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Of course it is possible to convert easily to black-and-white by simply desaturating or converting to grayscale, but these methods toss out all of the color information and force you to accept only one of the infinite number of grayscale interpretations that are possible. Back in the film days we used to carry around a plethora of colored filters to attain some degree of control over the tonal relationships in monochromatic film. Red filters would darken the shadow areas relative to the other areas, and would darken blue skies. Blue filters would open up shadows relative to midtones..etc etc. Now this kind of control is more powerfully and more easily accessed in Photoshop, if you preserve the original RGB color information, by using one of these methods.

method 1 THE TWO-HUE/SATURATION LAYER METHOD

Create two hue/saturation layers on top of the background image layer. In the layers palette set the saturation of the TOP layer to Zero. Also in the layers palette with the BOTTOM Hue/Sat layer selected, set the blending mode to "color". By adusting the Hue slider in this layer (the lower Hue/Sat layer) you can simply dial in the tonal variations and choose which interpretation works best. Add a Levels layer above the 2 Hue/Sat layers and make a final decision only after adjusting levels and the eye is easily fooled by the changing brightness range when making the Hue adjustment. This technique also can be useful as a way to reduce noise is the low-exposure areas by choosing a filtration that minimizes the problem. If you are trying to save an underexposed or high ISO image, particularly in the skin tones where noise is unacceptable, this can be very effective. This is all possible because noise tends to concentrated in a single color channel and this adjustment is simply varying the relative contribution of each of the color channels.

method 2 CHANNEL MIXER METHOD (adapted from a published article)

Go to Layer>New Adjustment Layer>Channel Mixer>click OK. Click on the Green Channel in the drop down menu at the top of the dialog box. The green channel is used because it is the one with the most information. Click or check the monochrome box at the lower left of the dialog box. Using the adjustment sliders set Red=20%, Green=70%, Blue=10%. ( Note that they total 100%) These are the approximate values PS would select if the image was desaturated or changed in grayscale. These settings are rarely the best, so adjust the sliders to optimize your image, just as if you had added a filter on the camera.

Once you have achieved the optimized best look, click OK in the channel mixer dialog box then select the background layer in the layers pallet. Click on Add New Layer at the bottom of the layers pallet (the icon with a half black and half white circle). Select a Hue / Saturation Layer. In the hue/saturation dialog box use the Hue slider to separate the image tones for the best look. Use the Saturation slider to adjust the strength of the filter applied. In the layers pallet you will now have, starting from the bottom, background layer, hue/saturation layer, and channel mixer layer. You may now click on the channel mixer icon (three circles) and readjust if necessary. The black and white rendition is still fully customizable, with your color image underlying it. .