This Photoshop tutorial is designed to instruct on the use of Photoshop's Curves dialog box to set the correct white point, gray point, and black point in your photograph.
Using the White Point Eye-Dropper
A good starting point when using Curves is to first set the "White Point" of your photograph by using the White Point Eye-Dropper Tool located in the lower right corner of the Curves dialog box.
Technical Note: Before making any changes to your photograph do two things: 1. Save the photo as a new .PSD file using "Save As," and 2. make all of your changes on a new duplicated layer by using Command>A to select ALL (the entire image), and then using Command>C to paste the copy to a new layer. Always make sure that the 'Preview" radio button is checked.
The White Point Eye-Dropper needs to be used with caution as it will make whatever it touches the new "white point," destroying all pixel information below it. This tool will also fix any underexposure issues with the photograph, but I would use the Curves "Slider" for this problem (see: Using Curves Diagonal Graph Slider" below).
To set the white point use the Eye-Dropper Tool to select the whitest area of the photograph. In the example above we have chosen the highlight in the headlight area. If the first selection produces no noticeable results, move to the next brightest area. Again, cation is needed as this can blow out the highlights in unwanted areas.
Technical Note: Always set the Eyedropper Tool sample-size to 5 x 5 pixels, especially important with photos that contain a high amount of noise.
Neutral Gray Eyedropper
The Neutral Gray Eyedropper Tool (center eye-dropper at right of Curves dialog box) is also useful to remove unwanted color casts in areas that are supposed to be a "neutral gray." The grey value you select in your photograph should be approximately half way between pure white and black - 50% gray.
Black Point Eyedropper
Use the Black Point Eyedropper Tool to fix photographs that are washed out from glare, haze, or overexposure. Use the eyedropper to the left to select the deepest shadow area in your photograph.
Using Curves Auto Color Correction Options
You can always let Photoshop use its auto correction algorithm to do the thinking for you, and sometimes this can yield good results. By selecting the "Options" button to the right of the Curves dialog box you will bring up the Auto Color Correction Options dialog box.
By selecting the Shadows, Midtones, and Highlights box you will bring up the Photoshop Color Picker pallet. By sliding the Circle Cursor in the "Select Target Highlight Color" box you can manually play with the target color and brightness of the White Point.
When all of your corrections have been made to your duplicate layer (described in the fifth paragraph (Technical Notes), assess the results by toggling the layer on and off so that you can compare your altered version to the original version below. I have found that the more corrections you make, and the further you stray from the original photograph, you can easily overdo the corrections. If this is the cast it is an easy fix as you can dial down the opacity of the top altered layer until you have a "natural" looking result.
On To: Tonal & Color Correction with Curves
