Adobe Photoshop CS5 for Photographers - Martin Evening [157]
Channel selection shortcuts
Along with other changes to channel selection shortcuts, those for selecting the color groups in Curves have changed since CS3. You now need to use to select the composite curve channel, use to select the Red channel and so on. Likewise, you can use to load the composite (luminosity) channel as a selection, to load the Red channel as a selection, etc. For what it is worth, Adobe have made available a plug-in which, when enabled, allows users to restore the old channel selection behavior. This does of course override the new shortcuts described here. Personally, I think it is better to adapt to the new ‘universal’ shortcuts as these are here to stay. These resolve some of the international keyboard conflicts and help unify the user experience between the other Creative Suite programs.
On these pages I show how you can use the Threshold mode analysis technique to discover where the shadow and highlight endpoints are in each of the three color channels, and use this feedback information to set the endpoints. This is a really good way to locate the shadows and highlights and set the endpoints at the same time, because once you have corrected the highlight and shadow color values in each of the color channels independently, the other colors usually fall into place and the photograph won't require any further color correction.
1.
This photograph has a blue cast in the shadows and a magenta cast in the highlights. The first step was to go to the Adjustments panel and click on the Curves button (circled) to add a new Curves adjustment layer.
2.
I went to the Channel menu, selected the Red channel and adjusted the shadow and highlight Input sliders until the red shadow and red highlight points just started to clip. If you hold down the key as you do this you will see the Threshold display mode (shown here) which can help you locate the shadow and highlight points more easily. I then repeated these steps with the Green and Blue channels until I had individually adjusted the shadow and highlight points in all three color channels.
3.
Finally, I added a midpoint in the Blue channel and dragged it to the right to add more yellow. This technique of adjusting the color channels one by one can help you remove color casts from the shadows and highlights with greater precision. The trick is to use the Threshold display mode as a reference tool to indicate where the levels start to clip in each channel and consider backing off slightly so that you leave some headroom in the composite/master channel. This then allows you to make general refinements to the Curves adjustment, ensuring that the highlights do not blow out.
Precise color corrections using Curves
The basic Levels/Curves adjustment technique I have just shown you can be quite effective at correcting the color at the shadows, highlights and midtones. It therefore follows from this that you can utilize the versatility of Curves to adjust the individual channels with even greater precision. As a general rule, once you have corrected for a cast in the highlights and shadows, all the other colors should more or less fall into place and a further tweak to the midtones may be all you need. However, there are still plenty of situations where you may wish to exploit the full potential of a Curves adjustment to improve the color appearance. In the steps shown opposite I found that I needed to make two carefully targeted midpoint Curves adjustments to the Red and Blue channels in order to correct for the blue/cyan cast.
Direct Curves shortcuts
Since Photoshop CS3, the Curves Adjustment panel shortcuts have been adapted to take into account the fact that such