[REQUEST] 72 DPI vs 300 DPI, and Exporting at 100% Quality

[Appletax]

I take pics with my iPhone 14 Pro. I have been using Luminar Neo and want to also try Lightroom to make simple photo improvements. The pictures are set by Apple to 72 DPI. I learned that the DPI is irrelevant unless you're printing on a large medium - it's irrelevant when viewing it on a screen - then only pixels matter. When I export my photos, should I set them to 300 DPI so that they can work great on any device or paper/medium? I have never printed anything larger than an 8 x 10, so perhaps I should just stick with 72 DPI and I could change it to 300 DPI in the unlikely event that I would need to.

Also, when I export a photo as JPEG 100% quality, why does that image take up multiple times more space than the original? You'd think it would be around the same size as the original photo. It's as if the edited photo is even higher quality than the original. I get similar file sizes when I choose 90% quality, which I'm guessing is worse quality than the original.

 

[Markverhyden]

JPEG reduction basically has nothing to do with DPI. The reduction represents consolidation/discarding of data being used to compose the picture. Each dot in the DPI uses the same data fields but reduced number of dots when the DPI is reduced.

Back when all this stuff started being used I seem to remember that the average human eye, which is not 20/20, generally can't differentiate changes in DPI above 300 or so. It's also useful to point out the fact that most of the time a photograph is used in it's entirety which means it's being looked at 2"+ feet away.

 

[Philippe]

I learned that the DPI is irrelevant unless you're printing on a large medium

Useful if you want to zoom in. For printing, think about printers' resolution: 300+ dpi.
You can calculate the dpi (dot or pixel per inch) for your screen. The number of pixels is ResY x ResX, and the surface is length x height.
So DPI = Square root of (number of pixels / surface of the screen)

and I could change it to 300 DPI

Nope. Once they're gone, they don't come back.

Also, when I export a photo as JPEG 100% quality, why does that image take up multiple times more space than the original?

Maybe the originals are not at 100% quality (i.e. 90% jpeg), are using a more efficient compression algorithm or/and the processing of the picture is done with very high quality, so the resulting picture has more information / quality.