Why downsample to 300 ppi only for images above 450 ppi - why not for all images above 300 ppi?

Question

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):

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.

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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"?

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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).

Answer 1

Downsampling does not come without a cost.

Badly and inappropriately done downsampling can ruin your print. To be successful there needs to be enough extra data to rebuild the signal in the new size without much problems. So if you have only a little more data like say 5-10% its like just applying a blur on your image. Even less data leads to unpredictable results, you might be fine or not.

300 DPI is also more of a guideline than a actual rule, it is perfectly normal for a seasoned print designer to go over or under 300 dpi. In fact 300 DPI is a bit of a answer that you give to people when you don't want to explain too thoroughly.

As a result you don't really want automatic processes to kick in just because you exceeded the limit a bit. Also when the image is sufficiently oversized then you can expect downsampling to succeed reliably.

PS: Entirely, another qestion is whether you ever want it to kick in or not in fact my pdf exports never downsample anything.

Answer 2

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