dpi vs file size

I felt the urge to post this after a way-too-long discussion with someone else about dpi, images, and file size. It seems there was some sort of a misunderstanding, and in retrospect I'm going to guess he was thinking about scanning an image, and I was thinking about saving an image again that had already been scanned.

It was started while talking about an image on someone's iphone. He didn't like the detail. He said that maybe he ought to save it with a different dpi, figuring that he had probably saved it with something like 72 dpi, after having had scanned it with 300. I said saving a 300 dpi image with a different dpi ought not make a difference. I was told that wasn't really true, to which I said changing the dpi will only change the detail of the image if the program you're using "preserves the inches" while you're changing the dpi. Somehow, he disagreed with that. In retrospect, I'm going to guess (hope) that we were simply on different pages. Maybe he was talking about scanning at a different dpi (I was talking about saving an image under a different dpi than the one at which it was loaded). My point was that dpi simply doesn't change the file size (or detail for that matter - the lone exception being if the program in changing the dpi alters the "pixels" while preserving the dimension in inches - which I have seen an application do before). While explaining that, I was told inches were meaningless.

Anyhow, to set the record straight, I still feel that dpi is meaningless unless you're dealing with something that will eventually end up on the printed page. Anything that displays images on the screen is going to display at whatever the screen dpi happens to be, not at whatever the image says it's supposed to be. The discussion hinged on an image that was only going to be viewed on an iphone and zoomed as needed for detail (does the iphone even care about dpi??). Why anyone would scan such an image in and then change the dpi is beyond me. It won't change the file size unless you're dealing with an application that is print-oriented and opts to preserve the dimensions in inches by altering the pixel dimensions. And if that's what you want, you're better off simply resizing the image. If I'm wrong, let me know - if not by commenting here, then by email, or you can even contact me a different way if you wish.