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CMYK HAS FEWER PRINTABLE COLORS THAN RGB
It's Device Dependent
CMYK is known as "Device Dependent" which simply means, all output devices that require a CMYK file will require a slightly
different mix of CMY and K in order to produce similar, accurate color. No two digital color devices handle color exactly the
same way, just as no two film stocks from different manufacturers are the same.
Many desktop users incorrectly believe that all that is necessary to convert a file to CMYK is to simply go under the Mode Menu
in Photoshop and pick "CMYK". Nothing could be further from the truth! One could fill pages on setting up Photoshop’s various
preferences for controlling how it converts to CMYK. Blatner and Fraser’s "Real World Adobe Photoshop 6" published by Peachpit
Press an excellent source for this information.
Even if one takes the time to create all the various preferences for Photoshop, another issue needs attention. By it’s very nature,
CMYK has far fewer printable colors then RGB. This range of colors is known as color gamut. If an output device has a narrower color
gamut then the file it receives, some colors will not be correctly reproduced; the colors will be clipped. This explains why so many
desktop users are disappointed when very saturated and vibrate colors they see on their monitors don’t look the same when printed.
In the best situations, the hue (the actual color) remains similar but the saturation is far lower, in the worst case, the color
simply doesn't get close to matching. When vibrant blue skies print magenta, someone is going to be very unhappy.
So keep in mind that all output devices require different mixes of color to produce the expected output. Also keep in mind that all
output devices will have different abilities to reproduce wider or narrower palettes of colors. This is true for RGB devices but
far more so for CMYK devices. Furthermore, many of the devices used in the imaging chain can only output RGB data, such as a color
monitor. Not only does a monitor need to be calibrated so it is set in a known state, software provisions are necessary so that
when this monitor displays a CMYK file, it does so in an accurate fashion. In order to do this, the software must in essence convert
CMYK data back to RGB in such a way that the data on screen is displaying what is truly representative of the file. Photoshop’s
monitor’s preference is one area where this can be accomplished assuming the proper preferences are created and loaded.
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