Some photographers who have recently graduated from shooting film are surprised that their expensive new digital SLR doesn't provide any way to shoot in black and white. After all, cheaper consumer models often do, and it seems to be an easy trick for a digital camera. But the truth is that even if your camera can shoot directly in black and white, you're much better off sticking with color, and converting to B&W in Photoshop after the fact.
Black and white film, as you likely already know, is not sensitive to color. It records only how bright or dark things are a property of light known as luminance. Unlike color film that allows us to differentiate objects based both and color and contrast, B&W film causes adjacent objects of different colors to appear to merge if each has the same luminance. A medium toned red object will stand out on top of a medium toned green one on color film but will blend in grayscale. To compensate for this, B&W shooters often use colored filters to alter how much light from various parts of the spectrum is allowed to pass through to the film. Original color imageA yellow filter will reduce the blue of shadows and improve contrast, enhancing landscapes and many other subjects. An orange filter will darken the sky by passing very little blue (the complement color to orange). Green filters lighten leaves and foliage and were also traditionally used to improve skin tones, although the need for this diminished considerably over the years with the introduction of more balanced emulsions. The quest to get good results with black and white film generally began by learning these rules, although the real challenge was in seeing the possibilities of a scene and selecting the best filter to maximize its possibilities.
Digital cameras that do provide a black and white mode generally do so simply by ignoring color and recording luminance only. This would give you results equivalent to some hypothetically "average" black and white film emulsion. In order to modify the responsiveness of the camera to different colors, you'd need to rely on colored filters, exactly as a film shooter would. While this would certainly work, it fails to tap into improvements digital is capable of.
The two easiest ways to convert to black and white in Photoshop aren't much better. Image >> Mode >> Grayscale converts an image to black and white by discarding all color information. The traditional three channels of red, green and blue are replaced with a single gray channel. The results though often tend to be fairly boring, constituting mainly of various gray tones with few blacks and whites. If instead you open the Image >> Adjustments >> Hue/Saturation dialog and move the Saturation slider to the far left you can strip all the color information out of an image. Results from doing so though tend to be equally lacking in impact. Both methods also provide no means of altering how color contrast is rendered without once again falling back on the use of colored filters over the lens when shooting in the field. Not really what we are after with the promise of digital.
The answer to getting good black and white conversions in Photoshop comes in the form of something known as the Channel Mixer. Found both on the Image >> Adjustsments menu and as an Adjustment Layer, it allows us to visually choose how much of the information in the red, green and blue channels goes into the final grayscale version. If you're a regular Earthbound Light reader, you can probably already guess that the Adjustment Layer route is the better choice as it lets you tweak the effect later without actually altering any of the underlying image pixels. Open your favorite color image and then click on the half-black-half-white circle New Adjustment Layer icon at the bottom of the Layers palette to create one. Select the option from the resulting pop-up menu and you'll be presented with the Channel Mixer dialog.
To use the Channel Mixer, first check the Monochrome checkbox at the bottom of the dialog since we want to convert to black and white. Be sure that the Output Channel drop-down list is set to the Gray channel and turn on the Preview checkbox so you can see the results on your main image. Now adjust the Red, Green and Blue sliders so that they total somewhere around 100. If they total considerably more than that you will end up with a completely washed out image. Considerably less will likely yield a very dark or even black image. Depending on the image, you may need a bit more or less than 100 to get the overall brightness where you want it, but don't stray too far from that total or you will find it difficult or impossible to get a good image. If desired, you can also modify the overall brightness by means of the bottom Constant slider.
Since you can see the results as you move the sliders, you can often achieve a good conversion simply by playing around a bit, but some analysis of the colors in your image can help steer you in the right direction more quickly. If you want to make a certain object brighter, you need to include more of the channel that includes the majority of its color information and less of the others channels. For example, a red object gets brighter if you move the red slider to the right and compensate by lowering the green and blue channels to get your total back around 100. Alternatively, you can darken that red object by doing the reverse. Indeed, you can convert the same scene with different settings to achieve radically different interpretations of your subject.
The Photoshop Channel Mixer provides you with visual feedback when converting to black and white so you can tailor the look of the results to your exact preferences. Once you try it, you'll find this to be a huge advantage over any in-camera method, film or digital.