How to export grayscale image as grayscale in PDF?

[feklee]

In acrobat, converting it to grayscale yet again is another processing.

I’m fully aware of this, and that’s why I intend to replace the image in the PDF with the original grayscale image from the scan. Of course, the scan has already been converted from RGB as the scanner is an RGB device – thanks for pointing that out. Also I stretched the histogram to give maximum contrast.

When attempting to replace the image, I ran into something that I didn’t understand:

Just playing with the test version of Acrobat. It's relatively simple to replace an image with its full-resolution grayscale version. Only, then there is loss of contrast.

Thanks to shettler’s answer on Graphic Design Stack Exchange, I now understand what is happening. The RGB image is separated into all four CMYK channels. In fact this can be verified using Output Preview in Acrobat’s Print Production tool. The result is a rich black but possibly with a color shift, which in fact is visible in the test print that we ordered. The grayscale image is mapped to the K channel, avoiding the color shift and – I can imagine – rendering to nicer gradients in print.