>There is a very good explanation of this at
> - Wayne Fulton makes the point that
>not all screens display at 72dpi, and especially not those on this list.
In addition to Wayne Fulton's excellent material here's a little more on the 72
dpi heritage, from a note posted to scan-leben list last fall:
----------------------
>I just read a bunch of posts from back in April '00 about the claimed
>72 dpi resolution of Macintosh monitors, and think I can throw some
>light on the subject for any who still care (Note: I'm going to be
>sloppy about my use of the terms "dots" and "pixels", but I think
>you'll know what I mean).
>
>(1) The original Macintosh (with it's dinky B/W screen) was designed
>(and adjusted at the factory) to display dots at a pitch of 72 dpi.
>
>(2) The original Macintosh printer was a dot matrix printer which
>printed at a pitch of 144 dpi (or twice 72 dpi).
>
>(3) Thus the original combination of Macintosh monitor and printer
>were measurably WYSIWYG (what you see is what you get), meaning that
>a straight line comprised of 72 dots (or pixels) would measure 1"
>long both on the screen and on the printed page. (Note: there were
>cases where the printer software would use the extra resolution of
>the printer to smooth fonts, and curved or diagonal lines, so they
>wouldn't have such jagged edges.)
>
>(4) 72 dpi was chosen as a happy convergence of three things: (a)
>Fonts are sized in "points" of approximately 1/72 of an inch, (b) At
>normal viewing distances (approx 20"?) dots on the screen spaced at
>approximately 80 dpi visually fuse together, and (c) The resolution
>of the dot-matrix printer mechanism Apple used was exactly twice 72
>dpi, making it easy to do WYSIWYG printing.
>
>(5) Most image file formats have internal fields for specifying the
>pixel pitch of the image. This is true for JPEG, Windows BMP, TIFF,
>Macintosh PICT, PNG, and others. I'm not sure about GIF.
>
>(6) Initially all Macintosh image files where created at, and
>internally specified as, 72 dpi. But over time laser printers
>(initially 300 dpi), color editing programs, and high-resolution
>monitors were introduced, yielding more and more cases where images,
>printers and monitors were no longer 72 dpi. Even more variation was
>introduced with multi-scan monitors, and video cards that could drive
>them with a wide variety of resolutions: the more dots you put on a
>screen of a given size, the more closely the dots are spaced, and the
>smaller (as measured by a ruler) a so-called 72 dpi image becomes.
>
>(7) The legacy of 72 dpi lives on in the Mac in these two ways: (a)
>Any image file that does not explicitly specify its pixel pitch is
>assumed to have a pitch of 72 dpi. (b) A 72 dpi image is, by
>default, displayed such that each pixel in the image file is
>displayed as one pixel on the screen (unless a program overrides this
>default and scales the image in some way). Thus the Mac, by default,
>BEHAVES as if monitors display dots at 72 dpi.
>
>(8) If you were to display a line 72 pixels long, and then measured
>it with a ruler, it's length would be 1" on a 72 dpi screen. But in
>fact most people are running their monitors with more closely spaced
>dots: say 80 to 100 pixels per inch, so the Mac is no longer
>MEASURABLY WYSIWYG. However It's rare for people to use rulers to
>measure the size of things on their screens, so I don't think there's
>any practical consequence to this "inaccuracy".
>
>(9) Unlike monitors, most printers will scale images when printing
>such that when you print an image at 100% it will be approximately
>the size you would expect (as measured with a ruler), and will use
>more or fewer dots as needed to achieve this.
>
>(10) By the way, Windows PCs use 96 dpi as their default for the dot
>pitch of images and monitors. I don't know why. But one of the
>results of Mac assuming 72 dpi and Windows assuming 96 dpi is that
>websites created by Windows users usually display on Mac screens with
>tiny text...but I digress.
>
>Well, I hope that some of you find this interesting...
