A recent question from a client:
“I have prepared a few images for my winter publication, exported out of LR and sent them on to my designer. They need CMYK. So, at what point do I make that conversion. Would I do an edit into Photoshop, make the conversion and then save it from there?”
“Conversion to CMYK needs to be done in Photoshop. When you’re done working on your master file (either in Lightroom or Photoshop), make sure to save it, then convert to CMYK as part of your process of generating the derivative file. You can do this my choosing Image > Color Mode > CMYK. (Be sure to retain your original RGB master!)
When you do this, the CMYK color space that will be used by Photoshop is determined by what’s set in Color Settings. You should use a CMYK profile that is as close as possible to the color space of the printing press being used; for example, if printing on a web press, use US Web Coated etc.. Try to get a custom profile from the printer, made specifically for their press. If you can’t get one, ask them what CMYK profile to use.
In Photoshop, You can also convert to any profile on your computer (CMYK or otherwise) under Edit > Convert to to Profile > and then select the profile from the menu. Sometimes this provides a better method than simply changing the mode due to the available options for choosing different rendering intents and a live preview.”