Photoshop's Levels command opens a tonal-manipulation powerhouse (see Figure 6-14). This deceptively simple little dialog box lets you identify the shadow and highlight points in the image, limit the highlight and shadow dot percentages, and make dramatic changes to the midtones, while providing real-time feedback via the on-screen image and the Info palette. For more detailed tonal corrections, we use the Curves command; but there are a couple of things that we can do in more easily in Levels, and for a considerable amount of grayscale work, it's all we need.
Figure 6-14. How Levels works
The Levels dialog box not only displays a histogram of the image, it also lets you work with the histogram in very useful ways. If you understand what the histogram shows, the workings of the Levels controls suddenly become a lot less mysterious.
The three Input Levels sliders let you change the black point, the white point, and the midtone in the image. As you move the sliders, the numbers in the corresponding Input Levels fields change, so if you know what you're doing, you can type in the numbers directly. But we still use the sliders most of the time, because they provide real-time feedbackby changing the image on screenas we drag them. Here's what they actually do.
Black- and white-point sliders
Moving these sliders in toward the center has the effect of stretching the dynamic range of the image. When you move the black-point slider away from its default position at 0 (zero) to a higher level, you're telling Photoshop to turn all the pixels at that level and lower (those to the left) to level 0 (black), and stretch all the levels to the right of the slider to fill the entire tonal range from 0 to 255.
Moving the white-point slider does the same thing to the other end of the tonal range. As you move it from its default position at level 255 (white) to a lower level, you're telling Photoshop to turn all the pixels at that level and higher (those to the right of the slider) to level 255 (white), and stretch all the levels to the left of the slider to fill the tonal range from 0 to 255 (see Figure 6-15).
Figure 6-15. Black- and white-point tweaks
The gray slider lets you alter the midtones without changing the highlight and shadow points. When you move the gray slider, you're telling Photoshop where you want the midtone gray value (50-percent gray, or level 128) to be. If you move it to the left, the image gets lighter, because you're choosing a value that's darker than 128 and making it 128. As you do so, the shadows get stretched to fill up that part of the tonal range, and the highlights get squeezed together (see Figure 6-16).
Figure 6-16. Gamma tweak
Conversely, if you move the slider to the right, the image gets darker because you're choosing a lighter value and telling Photoshop to change it to level 128. The highlights get stretched, and the shadow values get squeezed together. (David likes to think of this as grabbing a rubber band on both ends and in the middle, and pulling the middle part to the left or right; one side gets stretched out, and the other side gets bunched up.)
The number that appears in the slider's edit field is a gamma valuethe exponent of a power curve equation, if that means anything to you. Values greater than 1 lighten the midtone, values less than 1 darken it, and a value of 1 leaves it unchanged. If you only adjust the midtone slider, you really are applying a pure gamma correction to the image, but if you also move the endpoints, you aren't: instead, you're applying an arbitrary three-point curve correction. If you want a more detailed mathematical understanding of gamma encoding and gamma correction, a good place to start is http://chriscox.org/gamma/, written by Adobe's own Chris Cox.
The Output Levels controls let you compress the tonal range of the image into fewer than the entire 256 possible gray levels. In the days before ICC profiles, we used to use these controls to make sure that our highlights didn't blow out and our shadows didn't plug up on pressthe sliders let you limit black to a value higher than zero and white to a value lower than 255. Good ICC profiles tend to make this practice unnecessary, since they take the minimum and maximum printable dot into account.
However, even though grayscale is a first-class citizen in good color-management standing in Photoshop, very few other applications recognize grayscale profiles. When we have grayscale images with no specular highlights (the very bright reflections one sees on polished metal or water), we still use the output sliders in levels to limit our highlight dot, andin images with very critical shadow detailour shadow dot. For images with specular highlights, we use Curves instead (we discuss that later technique in this chapter).
We also use the output sliders when we're preparing images for slide shows that we burn to DVD and play on TV sets, and for producing "ghosted" images. And on those rare occasions when we're forced to deal with old-style legacy CMYK setups that use a single dot gain value, we may still use the output sliders to make sure that we don't force our highlight or shadow dots into a range that the output process can't print.
Black Output Levels
When this slider is at its default setting of 0, pixels in the image at level 0 will remain at level 0. As you increase the value of the slider, it limits the darkest pixels in the image to the level at which it's set, compressing the entire tonal range.
White Output Levels
This behaves the same way as the black Output Levels slider, except that it limits the lightest pixels in the image rather than the darkest ones. Setting the slider to level 240, for instance, will turn all the pixels at level 255 to 240, and so on (see Figure 6-17).
Figure 6-17. Output levels
You might think that compressing the tonal range would fill in those gaps in the histogram caused by gamma and endpoint tweaks, and to a limited extent it will; but all that number crunching introduces rounding errors, so you'll still see some levels going unused.
