Color Correcting Images
Open the curves dialogue box. Double click on the shadow eye dropper. Enter 10 for all the RGB values.
Double click on the greytone eye dropper to set the midtones to 130. Double click on the highlight eye dropper to set the highlights to 245. “Ok” out of the dialogue box.
Click on the ‘adjustment layer’ half moon icon in the layers pallet and select ‘threshold’ to find the darkest part of the image. Select (shift + click) the darkest spot with the eye dropper.
Repeate the above step for the whitest spot in the image. Then cancel/delete the ‘threshold’ layer.
Create a new layer and fill (tip: shift + F5) it with 50% grey. Change the ‘blend mode’ to ‘difference’. Difference compares the layoer you are on to the one bwlow. With 50% grey the midtones show up darkest.
Add a ‘threshold’ adjustment layer. Find the darkest spot, which is the “most grey” and select (shift + click) the point with the eye dropper.
Open the Curves dialogue box. Use the first eye dropper (shadows) and click directly over the dargest spot selected previously. Repeate with the midtone and highlight eye droppers. (tip: use the capslock key to help selecting the points)
Select the ‘color sample tool’ click on the clear button to remove the targets.
note: For CMYK color reproduction use these values: Shadows[75 63 62 90], Midtones[50 40 40 10], Highlights[5 3 3 0]
LayersMagazine.com (based on this article and modified the steps with my own PS knowledge)