Whisky-dave
Do you happen to know whether or not this preview file is a copy of the original file but 'rendered' at 72DPI rather than the final copy which is mostly likely to be 300+ DPI for printing ? A little like doing a preview of soemthing you want to scan.

Sandman
DPI is irrelevant in this context.

Whisky-dave
So what is the point of a preview file, you've not explained that.

And would this preview file be differnt (differnt DPI) if you were using a retina or 4K/8K screen.

Sandman
You don't know how DPI works. The *screen* is of a certain DPI, not the image. Everything shown on a screen is shown in a specific DPI. This has nothing to do with the preview file.

Whisky-dave
Above you've said nothing .

So what purpose does the preview file have, or why create a preview file ?

Sandman
For an image file, "dpi" is just a value that has nothing to do with the actual image data.

Whisky-dave
again saying very little, or is it that you don't understand ? Are you saying that if you pixilate or use filers on a 1Mb image and a 1Gb image the time to process this will be the same.