Adobe Photoshop CS4 - Manual (Page 199)

192 USING PHOTOSHOP CS4
Color and tonal adjustments
Last updated 1/10/2010
2 Alt-click (Windows) or Option-click (Mac OS) the Auto button in the Adjustments panel.
3 Specify the algorithm you want Photoshop to use to adjust the overall tonal range of an image:
Enhance Monochromatic Contrast Clips all channels identically. This preserves the overall color relationship while
making highlights appear lighter and shadows appear darker. The Auto Contrast command uses this algorithm.
Enhance Per Channel Contrast Maximizes the tonal range in each channel to produce a more dramatic correction.
Because each channel is adjusted individually, Enhance Per Channel Contrast may remove or introduce color casts.
The Auto Tone command uses this algorithm.
Find Dark & Light Colors Finds the average lightest and darkest pixels in an image and uses them to maximize contrast
while minimizing clipping. The Auto Color command uses this algorithm.
4 Select Snap Neutral Midtones if you want Photoshop to find an average nearly-neutral color in an image and then
adjust the gamma (midtone) values to make the color neutral. The Auto Color command uses this algorithm.
5 To specify how much to clip black and white pixels, enter percentages in the Clip text boxes. A value between 0.0%
and 1% is recommended.
By default, Photoshop clips the black and white pixels by 0.1%—that is, it ignores the first 0.1% of either extreme when
identifying the lightest and darkest pixels in the image. Because of the better output quality of modern scanners and
digital cameras, these default clipping percentages might be too high.
6 To assign (target) color values to the darkest, neutral, and lightest areas of an image, click a color swatch.
7 Do one of the following:
• To use the settings in the current Levels or Curves adjustment, click OK. If you then click the Auto button,
Photoshop reapplies the same settings to the image.
• To save the settings as the default, select Save as Defaults, and then click OK. The next time you access Levels or
Curves in the Adjustments panel, you can apply the same setting by clicking the Auto button. The Auto Tone, Auto
Contrast, and Auto Color commands also use the default clipping percentages.
Note: When you save the Auto Color Correction options as defaults for Auto Color, Auto Tone, and Auto Contrast, it
does not matter what algorithm you select in step
2. The three auto-correction commands use only those values that you
set for the target colors and clipping. The only exception is that the Auto Color command also uses the Snap Neutral
Midtones option.
Apply the Variations command
The Variations command lets you adjust the color balance, contrast, and saturation of an image by showing you
thumbnails of alternatives.
This command is most useful for average-key images that don’t require precise color adjustments. It does not work on
indexed-color images or 16-bit-per-channel images.
1 Choose Image > Adjustments > Variations.
The two thumbnails at the top of the dialog box show the original selection (Original) and the selection with its
currently selected adjustments (Current Pick). When you first open the dialog box, these two images are the same. As
you make adjustments, the Current Pick image changes to reflect your choices.
2 Select the Show Clipping option if you want to display a preview of areas in the image that are clipped—converted
to pure white or pure black—by the adjustment. Clipping can result in undesirable color shifts, as distinct colors in
the original image are mapped to the same color. Clipping does not occur when you adjust midtones.
3 Select what to adjust in the image:
Shadows, Midtones, or Highlights Adjusts the dark, middle, or light areas.