How to Convert RGB to CMYK in Photoshop
When you take a photograph with a digital camera or scan an image using a digital scanner, the resulting file will use the RGB (Red Green Blue) colour space. You can then view it on your computer monitor and print it out using your desktop printer and get a good representation of the colour in the original image; this is because your monitor and printer use the same RGB colour space.
If you need to have your photograph printed by a commercial printer using 'full colour process printing', you will have to convert RGB files to CMYK, as a printing press uses the CMYK (Cyan Magenta Yellow blacK) colour space.
The problem is; the range of colours, referred to as gamut, that can be produced using CMYK colour inks on paper, is a lot smaller than what can be represented using RGB. This can result in a photograph printing with some of the colours changing hue and looking dull. This often happens to the bright rich blue found in sky areas of an image.
The image below left is the original RGB, to the right, the image has been converted to CMYK. Notice the colour shift. The larger photo has been adjusted slightly to bring out the colour.
We have a further problem. There is no standard fixed CMYK gamut; so the same percentages of CMYK inks will not always produce the same colour. There are variations in the inks, the absorbency of the paper, the printing conditions, and different press setups which vary in different parts of the world.
The black ink in CMYK printing is used because the CMY inks do not produce a pure black when added together: they produce a brown colour due to impurities in the inks. The black ink when added gives more contrast to the image with darker shadow areas.
It is possible to replace any grey areas of an image (represented by equal amounts of CMY inks) with an equivalent percentage of black ink, thus reducing the amount of ink used. This is referred to as undercolour removal (UCR).
Therefore, there are many possible combinations of CMYK to achieve the same image as in the original RGB file.
Converting RGB files to CMYK
Thankfully, Adobe Photoshop, Illustrator and InDesign, are design programs that provide CMYK Prepress presets recommended for press setups across the world.
In Photoshop, select 'Edit' then 'Color Setting...' and look at the dropdown list under 'Settings'. Here you will see presets for Europe, North America and Japan.
Select a CMYK profile that best suits the final printing conditions, for example, 'ISO Coated v2 (ECI)', which is used for standard ISO printing based on the FOGRA39L characterization dataset applicable to reference printing conditions according to the international standard ISO 12647-2:2004.
Clicking on the 'More Options' button will allow you to set the rendering intent to use when converting from RGB values to CMYK. For continuous tone images such as photographs, selecting 'Perceptual' will generally give the best results while preserving the visual relationships of the source image.
Once you have setup the best colour settings above, you can open the RGB image you want to convert.
A photograph from a digital camera will probably have an embedded RGB colour profile (sRGB IEC61966-2.1). If Photoshop presents a window with 'Embedded Profile Mismatch' then select 'Use the embedded profile'.
If you need to make any adjustments to the image or apply any filters etc., do it now while the image is still in the RGB colour space.
To see which areas of the image that are 'out of gamut'; select 'View' - 'Gamut Warning'. Any colours that turn grey are colours that can not be produced in the CMYK colour space and Photoshop will have to select the nearest colour it can, depending on the rendering intent you selected earlier.
To covert from RGB to CMYK; select 'Image' - 'Mode' - 'CMYK Color'. You will probably notice that some of the colours change and become much duller. As shown in the images above. Here the colour of the sky has changed dramatically from a bright saturated blue to a duller purplish blue.
It is possible to fine tune the CMYK image to improve the colour of the sky and make it a brighter blue. To arrive at the larger image above, I used Photoshop's 'Replace Color' adjustment tool, selecting the sky colour and adjusting the saturation, lightness and hue controls.
If you have a lot of images that you want to convert automatically, then see my article on batch converting.
You might also be interested in my article on choosing a colour profile for commercial print.