Quadtones can add a nearly sculptural depth to black and white portraits. While the shadow areas of the image are saturated with process colors, only a small amount of color appears in the midtone and highlight areas. The result is a rich neutral image that appears to rise from the page.

The original grayscale image was copied into each of the four channels of a blank CMYK image. (Note that the full range was copied into each channel and not pasted into the CMYK layer, which would result in a conventional separation.) Next, a top layer filled with C60, M35, Y35 and K0, was added. The Blend mode was set to Color and the Opacity was set to 15%.

More info:

Quadtones »
Tonal Range »

Ed #2 Quadtones

Image Credit: Charlie Simokaitis