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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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3 Answers 3

up vote 7 down vote accepted

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
1  
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

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

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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