Converting Color to Black and White

From time to time you'll need to convert a color picture to black-and-white, whether the original was taken on film or in pixels. Some digital cameras have a black-and-white option, perhaps augmented by a sepia option, too. You'd think they'd do a good job of creating a B/W picture because, technically, a digital camera's sensor is totally blind to color. The sensors themselves are strictly black-and-white components. They become "color sensitive" because of the color filters that are placed over each photosite on the sensor.

If there were some way to remove those color filters, then a digital camera could, theoretically produce a great-looking black-and-white image. Indeed, some vendors, such as Kodak, actually sold black-and-white-only digital cameras during the previous millennium. Because every pixel in the sensor could be used without interpolation (as is required for color digital pictures with cameras other than the Foveon-using Sigma and Polaroid cameras), a black-and-white digital shooter maximized the available resolution.

Most of the time, you'll need to convert an existing color digital or film image to monochrome. Photoshop makes it very, very easy to convert a good color photo into a bad black-and-white image. All you need to do is select Image > Mode > Grayscale from the menu bar, and presto change-o, your color image has been converted to an inaccurate black-and-white rendition. Or, perhaps, you decide to use Image > Adjustments > Desaturate, which does much the same thing, but only operates on a particular layer or selection.

Of course, images converted this way always seem to have low contrast. So, your next step probably would be to use Image > Adjustments > Brightness/Contrast to boost the contrast a bit. In a process that took only a few seconds, you've managed to convert a good color image into an excessively contrasty black-and-white photo that doesn't necessarily offer a good representation of the original. What happened? You've fallen for the same trap that has snared photographers for decades. It has long been common to increase contrast when making a black-and-white print from a color negative, and the practice has become standard operating procedure in the digital world, too.

The fallacy lies in the fact that in a black-and-white photo, the contrast, or apparent differences between objects in an image that makes them distinct, is determined solely by the relationship between the light and dark tones. This is important: In a black-and-white picture, the only way to separate various objects in a picture is through the use of the monochrome tones, the variations between them, and how they provide a three-dimensional look as they represent the lighting that illuminates the objects. There are no other visual cues to differentiate between, say, a green Granny Smith and a Red Delicious apple.

That's not true when an image is presented in color. In a color photo, three separate factors determine true visual contrast among objects. Those include the hue (the various colors of the image), saturation (how rich they are), and brightness (the lightness or darkness of a tone). I see this glossed over in most books about Photoshop, so I'm going to take the time to clarify the inherent problems behind color to black-and-white conversions. Understanding the problems will help you avoid them.

The following illustrations should make the situation abundantly clear. I'll use the image shown in Figure 7.3, a landscape photo of a barren dirt field with mountains and sky in the background. When converted to grayscale using Photoshop's Mode changing operation, the image looks like Figure 7.4.

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