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I'm quite confused by the nomenclature; what does DPI really mean?

The way I interpret it, it says how many pixels there are per inch of the display. Now naturally, you can't change this (not on LCDs, anyway). But operating systems have a setting to "change" the DPI... but what does that mean?

If I "change" my DPI from 96 to 110, shouldn't that make everything smaller, because I'm supposedly fitting more pixels to every inch on the display (by definition)? Why does everything get bigger instead?

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Text size is normally expressed in points (of which there is a fixed value of 72 per inch on digital displays), not pixels. Therefore 12-point text will take 12/72*96=16 pixels on a 96 DPI display, but 12/72*110=18.3 pixels on a 110 display. Unless your display is capable of changing its native resolution on the fly (which would be a very neat trick to see), your text will appear 110/96-1=14.6% larger.

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Wait, so "dot" == "pixel" or "dot" == "point"? – Mehrdad Apr 23 '11 at 21:35
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On a digital display, a "dot" is a pixel. The "point" is an abstract measurement of text brought over from the printing press. en.wikipedia.org/wiki/Point_%28typography%29 – Ignacio Vazquez-Abrams Apr 23 '11 at 21:35
But if this is all about text size, then why do graphics also get bigger? Is it just to maintain flow with the text? – Mehrdad Apr 23 '11 at 21:36
That is correct. – Ignacio Vazquez-Abrams Apr 23 '11 at 21:38
Ah, okay, thanks for the explanation. :) +1 – Mehrdad Apr 23 '11 at 21:39
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up vote 2 down vote

What you change isn't the physical number of dots per inch on the display, it changes the number of dots your system believes are in an inch. So for a 12 point font, that's 1/6 of an inch, so it's 1/6 of however many pixels your DPI number is.

The same is done with images in some contexts which have a 'native' DPI [or are assumed to be 96 dpi] which is scaled to your system's DPI.

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up vote 2 down vote

Dots per inch comes from printing the image out onto paper.

So an image that is 800x600 pixels at 100 dots per inch will print at 8 inches x 6 inches.

The display uses this as a basis for determining the size of text on the display (as per Ignacio's answer).

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+1 thanks for the info. :) – Mehrdad Apr 23 '11 at 21:39
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