Here's something that's puzzelled me for a long time. Here's the default on InDesign's Export to PDF (and also Acrobat's PDF optimiser):

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Obviously 300 ppi is the default it downsizes to because 300 ppi common resolution for print. But what's the benefit in only downsampling images that are above 450 ppi?

Why not "downsample to 300 pixels per inch for images above 300 pixels per inch"? I don't see what benefit there is to the two figures being different.

If we're assuming that any detail beyond 300 ppi won't make any difference to the print, why not downsample every raster image that is 301 ppi or over? If, however, it expects the difference between 440ppi and 300 ppi to be significant, why does it downsample higher quality images to 300 ppi instead of 450 ppi?

What's the benefit in leaving a 440 ppi image at 440 ppi, but downsampling a 460 ppi image to 300 ppi?

Why are the two figures different? Is there any reason not to always switch this default to "downsample to 300 ppi for images above 300 ppi"?

All I can think is that maybe downsampling small amounts might lead to mistakes - but I'm sure I've heard it said back in the olden days that downsampling in steps of 10% at a time is a good way to preserve image quality (I think that's no longer true because these days the software will do that anyway if there's any benefit).


Downsampling re-processes the image, which isn't something you'd always want to happen automatically.

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