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     áòèé÷ :: Filmscanners
Filmscanners mailing list archive (filmscanners@halftone.co.uk)

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filmscanners: 72 dpi



>There is a very good explanation of this at
>http://www.scantips.com/basics1a.html - 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... 




 




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