like this, with one edit attempting to undo the effect of the previous one. (And now that you've read this, you won't either!) The two immediate lessons to be drawn are:
In short, whenever you edit tone and color, something has to give. See the sidebar, "Difference Is Detail: Tonal-Correction Issues," on the next page.
Data Loss in Perspective
Photography has always been about throwing away everything that can't be reproduced in the photograph. We start with all the tone and color of the real world, with scene contrast ratios that can easily be in the 100,000:1 range. We reduce that to the contrast ratio we can capture on film or silicon, whichif we're exceptionally luckymay approach 10,000:1. By the time we get to reproducing the image in print, we have perhaps a 500:1 contrast ratio to play with.
This loss of image information may seem scarier than it really is. Nonetheless, we should hammer home the following lessons and the ensuing pieces of advice.
Get good data to begin with
If you've got a high-bit capture device, (which you almost certainly do), it's better to get the image as close to "right" as you possibly can while the image is in high-bit form. The whole point of high-bit capture is to let you manipulate the high-bit data, so that you get the right 8-bit data when you downsample to 8 bits per channel.
Use adjustment layers
You can avoid some of the penalties incurred by successive corrections by using an adjustment layer instead of applying the changes directly to the image. Adjustment layers use more RAM and create bigger files, but the increased flexibility makes the trade-off worthwhileespecially when you find yourself needing to back off from previous edits. But since the various tools offered in adjustment layers operate identically to the way they work on flat files, we'll discuss how the features (Curves, Levels, Hue/Saturation, and so on) work on flat files first. For a detailed discussion of adjustment layers, see Chapter 7, The Digital Darkroom.
Use high-bit data
If your scanner or camera allows it, you can bring the 10-, 12-, 14-, or 16-bit-per-channel data directly into Photoshopyou are, in effect, telling your capture device "just give me all the data you can capture"and then edit it in Photoshop. In the 16-bit-per-channel space, you have much more editing headroom before you run into posterization. Photoshop CS2 lets you do just about anything to a high-bit file that you can do to an 8-bit one, so the main reason for downsampling is to keep file sizes managable. If you bring digital captures into Photoshop using Camera Raw, note that Camera Raw always operates on the high-bit data, and offers the option to bring the edited high-bit data into Photoshop.
Minimize tonal correction
Small tonal moves are much less destructive than big ones. The more you want to change an image, the more compromises you'll have to make to avoid obvious posterization, artifacts due to noise, and loss of highlight and shadow detail.
Be decisive, or use adjustment layers
If you push an edit too far, don't try to correct it with a second editgo back and fix the problem instead. You can use History to step back before the edit, or if you use adjustment layers, you have the freedom to keep changing your mind right up to the point where you flatten the image.
Since the data you lose is irretrievable, leave yourself a way out by working on a copy of the file, by saving your tonal adjustments in progress separately (without applying them to the image by applying your edits using adjustment layers), or by using any combination of the above. Or you can use the History feature to leave yourself an escape routejust remember that History only remembers the number of states you specify in the Preferences dialog box, and it can consume mind-boggling amounts of scratch disk space.
Data loss can be good
Sometimes you want to throw away information. For example, none of these restrictions applies when you're working on masks or alpha channelsin fact, you usually want to throw away data on those, since you're often trying to exaggerate a feature or isolate it from its background. (See "Step-by-Step Silhouettes" in Chapter 8, Making Selections.)
With all these caveats in mind, let's take a look at Photoshop's tonal-manipulation tools.