Originally posted by Lowell Goudge In terms of DPI and this has come up many times, for some reason pentax elects to relate the physical image size in inches to 72 DPI.
Someone recently pointed me to the actual EXIF specification, and it's actually suggested/mandated right there that 72dpi be used.
Anyhow, here's the deal as simply as I can put it: the number stored in that field is *meaningless*. It has no relevance whatsoever; it's just there because there is place for a number that is relevant for a *scanner*, not a camera, but the camera has to put a number there, and the EXIF spec suggests/mandates it be 72. Could just as well be 7, 42, 666, or 2012 for all the difference it makes.
That's because the *actual* dpi of your image has nothing to do with the number in that field. it's the product (ok, literally, quotient) of two things and two things only: the number of *dots* in your image, and the size of your image in *inches*. That is, it is quite literally *dots per inch*. Your images don't *have* a resolution in dpi until you actually print them, any more than your car has has a speed in mph until you actually drive it. Your image might be 300dpi, 72dpi, 100dpi, or 20dpi depending on how big you print it, just as your car might go 72mph, 100mph, or 20mph depending on how fast you drive it (good luck on the 300 there, though :-).
So if you want a print of at least 300dpi, then find out how many dots (pixels) wide your image is, divide that by 300, and that is the size you can print at in order to get 300dpi. The old 6MP cameras made this easy: their images were 2000 dots tall and 3000 dots wide. 3000 / 300 = 10 . Meaning a 10 inch wide print would be exactly 300 dpi, because a 10 inch print would be 3000 dot / 10 inches = 300 dots per inch. Simple as that.
Your camera probably has more pixels/dots than that (check the specs; or the images themselves - the actual pixel dimensions are usually displayed by most photo viewers. Meaning you can print big than 10 inches wide and still have 300 dots per inch.
So if theyy are asking you to submit an image that will be 300dpi when printed, you need to know how big they plan to print it in inches. Multiply those numbers by 300, and you'll know how big your image needs to be in pixels. But that number in the EXIF field won't have anything to do with any of this.