Black/White/Gray Point Selection

by Hugh Stockton

In his book, Welcome to Oz, Vincent Versace asserts that all sensors have a color cast. The following is adapted from his 'fix' for the problem.

Ron Donson sent me a version of this from Dave Cross that includes finding 'Gray', I've incorporated it below.

Panels which need to be visible - Tools, Layers, Adjustments

1. Open the image in PhotoShop
   
   
2. Make Background Layer active - click on it
   
   
3. From Tools Panel - select Eyedropper Tool
   
   
4. Select Caps Lock to make the cursor cross-hairs
   
   
5. In Adjustments Panel - select Threshold, image will become black/white
   
   
6. Move slider all the way left - image will go all/mostly white
   
   NOTE: any areas that are not white will be areas of zero detail - all black, and should NOT be selected in the steps below.
   
   
7. Very slowly move slider right until the first black appears
   
   
8. Zoom in very tight and tease the slider back and forth until you find the first, black pixel or first group of black - Shift-Click on it/them - a small circle with number 1 will appear.
   
   
9. Move the slider to the middle, zoom out so you can see all of the image - still black/white
   
   
10. Move slider all the way right - image will go all/mostly black.
    
    NOTE: any areas that are not black will be areas of zero detail - all white, and should NOT be selected in the steps below.
    
    
11. Very slowly move slider left until first white appears
    
    
12. Zoom in very tight and tease the slider back and forth until you find the first, white pixel or first group of white - Shift-Click on it/them - a small circle with number 2 will appear.
    
    
13. Close/delete/trash the Threshold tool - the image will return to color. Click OK to delete the Threshold layer.
    
    
14. In the Layers Panel, be sure the Background Layer is active.
    
    
15. Click the small rectangle icon at the bottom of the Layers Panel to create a blank layer.
    
    
16. Edit>Fill - select 50% Gray from the drop-down.
    
    
17. From the Blend Mode drop-down, select "Difference." Ignore the color change in the image.
    
    
18. In Adjustments Panel - select Threshold. Image will become black/white.
    
    
19. Move slider all the way left. Image will go all/mostly white.
    
    
20. Very slowly move slider right until the first black appears.
    
    
21. Zoom in very tight and tease the slider back and forth until you find the first black pixel or first group of black. Shift-click on it/them. A small circle with number 3 will appear.
    
    
22. Delete the Threshold panel.
    
    
23. Delete the Fill Layer.
    
    
24. In the Layers Panel, select the Background Layer.
    
    
25. From the Adjustment Panel, select Curves.
    
    
26. In the Curves panel, double-click the Black/Top Eye-Dropper. The color picker will open.
    
    
27. From the Tools Panel, select the Foreground Color Picker. Change the R, G, and B boxes to 10. Click OK.
    
    NOTE: 10, 10. 10 is Black enough for Dave Cross
    
    
28. Zoom in very close on Circle 1. As precisely as possible, superimpose the cursor on the circle, and click.
    
    
29. In the Layers Panel, select the Background Layer
    
    
30. From the Adjustment Panel, select Curves.
    
    
31. In the Curves panel, double-click the White/Third Eye-Dropper. The Color picker will open.
    
    
32. Change the R, G, and B boxes to 243. Click OK.
    
    NOTE: 243,243,243 is White enough for Dave Cross.
    
    
33. In some cases the result may not be acceptable to you. If so, simply drag the Curves layers to the trash can at the bottom of the panel.
    
    
34. In the Layers Panel, select the Background Layer.
    
    
35. From the Adjustment Panel, select Curves.
    
    
36. In the Curves Panel, double-click the Gray/middle Eye-dropper. The color picker will open.
    
    
37. From the Tools Panel, select the Foreground Color Picker. Change the R, G, and B boxes to 133. Click OK.
    
    NOTE: 133, 133. 133 is Gray enough for Dave Cross
    
    
38. Zoom in very close on Circle 3. As precisely as possible, superimpose the cursor on the circle, and click.
    
    
39. From the Tools panel, select the Color Sampler Tool - right click in the Eyedropper and select from the fly out. Move the cursor over the circles one at a time and Alt-click to remove them.
    
    NOTE: The closer your scene matches the White Balance of your sensor, the less effect you will see. the most difference I've found is in low light with multiple light sources.
    
    

Please let me know if you have questions. I'd be very interested in your results.

hugh@shutterfreaks.com