Modern PDF workflows using the PDF/x-4 standard fully support RGB. There
is no reason to convert to CMYK and even if you do need that done, the
export using the press setting in InDesign will convert it on export.
Converting the images to CMYK is something I would not do anymore. It's
Because Photoshop and InDesign use the same color settings (synchronized with Adobe Bridge Color Settings), there is absolutely no advantage to doing it in Photoshop. If you provide a customized print settings file, it can be used as well in InDesign as in Photoshop.
And as Bob suggested, in modern print workflows it's best to leave the images as RGB.
The reason for not converting to CMYK in Photoshop is that it doesn't limit your file to output on a single device, so you can repurpose for digital printing, a sheetfed press in the US or a web press in China, or even an on-screen display and get optimal color from a single file, as long as you specify the correct output profile during export to match the output conditions.
The upside to this workflow is that you get the same conversion you would get by opeing the file in Photoshop and doing a Convert to Profile. That's also the downside -- there's no addtional opportuinty to tweak anything, so for projects that are REALLY color critical, like museum catalogs, I still do my conversions one by one in Photoshop and replace the RGBs in the .indd file. If I think I'll ever need to repurpose, though, I'd save an all RGB version, too.
Thank You Peter,
This has given me a clearer explanation to pass on to our customers and has been quite helpful.
Coming from a background in fine art reproduction the concept of not needing to "see" the conversion to CMYK for each image individually was a bit hard for me to grasp.
Although some of our clients do have online editions most of what we do here are image heavy catalogs and magazines of modern art and sculpture for clients that are extremely color critical. (we even have a few that check their proofs against originals with a spectometer)
Many of them have us do all of their image editing for them and we always keep an RGB version for back up or repurposing,
but for the few who provide print ready pdf's I was concerned that they would get unexpected results if they stopped converting their images first .