LevelsPhotoshop'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[View full size image] Input LevelsThe 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 slidersMoving 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[View full size image] Gray sliderThe 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 tweakOutput LevelsThe 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 LevelsWhen 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 LevelsThis 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
Levels Command GoodiesThere are a few very useful features in the Levels dialog box that aren't immediately obvious. But they can be huge time-savers. PreviewWhen 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. Instant before-and-afterIn 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 displayBlack-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[View full size image] AutoAuto 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). Auto-resetIf 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 ColorWhen 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[View full size image] We also use Levels' Auto Color feature to make quick major corrections (see "Auto Color," later in this chapter). The Levels composite histogramLike 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[View full size image] How Levels works on color imagesAs 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[View full size image] Auto ColorThe 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[View full size image] In the example shown in |