Photoshop is a wonderful tool for handling color, but we don't live by color alone. Grayscale images have a magic all of their own, and many photographerseven those who print exclusively in grayscalefind that they can produce much better grayscale images from color captures than they can from black-and-white captures, whether they're shooting with film or digital cameras.
However, if you convert images by selecting Grayscale from the Mode menu, there's a good chance you're not getting the best-quality images you can. Many color images contain a great grayscale version, but you often have to wrestle to find it. The obvious method isn't always the best, so let's first take a look at the nonobvious ones too.
Tip: Scan in Color
Almost every scanner on the market these days is built to scan color images. If your original image is a color picture, you'll often get a better final result by scanning it in color and then converting it to grayscale in Photoshop using one of the techniques belowit's like using color filters when you shoot black-and-white film. If you're scanning a grayscale picture, you may also get a better result scanning in color; it depends on how neutral gray the image really is. For instance, we're more likely than not to scan an ancient yellowed black-and-white photograph as an RGB color image.
Convert to Grayscale
The most obvious way to convert an image to grayscale is simply to choose Grayscale from the Mode submenu (under the Image menu). When you do so, Photoshop mixes the red, green, and blue channels together, weighting the red, green, and blue channels differently (according to a standard formula that purports to account for the varying sensitivity of the eye to different colors). If that produces the result you want, great, but if it doesn't, there are many alternatives.
Take a channel, any channel
Look at the individual color channels in the image. Occasionally you'll find the grayscale image you want sitting in one of them. Then you can copy and paste it, or use Duplicate Channel from the Channel palette's popout menu to save it into a new document. Or you can just delete the other two channels by first displaying the channel you want, and then selecting Grayscale from the Mode menu.
You can select Desaturate from the Adjustments submenu (under the Image menu, or press Command-Shift-U). This is the same as reducing the Saturation setting in the Hue/Saturation dialog box to zeroit literally pulls the color out of each pixel in the document. The image is still RGB, but if you convert it to grayscale you'll get a different result than if you'd simply converted it to grayscale without desaturating first.
Convert to Lab
For a more literal rendering of the luminance values in an image, you can convert the image to Lab, then discard the color channels (a and b). This gives you yet another different rendering.
Load the luminance mask
One of David's favorite methods for squeezing a grayscale image out of a color photograph is to Command-click on the composite color channel (the RGB or the CMYK tile in the Channels palette), which loads the file's luminance map (this is different than the L channel of a Lab file). You can then tell Photoshop to save this selection as a new file by choosing Save Selection from the Select menu. We find that this often provides a much better grayscale image than simply converting to Grayscale mode.
Sometimes none of the above methods provides the grayscale image you want. Photoshop offers some more devious alternatives. In the past you had to use the dreaded Calculations dialog box to mix channels; we still use Calculations sometimes, because it lets you do things you can't do any other way, but the Channel Mixer feature offers an easier way to blend channels, so we often turn to it first.
The Channel Mixer dialog box is more utilitarian than you'd hope from a program like Photoshop (see Figure 12-1), but it lets you do one thing very well: mix the color channels of your image. You mix channels by percentage, and the result is a single channel (you can choose which channel the result will end up on in the Output Channel popup menu).
Figure 12-1. Channel Mixer dialog box
When converting an RGB image to grayscale, remember two things. First, the percentages in the dialog box should always add up to 100 percent to maintain the same overall tone of the image (though there may be situations where you don't want to maintain the overall tone of the image). Second, turn on the Monochrome checkbox to ensure that the result is neutral gray.
(Note that the Channel Mixer works fine with CMYK images, but it's much harder to maintain the image's tone. We prefer working from an RGB image when building grayscale images with the Channel Mixer, even if it means converting from CMYK to RGB first.)
Tip: Channel Mixer Adjustment Layers
We pretty much always use the Channel Mixer on an adjustment layer rather than applying the effect directly to an image, because the adjustment layer makes it very easy to editjust double-click on the tile in the Layers palette, and you can change the Channel Mixer settings. Even better, you can combine a Channel Mixer adjustment layer with a Levels or Curves adjustment layer to really bring out your image's most important tones. (See Chapter 7, The Digital Darkroom, for much more on adjustment layers.)
Like the Channel Mixer, selecting Calculations from the Image menu lets you mix and match a new grayscale image from the existing channels, but with much more power and flexibility (see Figure 12-2). The options in the Blending popup menu are the same as the ones in the Layers palette, and the Opacity field serves the same function as the Layers palette's Opacity slider. Calculations lets you combine the channels in much more complex, albeit less-intuitive ways than the simple addition and subtraction offered by the Channel Mixer.
Figure 12-2. The Calculations dialog box
Tip: Use Preview in Calculations
At first glance, the Calculations dialog box seems a lot less interactive than the Layers palette. But it doesn't have to be that way. When you turn on the Preview checkbox, you can actually watch how the various combinations work in real time (or at least as fast as your machine can compute). For instance, when you change opacity with Preview turned on, you see the effect almost immediately. You can type a new Opacity setting almost as quickly as you can move the slider in the Layers palette.
Figure 12-3 shows the results of the various grayscale conversion techniques on a Macbeth Color Checker (so that you can easily see how the colors convert to tone), and on an actual image.
Figure 12-3. Finding the hidden grayscale