Be sure the Info and Histogram windows are open in the Palette Bin. Select the Eyedropper.
Be sure Sample Size is set to 3x3 Average.
Use Layer->New Adjustment Layer->Levels, or Alt + Layers Palette->Create adjustment Layer->Levels to bring up a New Layer dialog box.
Type in a useful name, like blackPoint, as we will set the true black in this step.
Select the shadow eye dropper.
Find a good dark shadow which you want to make your pure black. Watch the pixel values in the Info window. When all the shadows have one of the RGB values either lower or higher than the other two, that is a good indication of a color cast in the shadows. Click one of these points and see what happens to the image. Note that the shadows have been darkened and had a color added or subtracted to balance them. Hit the Reset button on the Levels dialog box.
Try another point. Keep it up until you find one you like. Do not select a pure black (0.,0,0) shadow, as that will change nothing.
If you want to darken all shadows and midtones, move the black Input Levels slider to the right. Watch what happens to the image & the histogram.
To lighten the shadows, move the black Output Levels slider to the right. Watch what happens to the image & the histogram.
If, as you examine pixel values in the Info dialog box, you cannot find an evenly-balanced sample point, i.e. the RGB values are not close, try the following: the goal is to get the RGB numbers to be close in the darker areas by adjusting individual channels.
Select a channel you need to change such as Red. Use the black Input Levels slider to reduce dark and midtone reds (increase cyans).
Use the gray Input Levels slider to adjust red midtones. Use the black Output Levels slider to increase dark reds. Repeat with one other color if necessary, but remember the goal is to make as few adjustments as necessary, because, to some extent, each color changed, also impacts the others. Recheck using the Info window.
When you think you’ve got it, click OK on the Levels box to add the layer. Click on Opacity at the top of the Layers box and do further experimentation until it looks good. Remember you’re not through, so do not run wild.
Now do the same procedure by creating a whitePoint Levels layer. Use the highlight eyedropper and an appropriate near-pure white highlight. Do not select a pure white (255,255,255) highlight. Reset and repeat until you are close. If you want to brighten all highlights & midtones, the white sliders work as you would expect.
Now bring up one more Levels box and name it grayPoint. If there is a good near-medium gray (ca. 127,127,127) in the image, try selecting the gray eyedropper and clicking it on the near-gray point. If this does not work, reset it. Try taking the middle, gray slider in the Input Levels box and sliding it left or right to adjust midtones. Get it to a point you like, and click OK on the Levels box.
Finally, because these are layers, you can adjust the opacity on any of them.
You can also make them invisible in different combinations until you get the version you like the best.
