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Fine-tune image adjustments with Photoshop channels

Added on Wednesday 20th of January 2010 02:24 am EST
 

Application:

Adobe Photoshop CS/CS2/CS3/CS4

Operating Systems:

Macintosh, Microsoft Windows

 

While they might seem daunting, channels aren’t difficult to understand and edit. With a little practice and experimentation, you’ll be amazed at just how much adjustment control you have for fine-tuning your images.

 

To adjust an image with channels, we’ll:

     Provide a brief explanation of channels.

     Experiment with different Brightness and Contrast settings on individual channels to see how each affects the image.

     Select a portion of an image and apply a gradient fill to different channels to alter the image’s appearance.

 

Mention the words channels and image adjustments in the same sentence and the reaction you’re likely to get from most people is a glazed stare. That’s because, for whatever reason, channels are thought of as the quantum physics of image editing. But they really aren’t. In fact, channels are the very building blocks of digital images, and by using them to make critical image adjustments, as shown in Figure A, you can achieve a number of terrific results that you just can’t get any other way.

 

A

 

Channels demystified

Just in case you’re a bit unclear as to what channels are, let’s take a few moments to review them. When you capture or create a digital image, you first select its color depth. The color depth you choose in turn determines the number of color components you split your image into: one for grayscale, three for RGB, or four for CMYK. Each of the color components of your image is stored as a separate grayscale bitmap, called a channel, in your image file.

 

In addition to the one-channel-per-color component, a component combination channel is present in RGB and CMYK images. So, for example, an RGB image file contains a channel for red, green, blue, and RGB. When you open a digital image in an application, such as Photoshop, you can not only view each channel individually, but you can edit each channel separately as well. This enables you to critically adjust such image properties as brightness, contrast, and levels.

 

 

Open the channels of an image file

To demonstrate, let’s open an RGB file and then work with its channels. We’ll first make several adjustments to improve the image’s appearance, and then we’ll add a gradient fill to give depth to its background area. To follow along using our example, download the file channel.zip from the URL given at the beginning of this article, extract the file sails.psd, launch Photoshop, and open the image shown in the “before” example in Figure A. (Images provided by PhotoSpin. Some images modified for educational purposes.)

 

As you can see, the image requires a slight color balance and contrast adjustment. Select Window > Channels to open the Channels panel, as shown in Figure B. The image has both a slight reddish cast to it, which you can adjust using the red channel, and a flat appearance, which you can adjust using all of the channels. Let’s remove the reddish cast first.

 

B

 

Adjust the color balance via the red channel

You could adjust individual channels using the Brightness/Contrast command. However, the brightness and contrast adjustments change the appearance of an image channel in different ways. To see which adjustment will work best, let’s test each one separately:

 

1.       Select the Red channel to make it active, which also turns the visibility off for the other channels.

2.      Select the RGB channel’s Eye icon so you can see how your adjustments affect the entire image. Verify that the only selected (highlighted) channel is the Red channel.

3.      Select Image > Adjustments > Brightness/Contrast to open the Brightness/Contrast dialog box. CS3/CS4 users: Select the Use Legacy check box every time you use this dialog box for this article.

4.      Move the Brightness slider to the right to +100 and then to the left to -100 and examine the results, which are shown in Figure C. Reset the Brightness slider to 0.

5.      Move the Contrast slider to +100 and then to -100, and examine the results, which are shown in Figure D. Because the Brightness adjustment affects the image color balance more evenly in the highlight, mi...