For those of you who don't know, I teach graphics and multimedia classes for a living (Flash, PhotoShop, etc.) For several years now, I've been explaining to students how to optimize graphics for the web, and inevitably the subject of dpi/ppi comes up. I usually tell students to save files (or resample files) to 72dpi, because dpi stands for dots-per-inch and there are 72 dots (pixels) per inch on computer monitors. (We've discussed the PC/MAC difference here before: PCs display at 96dpi -- actually many MAC monitors do too nowadays. Xara is a PC app, so its default resolution is 96.)

Trouble is, this isn't true. Monitors DON'T display 72 pixels per inch. Nor do they display 96 pixels per inch. In fact, the concept of an "inch" doesn't make much sense when talking about monitors. For instance, if you design a 72 pixel by 72 pixel graphic at 72dpi, this should display at 1 inch x 1 inch, right? You should be able to hold a ruler up to your monitor and measure it at 1 inch x 1 inch, right? Okay, try displaying it on one of the monitors in Times Square. It will probably measure 1 yard x 1 yard.


Pixels have no set size! Monitors have no set size. So what, exactly, does the 72 (or 96) dpi setting (in Xara, PhotoShop, etc.) mean?

As best as I can understand it, 72dpi means "if you have a 13-15inch monitor, every inch of this image will contain 72 pixels. If you have a larger monitor, every UNIT will contain 72 pixels, and it's up to you to figure out what a UNIT is."

Does anyone here have a REALLY good gut understanding of dpi (a.k.a. ppi)? Say I'm designing a graphic that's going to appear during the title sequence of a major motion picture (shown in the cinema). What does 72dpi mean to me?

I've searched the web for details, but almost every graphic site says "set resolution to 72dip, because monitors display at 72dpi."

Hope this doesn't seem wildly off-topic.

Marcus Geduld
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