When I tell you that I once spent a day trying to figure out how to change the NETBIOS hostname on a Macintosh, you know what I mean when I say understanding dots per inch (DPI) is best if you don't think about it. By the way, the hostname is in the network configuration for Macintosh... which makes perfect sense if you discount the possibility of a non-networked computer. And that little bit of knowledge may be the last time in this post that I state anything useful, the rest may be wandering thoughts about pixels... you are warned.
I have had the discussion about DPI many times with different professionals in different professions. None of them appear have the slightest clue about any resolution other than what they are used to. Print folks always want to see 300 DPI. Don't ask me why. The personal laser printer I bought in 1992 had the capability to output files 600 DPI, why would I want to produce files at half that resolution? Modern ink jet printers are capable of physical resolutions of 1200 DPI and they claim that through software to achieve resolutions twice that high. Yet in speaking to the fine art printer who is putting some images on canvas for me, their practice is to use their software (standard commercial stuff) to adjust the resolution and image to print always at 300 DPI.
And for the screen, we really don't care. Anything over 72~90 DPI is lost anyway. Thats how many dots per inch are on a computer screen. (For reference your large screen HD television is even lower resolution than this, but since the pixels are "moving" you tend not to notice.)
Because I am so often designing for the computer screen these higher resolutions and measurements that are relevant to hard copy output confuse me. In the web world, we use absolute measurements such as pixels. Everything is so many pixels wide by that many pixels high. The size it is on your computer if you put a ruler next to your screen is also effected by the size of your actual screen in inches, and the resolution you set your video card to.
Or a camera may be rated by the number of megapixels it gives. This number describes the overall number of pixels created by a camera, so if my camera can make images that are 2560 X 1920 pixels it is a 4,915,200 pixel camera, or roughly 5 megapixels. You got mega on disks, which is 1024K and mega on the wire that is 1000K and this marketing mega, ~983K ... this does not help my confusion, how about you?
All of this came up, because I use the GIMP to manipulate images. In order to retain all the data in a file but change the size it prints at, I have changed the resolution of a JPEG file from 72 DPI. I have changed the resolution to 225 DPI so the image prints at roughly 8.5 X 11, instead of the "natural" print size at 72 DPI (???) which is 35.556 X 26.667 inches.
As I look at JPEG files using my web browser, it is not clear to me that any program other than the GIMP thinks about the print resolution of JPEG files. And as I think about this a little more, since there have been no similar problems with TIFF files, this might mean that output resolution is a part of the file spec there, and that explains why browsers don't natively show TIFF.
My head hurts now. If you know about this stuff, feel free to help us out by posting a comment.