If you’re scanning colour negative films - or you’re shooting digital with a dodgy white balance - there’ll come a time when it’s necessary to alter the colour balance of the image. You could do this by eye using Photoshop’s Hue/Saturation tools, but if you’re anything like me, that’s just too hit and miss. The Curves adjustment allows a much greater degree of control, but relies on being able to accurately identify the darkest darks, the lightest highlights and the true mid tone. Here’s a devious way of doing this automagically.

1. Open the image as an RGB file
2. Select Image/Adjustment/Curves
3. Double-click the black eyedropper, and when the colour picker dialogue box appears, enter 20 in each of the R, G and B fields. This resets the black point to a slightly lighter grey.
4. Click OK, then double-click the white dropper. This time enter 244 in each of the R, G and B fields.
5. Click OK, then do the same for the grey eyedropper - but use 133 for these values.
6. Close the Curves dialogue, and answer yes when asked if you want to save the values. You won’t need to do this again.
7. Create a new Threshold adjustment layer (Layer/New Adjustment Layer/Threshold)
8. When this layer appears, move the slider all the way over to the left - then as you move it back towards the right, some areas of black will appear. These are the darkest areas of the image - in other words, the true blacks.
9. If you do the same but move the slider to the right, you’ll identify the lightest areas in the image - in other words, the true whites.

Now for the clever bit, which will help you find the midtones:

1. Create a new layer.
2. From the Edit -> Fill menu, select 50% grey so that this layer is filled. You’ll have now covered the rest of the image with an inpenetrable 50% grey layer.
3. Change the blending mode of this layer to Difference.
4. Create a new Threshold Adjustment layer as above, and drag the slider all the way over to the left. Now when you move the slider back over to the right, the black areas that come back are the true midtones!

This entry was posted on Friday, September 8th, 2006 at 23:04 and is filed under Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.