Tip: Leave Some Room When Setting Limits
Always leave yourself some room to move when you set input and output limits, particularly in the highlights. If you move the white input slider so that your highlight detail starts at level 254, with your specular highlights at level 255, you run into two problems.
To avoid these problems, try to keep your significant highlight detail below level 250. Shadow clipping is less critical, but keeping the unoptimized shadow detail in the 5 to 10 range is a safe way to go.
Likewise, unless your image has no true whites or blacks, leave some headroom when you set the output limits. For example, if your press can't hold a dot smaller than 10 percent, don't set the output limit to level 230. If you're optimizing with the output sliders in Levels, set it to 232 or 233 so you get true whites in the printed piece. If you'll be optimizing later with the eyedroppers or Curves, set it somewhere around 237 or 240. This lets you fine-tune specular highlights using the eyedroppers or Curves, but brings the image's tonal range into the range that the press can handle. Again, we should emphasize that if you have a good ICC profile for your output, you don't need to compress the dynamic range using the output sliders because the profile will take care of it for you.
Levels Command Goodies
There are a few very useful features in the Levels dialog box that aren't immediately obvious. But they can be huge time-savers.
When you turn on the Preview checkbox in the Levels dialog box, Photoshop redraws the imageor the part of the image that is selectedto reflect any Levels tweaks you've made, so you can see the effect before you click OK.
In any mode, you can see instant before-and-afters by turning the Preview checkbox on and off. (This is true in any Photoshop dialog box that has a Preview checkbox.)
Black-point/white-point clipping display
Black-point and white-point clipping is the one feature that keeps us coming back to Levels instead of relying entirely on Curves to make tonal adjustments. It doesn't work in Lab, CMYK, Indexed Color, or Bitmap modesjust Grayscale, RGB, Duotone, and Multichannelbut it's immensely useful.
When you set the black and white points, you typically want to set the white point to the lightest area that contains detail, and the shadow to the darkest point that contains detail. These aren't always easy to see. Hold down the Option key while moving the black or white Input Levels sliders to see exactly which pixels are being clipped (see Figure 6-19).
Figure 6-19. The clipping display in Levels
Tip: Look for the Jumps When Clipping
When you're Option/Alt-dragging the input sliders to view the clipping display, watch out for big clumps of pixels turning on or off. You generally want to stay outside of these clumps of image pixels, because moving past them removes a lot of detail.
These types of jumps are also what we look for when we're evaluating scans and scanners. A good scan gives you a smooth growth of pixels as you Option-move the sliders. Lesser-quality scanners tend to provide scans with distinct jumps between gray levels.
Auto Levels and Auto Contrast work identically on grayscale images, though they differ in their handling of color ones. For grayscale, we advise avoiding both unless you want to auto-wreck your images. They automatically move the black and white input sliders to clip a predetermined amount of data separately on each channel. If you have a large number of images that you know will benefit from a preliminary round of black and white clipping, you may want to consider running Auto Levels, but you'll probably want to reduce the default clipping percentages from 0.50 percent to something lower (the minimum is 0.01 percent). To change the clipping percentage, click the Options button, and enter your desired percentages in the dialog box that appears.
Auto Color, however, is one of Photoshop's more useful features for making quick fixes to color images (see "Auto Color," later in this chapter).
If you hold down the Option key, the Cancel button changes to Reset (if you click this, all the settings return to their default states).
Levels in Color
When you work on color images, Levels lets you work on a composite channel (all colors) or on the individual color channels in the image (see Figure 6-20).
Figure 6-20. Levels in color
When you work in the composite (RGB, CMYK, or Lab) channel, Levels works very much the same way it does on grayscale images. It makes the same adjustment to all the color channels, so in theory at least it only affects tone. In practice, it may introduce some color shifts when you make big corrections, so we tend to use Curves more than Levels on color images. But Levels is useful on color images in at least two ways.
We also use Levels' Auto Color feature to make quick major corrections (see "Auto Color," later in this chapter).
The Levels composite histogram
Like the Histogram palette, Levels displays the histogram for an individual channel when you're viewing a single channel, and offers a Channels menu when you're viewing the composite image. The composite histogram it displays (labeled RGB, CMYK, or Lab, depending on the image's color space) is the same as the default composite histogram in the Histogram palette, but different from the Histogram palette's Luminosity histogram.
In the Luminosity histogram, a level of 255 represents a white pixel. In the RGB and CMYK histograms in Levels, however, a level of 255 may represent a white pixel, but it could also represent a fully saturated color pixelthe histogram simply shows the maximum of all the individual color channels. Fortunately, the Levels dialog box's clipping display makes this clear (see Figure 6-21).
