DPI, PPI, pixels and size
It’s surprising how often I see a call-out, a request, for a digital image something like “2000 x 3000 @ 72 dpi”.
That’s actually nonsense.
A digital image’s size is its size, period. “2000 x 3000 @ 72 dpi” or “2000 x 3000 @ 300 dpi” is not going to change the size of the file. The file is 2000 x 3000.
What it does do is say “If I print this at 72 dpi, the resulting print will be 27.7″ x 41.6″, but if I print it at 300 dpi, the print will be 6.6″ x 10″.
It would be very unusual for a person making a print to ask you to embed the dpi in a file, since that’s something they will set themselves.
However, it does make sense to use “dpi” if the size specification is in inches instead of pixels (as in dots per inch): “send me an image that’s 5″ x 7″ @ 300 dpi.” That file will be 1500 x 2100 pixels. (5 x 300 = 1500; 7 * 300 = 2100). Equally, if someone asks you to send them an image specified as just “5 x 7” – then you have no idea at all what to send (unless perhaps you send them an actual 5×7 print instead of a digital file.)
Digital files are specified by either just pixel dimensions OR by inches and dpi. Mixing one with the other makes no sense.
And finally, the proverbial “72 dpi” – which is probably wrong on a couple of counts, unless you’re printing a huge image, meant to be viewed from a great distance. It would never work for a printed photograph.
The person specifying “72 dpi” is probably thinking of a computer monitor screen, but as far as I know, no computer monitor has been made at 72 ppi (notice: PIXELS per Inch, not DOTS per inch) in well over a decade, perhaps 2 decades. Today’s monitors pretty much begin at 94 ppi and go up from there. And even then, specifying a ppi for a monitor is meaningless, since the pixels per inch of a monitor is fixed, and cannot change (whereas one can print at any chosen dpi.)
PPI (Pixels per inch) is used for displays, such as monitors and TVs; dots per inch (DPI) is used in the printing process. (It is usually true, with rare exceptions, that 1 ppi = 1 dpi.)