Photoshop - Converting a Color Image to Black & White

There are several ways to convert a color image to black and white, and each has its own advantages and disadvantages. As ever, feel free to right click the original image and copy it into your Photoshop.

RGB to GRAYSCALE

It's very tempting to go down this route. Simply IMAGE...Mode...Grayscale and that's it. However unless your image is destined for black-and-white output we'd advise against this method as it throws away two-thirds of the image information by condensing three channels (RGB) into a single greyscale channel.

An advantage of converting via grayscale is that it opens up the Duotone/Tritone/Quadtone capabilities, which can generate very subtle effects.

Photoshop lets you create monotones, duotones, tritones, and quadtones. Monotones are grayscale images printed with a single, nonblack ink. Duotones, tritones, and quadtones are grayscale images printed with two, three, and four inks. In these types of images, colored inks are used to reproduce tinted grays rather than different colors. This section uses the term duotone to refer to duotones, monotones, tritones, and quadtones.

To get this options box, go to IMAGE...Mode...Duotone and click the Type box to get the drop-down menu.

Make sure the "Preview" box is checked. Adjusting the graph lines alters the image. # Adjust the duotone curve for each ink by dragging a point on the graph or by entering values for the different ink percentages.

In the curve graph, the horizontal axis moves from highlights (at the left) to shadows (at the right). Ink density increases as you move up the vertical axis. You can specify up to 13 points on the curve. When you specify two values along the curve, Adobe Photoshop calculates intermediate values. As you adjust the curve, values are automatically entered in the percentage text boxes.

However, remember that this still doesn't alter the fact that valuable color information is being thrown away. With this in mind, always create a duplicate of the original image and apply grayscale/duotone etc. effects on the copy.

Desaturation
To achieve a black and white effect without losing color information, open up a color image and go to LAYER...New...Adjustment...Layer...Hue/Saturation. In the dialog box that appears, make sure Hue/Saturation is selected and click OK. This will bring up the Hue/Saturation dialog box. Make sure "Colorize" is unchecked and drag the Saturation slider to the left (-100).
An adjustment layer basically "sits above" the original allowing non-destructive changes.
The advantage of this method is that it maintains all the detail in your original image, resulting in a richer print when converted to CMYK and output on a process color page.
By adding a little bit of saturation back into the image you can create some subtle, aged looks that almost look like hand-tinted photos.

Channel Mixer
By far the most flexible ways to convert a color image into black and white is to use the Color Mixer. With a color image open, go to LAYER...New Adjustment Layer...Channel Mixer.

The box to the right will appear. Tick the Monochrome box and adjust the Source Channel sliders until the three add up to 100% (if less than 1005 the image may look underexposed, over 100% it may look overexposed).

Probably the best part of doing it this way is that you can have fun by making up the 100% which is not uniform across the channels. This is where you can make previously-hidden elements of an image appear. Have fun!