: Acrobat / Illustrator: Grayscale images lose contrast on import The original image I imported into Gimp, then converted it to grayscale by switching to grayscale mode. I exported it as Lenna-gray.jpg.
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:
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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