Set various image resolutions

Actually, they're different settings for reducing the size of overly large images. And only one set applies to monochrome images.

Since InDesign allows users to "scale" pixel images, it can create innate problems in outputting files. That's because InDesign really can't scale files, per se. Resolutions are locked into the pixel images upon creation, and reducing the amount of space — in height and width terms — artificially increases the resolution. So reducing, say, a 400 pixel per inch image to 10% of its original space within InDesign creates an effective ppi of 4000 pixels per inch. That's enough to crash most any commercial imagesetter.

So InDesign allows users to reduce those resolutions when creating PDF output files to reproduce the files easily.

What resolutions should you set? That depends on the final use of the PDF file. The generally-accepted rule for adjusting color and grayscale images is two times linescreen, expressed as lpi, for your final use. So if you're producing newspaper ads to reproduce at 85 lpi, you'd want to reduce the color and grayscale resolutions to 170 ppi. If you're producing fancy corporate annual reports at 175 lpi, you'd want to reduce the resolution for grayscale and color images to 350 ppi.

As for the settings you requested for monochrome — Honest to God black output dots or not, with no shades of grey in-between — you'd want to set it to the maximum resolution of your final output device. So if you're outputting your film/plates to print that fancy report on a 2400 dpi image or platesetter, that's the setting you'd want to reduce monochrome images to 2400 ppi. Though as a practical matter, anything finer that 1000 to 1200 ppi is effectively invisible to the naked eye.

Hope this helps,

Randy