Today’s Question: You said “a CMYK file prepared for printed output would normally have four channels”. But how could a CMYK file have anything other than four channels, for cyan, magenta, yellow, and black?
Tim’s Quick Answer: While a CMYK file would normally contain four channels (cyan, magenta, yellow, and black), it is possible to add additional channels to provide “spot color” support when printing.
More Detail: Commercial CMYK offset press printing typically employs four inks, which are generally cyan, magenta, yellow, and black. This is similar to the typical ink sets used in photo inkjet printers, though today’s inkjet printers often include varying shades of each standard ink color, and sometimes include other colors altogether in order to expand the color gamut.
While offset press printing commonly involves four inks, that isn’t a fixed requirement. In cases where you don’t need a full color gamut, fewer than four inks might be used. And it is also possible to employ more than the standard four inks to supplement the color range available.
A typical four-ink CMYK offset press print job would be capable of producing a full range of colors such as would be found in a color photograph. But you can also print using inks that go far beyond the capabilities of CMYK inks. For example, gold ink might be added to print a golden signature on a photographic print. In some cases a “spot color” ink is actually a varnish that coats part (or all) of the print.
So while four inks are certainly standard for CMYK printing, it is also possible to change the number of inks used for specialty print jobs. This is one of the reasons that photographers may still want to consider offset printing for certain photo projects, even if they typically print their own photographic images under normal circumstances.