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The original image I imported into Gimp, then converted it to grayscale by switching to grayscale mode. I exported it as Lenna-gray.jpg. After switching back to RGB mode, I exported again, as Lenna-rgb.jpg. When viewed in the Windows file browser, as expected, both JPEG images look identical.

In Acrobat Pro DC, I imported the images into a PDF document. Now, they look different. The grayscale JPEG has less contrast:

Screenshot of Acrobat showing the difference in images

Why the difference in contrast?

In Illustrator there's the same issue: The grayscale image imported using File / Place… shows loss of contrast, whereas with the identical RGB image, contrast is as in the original file.

I am preparing a document for digital print (laser).

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It's because they're different colour spaces converted into CMYK. The greyscale image only has info in the K channel (black). But the RGB image has info in all of the Red, Blue, and Green channels. When you place the RGB into a print-formatted PDF, it's converted into CMYK, and info goes into all four CMYK channels, giving you get a rich black. Your preview image looks like it's showing a rich black at left, and pure black at right.

If you print the RGB one on a press, it will be a challenging task for the printers to ensure the greyscale looks pure grey. Because there's cyan, magenta, and yellow ink going down on the page, too, you may get a slight (but noticeable) colour shift.

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  • Indeed that color shift – visible in a test print – is what I want to get rid of. Just checked individual channels using Output Preview in Acrobat’s Print Production tool, and they match your explanation. For the RGB image, there is information in all four CMYK channels. For the grayscale image, there is information only in the K channel.
    – feklee
    Commented Mar 16, 2016 at 0:34
  • @feklee Forget about RGB for anything print. It's Grayscale or CMYK for best results. Make sure your color profile match too. There's also "simulation profile" in the preview tool in Acrobat Pro.
    – go-junta
    Commented Mar 16, 2016 at 6:30
  • @go-junta For completeness sake, there are RGB print processes, primarily for photos.
    – feklee
    Commented Mar 16, 2016 at 9:29
  • so how do i convert that pure black to rich black?
    – Saravanabalagi Ramachandran
    Commented Oct 17, 2016 at 8:35
  • @ZekeDran There are a handful of ways you could do it. The most straightforward would be to turn the saturation down to zero. But you can also control how different colours get converted to black and white through other means. Gimp has a tutorial on how to convert to black and white. That's just the tip of the iceberg though. BW conversion is something you can definitely geek out over.
    – irodoramatic
    Commented Oct 21, 2016 at 5:11

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