Figure 6-21. Levels color clipping display
Note that saturation clipping isn't necessarily a problem. It's simply a signal that you should check the values in the unclipped channels to make sure that things are headed in the right direction. If you're trying to clip to white and the unclipped channel is up around 250, or you're trying to clip to black and the unclipped channel is under 10, you don't really have a problem. But if the values in the unclipped channels are far away from white or black clipping, respectively, you may actually be creating very saturated colors that you didn't want.
How Levels works on color images
As the composite histogram implies, any moves you make to the Levels sliders when you're working in the composite channels apply equally to each individual color channel. In other words, you get identical results applying the same move individually to each color channel as you would applying the move once to the composite channel.
However, since the contents of the individual channels are quite different, applying the same moves to each can sometimes have unexpected results. The gray slider and the black and white Output sliders operate straightforwardly, but the black and white Input sliders require caution.
The white Input slider clips the highlights in each channel to level 255. This brightens the image overall, and neutral colors stay neutral. But it can have an undesirable effect on non-neutral colors, ranging from over-saturation to pronounced color shifts. The same applies to the black Input slider, although the effects are usually less obvious. The black Input slider clips the values in each channel to level zero, so when you apply it to a non-neutral color, you can end up removing all trace of one primary from the color, which also increases its saturation.
Because of this behavior, we use the black and white Input sliders primarily as image-evaluation tools in conjunction with the Option-key clipping display. They let you see exactly where your saturated colors are in relation to your neutral highlights and shadows. If the image is free of dangerously saturated colors, you can make small moves with the black and white Input sliders; but be careful of unintentional clipping, and keep a close eye on what's happening to the saturationit's particularly easy to create out-of-gamut saturated colors in the shadows.
The image shown in Figure 6-22 is a good candidate for correction using Levels. It has no real color problems, and no dangerously saturated colors, but it's washed-out and flat. The Levels clipping displays reveal that the only data above level 232 is a tiny specular highlight, and clipping the shadows at level 10 introduces a hint of true black. A midtone adjustment with the gamma slider completes the jobthree quick moves make an immense difference to the image.
Figure 6-22. A quick fix with Levels
We usually use Levels to make only relatively small corrections like the one in Figure 6-22, because compared to curves, it's something of a blunt instrument. But some situations call for a sledgehammer rather than a scalpel, and Levels' Auto Color feature is a case in point.
The Auto Levels feature in the Adjustments submenu (in the Image menu) generally wrecks color images, causing huge color shifts. Its younger sibling, Auto Contrast, while less of a blunt instrument than Auto Levels, still leaves a great deal to be desired. However, the Auto Color feature can be very useful indeed for making major initial corrections, particularly on scans of color negatives or on images that need major adjustments in color balance and contrast.
Figure 6-23 shows a pretty desperate situation. (This is what happens when your bags get lost by the airline and your undeveloped film goes through numerous baggage scanners as it chases you around the world!)
Figure 6-23. Auto Color
If you simply use Auto Color's default settings (for example, if you simply chose Auto Color from the Adjust submenu), you'll typically get a less-than-desirable result. With very little help, though, Auto Color can quickly get you a lot closer to where you need to be. Here's how we use it.
In the example shown in Figure 6-23, we reduced the default clipping percentages to a lower value to avoid blowing out the highlights in the sky and plugging up the shadows. The default midtone color setting made the image too cold, so we chose an amber warming filter color, and adjusted it by dragging the target circle in the Color Picker. The image updates as you change the target values, so the process is quick and interactive.
You can adjust the midtone swatch color by changing the numbers in the Color Picker, or simply by dragging the target indicator in the color swatch. Neither method is better than the otheruse whichever you find more convenient.
Tip: Save as Defaults
When you dismiss the Levels dialog box, you're prompted to save the new settings as defaults. You may be tempted not to do so since the corrections are almost always image-specific. Save them anyway, because that way, the next time you click the Options button in Levels, it should be set to Find Light and Dark Colors with Snap Neutral Midtones turned on. You'll still probably want to adjust the clipping percentages and midtone target color, but you'll save yourself some time.
Tip: If Things Aren't Working Right
For some strange reason, the Options button in Levels sometimes defaults to Enhance Per Channel Contrast (Auto Contrast), and when you switch it to Find Light and Dark Colors (Auto Color), the Snap Neutral Midtones checkbox is turned off. If you're like us, you'll find yourself diving right in and adjusting the midtone target value, then wondering why nothing's happening. Don't forget to turn on that checkbox!
Other than in the situations covered by the two preceding tips, we don't usually bother saving the settings as defaults when prompted unless we're processing a bunch of images that need the same correction. More often than not, the settings are image-dependent. But we find that simply adjusting three valuesthe highlight clipping percentage, the shadow clipping percentage, and the midtone target colorlets us carry out powerful initial corrections with a minimum of fuss. We don't aim for perfection; rather, we use Auto Color to get the image into the ballpark, and fine-tune the results using the Curves dialog box.