...when it comes to converting RGB images to CMYK there are a few areas that determine how the conversion is made...
...you can, when exporting to PDF from indesign or printing to postscript, perform the conversion in the print or export stream (convert to destination, preserve color numbers)...
...the destination CMYK profile used will determine what CMYK color values will eventually be used, as too will what source RGB profiles are tagged to the imported RGB images...
...if no RGB tagged profiles are on the images then indesign will use your RGB working space profile...
...cmyk images imported into indesign with tagged profiles are ignored, which means that the actual CMYK numbers remain untouched (preserve color numbers)...
...there has always been a big debate as to who should be doing the color conversion to CMYK from RGB images, and it is safe to say that keeping them in RGB allows for greater flexibility, allowing for one source RGB color values to be converted to different output CMYK intents (such as the differences between magazine CMYK and newsprint CMYK fro example)...
...this is where you should really, where possible, open up discussion with you print provider to discover their preferred color workflow...
...many reputable print providers have spent thousands and thousands on color workflow systems that are geared up for consistent color management, and with these type of calibrated systems, keeping to a RGB workflow can not only save a lot of RGB conversions, but can also yield higher quality CMYK output...
...essentially all your RGB images (tagged with RGB profiles (usually sRGB), and typically Adobe RGB 1998 is the best working space for conversion to CMYK) can be converted by your print provider to more accurate CMYK profiles suited to the actual base CMYK targets they have in place for the different paper types they use...
...and as mentioned above, your source RGB images can simply be repurposed to a different printing condition when or if needed...
...not only that, but RGB images hold wider color ranges (or gamut) and are smaller file sizes, using three (RGB) color channels instead of four (CMYK)...
...there are many pros to using RGB profile tagged images, but all in all you are best to discuss with your print provider their preferred workflow...
...whatever the outcome, you can save yourself a lot of conversion time by performing that in your print/export stream from indesign...
andrew
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