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Performing Individual Channel Adjustments
Suppose that your Aunt Marge hands you a disk containing a digital photograph of your cousin Eddy, and she asks you to pretty it up for a holiday card. Because Aunt Marge is a nice lady and also has included you in her multi-million dollar will, you say, “Sure, no problem.” You take the file home (how hard can this be?), open it in Photoshop and find . . . a mess. The photo has too much blue, the flash created some nasty reflections that make highlighted areas look white hot, and little Eddy’s nose looks a bit like a squashed tomato.So you get to work trying to adjust the tone by using Levels, apply the Unsharp mask in an attempt to add definition to Eddy’s nose, and even try adjusting the image by using the Brightness/Contrast command (always a mistake — that command has wrecked more images than it ever helped). You start to get a bit nervous because sharpening the nose also added to the white hot highlight mess, and even though the blue is out of the photo, Eddy’s skin has taken on a sun-burnt hue. You don’t want to get written out of Aunt Marge’s will, so what can you do?Upon reflection (and some serious use of the History palette to undo all the previous attempts at correction) you think to yourself, “Hmm, too much blue, what about the Blue channel?” That leads you to take a look at the Blue channel in the Channels palette. You find that the Blue channel looks horrible — the edges aren’t sharp, some odd pixels are hanging around, and overall it’s way too dark. (This is a typical result when photographs are taken with lower-end digital cameras.)You start adjusting the photograph channel by channel, and voil! Little Eddy’s picture (and your place in Aunt Marge’s will) is restored.Adjusting the channels individually in an image gives you several advantages:
If a particular tonal area is off (for instance, the blue mentioned in the Aunt Marge story), you can target the problem color by using a specific channel — there’s no need to correct the Red and Green channels when the Blue channel is off.
Adjustments are more precise because you correct each channel’s problem individually, not the collective problems of the combined channels, which are different.
You control how the tonal values mix when the image is viewed with its composite channels. You can adjust the colors that you think need correction (instead of letting Photoshop do this for you automatically, which can lead to poor results).
The color and tonal adjustments commands (found on the Image>Adjustments menu) that you can apply to individual channels include Levels, Curves, Brightness/Contrast, Shadows/Highlights, and all the one shot tonal commands: Invert, Equalize, Threshold, and Posterize.For example, Figure 53-1 is an interesting photograph of the Brooklyn Bridge. The photograph is way too dark — the bridge columns in the background are almost indistinguishable from the shadows, and the sky looks murky.
Figure 53-1: An interesting photograph of the Brooklyn Bridge needs some adjustments.
If I apply Levels to all the channels at once (the composite), the best results that I can get without over-brightening the highlights are shown in Figure 53-2. To my eye, the photograph looks a bit washed out and it doesn’t have enough definition.
Figure 53-2: Although the Levels adjustment helps, it doesn’t add enough definition.
I went back to the starting board and adjusted the Levels of each channel individually. After fiddling for only five minutes, I was able to adjust the shadows and add the definition that I was looking for. As shown in Figure 53-3, the texture of the footpath has more definition and the bridge columns aren’t lost in the background anymore.
Figure 53-3: Applying Levels to each individual channel adds the definition that I’m looking for.
So anytime that you’re looking at a photograph and it just doesn’t have the clear quality that you want, the colors are dull, or overall it looks washed out, try adjusting each channel individually. You may be surprised at the great results that you get.
