March 27, 2011, 9 a.m.
posted by lordnikon
is that Auto Contrast does not work on a channel-by-channel basis but instead uses the composite (combined) channel made up of all the channels in the image. To apply the Auto Contrast command to an image, choose Image>Adjustments>Auto Contrast.
The Auto Color command adjusts contrast and color by searching the image and assigning 50% gray as the neutral midtone color, and then redistributing the highlights and shadows out from that midpoint. To apply the Auto Color command to an image, choose Image>Adjustments>Auto Color.
If the tone or color of the image still isn’t quite right after using one of the Auto commands, complete the correction using the Levels dialog box, as described in “Setting Levels to Adjust Tone and Color.”
You can also apply the three Auto commands as adjustment layers, though you do need to go through a few hoops to get there. An adjustment layer uses the same commands found on the Image> Adjustments menu, but they work as layer-based corrections, giving you an amazing amount of flexibility. Layer-based corrections are not permanent and you can modify them at any time. Turn to Technique 22 to find out more about adjustment layers.
To apply the Auto Levels, Auto Contrast, or Auto Color command as an adjustment layer:
In the Layers palette, select the layer you want the adjustment layer to appear above.
Click the Create New Fill or Adjustment Layer button at the bottom of the Layers palette and choose Levels from the menu.
The Levels dialog box opens.
The Auto Color Correction Options dialog box, shown in Figure, opens.
Use the Algorithms area to select an automatic color correction.
Choose Enhance Monochromatic Contrast to apply the Auto Contrast command.
Choose Enhance Per Channel Contrast to apply the Auto Levels command.
Choose Find Dark & Light Colors and select Snap Neutral Midtones to apply the Auto Color command.
Click OK to close the Auto Color Correction Options dialog box.
Click OK to close the Levels dialog box and apply the automatic correction as an adjustment layer.