I recently was asked a question by a colleague that is a pretty common issue in print design.
Question:
"Before you send a PDF-x1a or release a file to print with images do you convert the psd image file (for example) to CMYK or leave as RGB?
Any pdf file I send out for print will be CMYK (except when spot colors are required--more on that in another post), but I generally don't bother converting the images I've placed in Indesign to CMYK. If you use the X1a export settings in Indesign, it should be converting to a SWOP CMYK color space by default when you export the pdf. This is the same conversion that you would make if you were converting your image to CMYK in Photoshop.
The short answer:
Any pdf file I send out for print will be CMYK (except when spot colors are required--more on that in another post), but I generally don't bother converting the images I've placed in Indesign to CMYK. If you use the X1a export settings in Indesign, it should be converting to a SWOP CMYK color space by default when you export the pdf. This is the same conversion that you would make if you were converting your image to CMYK in Photoshop.
The longer answer:
Converting a layered psd file will usually require you to flatten your layers, especially if the there are adjustment layers. So I avoid converting to CMYK for that reason as well (if you merge all the layers, instead of flattening, you can avoid losing transparency/silos). However it is best to be working in AdobeRGB in photoshop, not sRGB (camera and web color space).
The only time I do sometimes convert everything is when I have to send packaged files to a printer, when they need my layout instead of just a pdf. Some printers can be finicky about converting for you. In a case like that I'll package the InDesign layout to my desktop, convert all the images, update the images in the desktop copy of the layout—making sure no silos were lost (from flattening of the images). Then it's ready to send out.
If you have a CMYK file, you can leave it as CMYK, but you'll want to be aware of ink density. When RGB images convert to CMYK SWOP, the total ink coverage in the darkest areas will be no greater than 300%. (Lower specified densities, such as SNAP for newsprint, are a different issue.) With a CMYK file, with the use of adjustments such as levels and contrast, it's possible for total ink coverage to reach as high as 400% (100% ink in all four channels). Densities exceeding 300% can become a problem on press, leading to ink not drying fully, rubbing off, or bleeding through the paper. The "registration" color swatch in your layout program is 400% of ink coverage, and is intended only for registration and trim marks on the outside of the trim area of the page, never for type or other elements that are part of a page design.
If you have a CMYK file that exceeds the maximum allowed ink density, what's the easiest way to fix the problem? Convert it back to AdobeRGB. Need it to be a CMYK image file you are sending with packaged files to you print vendor? Convert it back to CMYK. The ink limits were constrained when you went to RGB, but will not 're-expand' when converting back to CMYK. They will fall within the limits of the profiles used within Photoshop.
The only time I do sometimes convert everything is when I have to send packaged files to a printer, when they need my layout instead of just a pdf. Some printers can be finicky about converting for you. In a case like that I'll package the InDesign layout to my desktop, convert all the images, update the images in the desktop copy of the layout—making sure no silos were lost (from flattening of the images). Then it's ready to send out.
If you have a CMYK file, you can leave it as CMYK, but you'll want to be aware of ink density. When RGB images convert to CMYK SWOP, the total ink coverage in the darkest areas will be no greater than 300%. (Lower specified densities, such as SNAP for newsprint, are a different issue.) With a CMYK file, with the use of adjustments such as levels and contrast, it's possible for total ink coverage to reach as high as 400% (100% ink in all four channels). Densities exceeding 300% can become a problem on press, leading to ink not drying fully, rubbing off, or bleeding through the paper. The "registration" color swatch in your layout program is 400% of ink coverage, and is intended only for registration and trim marks on the outside of the trim area of the page, never for type or other elements that are part of a page design.
If you have a CMYK file that exceeds the maximum allowed ink density, what's the easiest way to fix the problem? Convert it back to AdobeRGB. Need it to be a CMYK image file you are sending with packaged files to you print vendor? Convert it back to CMYK. The ink limits were constrained when you went to RGB, but will not 're-expand' when converting back to CMYK. They will fall within the limits of the profiles used within Photoshop.
So why doesn't it matter if you switch back and forth? When you are designing for print it doesn't. RGB has a wider gamut than CMYK, so you can't get any 'narrower' in your color space than CMYK.
In the image above, the blue line shows the colors achievable in CMYK, and the yellow line the colors you can achieve in RGB. Images may appear brighter on screen (also you have light being projected at you, with the darkest part of the screen emitting no light at all. But with CMYK, you will always end up with ink on paper. the brightest part of the image will be the white of the paper, and ink will make it darker.
I hope you've found this helpful, rather than more confusing.
