The profile you assign to the image by selecting it and using Assign Profile should be the actual profile of the image (the color space in which it was saved), which may be different from the profile used on the press. You may, and probably should, use Edit > Assign Profile.. to set the working space of the document to the same profile the printer has provided for output so that any native objects will remain unconverted during export and your overprint preview will be accurate.
PDF/X-1a is set to Convert to Destination (Preserve Numbers) so when you export to PDF/X-1a your RGB colors will be converted to the closest possible match in the destination space you specify. Any CMYK images will have the color numbers preserved and if they were saved in a different CMYK space you will see color shifts.
It is presumed that you are specifying the correct output profile, so it is imperative that you get that correct profile from the printer.
Sorry Peter I am still a little confused.
I will try an explain a little clearer and hopefully I you can explain again so I get it right.
The image is set to ISO Coated V2 and is CMYK
I have inserted into the document.
When I go to Edit>Assign profile do I choose Assign Profile and then the profile of the image (like below)
OR the profile that I want it to be?
When I make the pdf I export it and specify the correct output profile in the dialogue box as below. Is that right?
OK, if the final destination is PSR_SC_PLUS_V2_PT, that is the profile I would assign in the Assign Profiles dialog. When you do your PDF export your ISO Coated numbers are going to be preserved, but they are going to be interpreted as if they are the PSR space, (kind of like listening to two different regional accents pronouncing the same word) so you should expect some color change, though I have no idea how dramatic this will be.
This is a big disadvantage of the PDF/X-1a workflow. Your file is flattened and it is presumed that everything is in the correct space. RGB images will convert better (to the extent that they are in gamut for the output space) than CMYK images that are saved in the wrong space. The preset defaults to Convert to Profile (preserve Numbers), but you can change that to Convert to Profile and let ID change the numbers to match the appearance better, but you MUST NOT do this if there something like black type that is not already in the destination space because it will convert to 4-color black and be a problem for the pressman.
Ok, so I did all that and the image still looked terrible. Then I thought about the fact that you said RGB images convert better which I had know but not realised why.
So, I converted the image to RGB in Photoshop and then did what you suggested to make the PDF and by heck it worked!!!!
I am not sure I should convert a CMYK image into RGB but it has done the job.
Do you think this will cause a problem? Is it something I should just not do?
If not this has solved a rather long, frustrating and costly problem and has made my rather stressful Tuesday so much better
While converting from CMYK to RGB won't get back any colors you lost when the original conversion was made, there's nothing "wrong" with doing it to preserve the current state.
Well then I think this is the best work around I can find and am thankful that you answered my post as this was just driving me totally nuts. Many thanks Peter.
For what it's worth, I often leave images in RGB through export, but if I know in advance what the destination profile is going to be I will, for very critical work where i need to make color adjustments to the CMYK, do the initial layout using the RGB images, then just before sending to print I'll make a new version and replace the RGB images with properly sized and tweaked CMYK-converted images that match the output space.