Steven Pemberton : Views and Feelings

September 2002

A Pixel is not a Point

Here is another irritating site that thinks a pixel is a point:

  p { font-family: Arial, Helvetica, sans-serif; font-size: 11px}

For a brief period at the beginning of the Macintosh era, a pixel was indeed a point. A point, a unit of typesetting, is 1/72nd of an inch. Fonts are specified in points, so a font of 72 points is one inch high.

The original Macintosh quite deliberately was designed with screens with a 'resolution' of 72 dpi. (Resolution, now there's a misued word. Too often it is used to mean the number of pixels on a screen. Adverts announce screens with "A resolution of 640x480". That would be fine if all screens had the same physical size, then resolution and number of pixels would be the same. But they don't. I have a screen with 1024x748 pixels, but a resolution of 125dpi.) The Macintosh decision was deliberately chosen to make an exact match between pixels and points. Unfortunately, it lead to a generation of people who thought they were the same thing. On my machine, with a resolution of 125 dpi, a 72 pixel font will not be one inch high. It will be 72/125 inches high, or 0.576 inches high. So a 72 pixel font on my machine is 72 x (72/125) = 41 points high.

And the font sizes specified as 11px on the above website are (11 x 72/125) points. That is, just over 6 points. Put another way: unreadable.

If Moore's Law works for LCD screens as well, and they're made of transistors, so why not?, we can expect the dpi of screens to increase by sqrt(2) every 18 months or so (at constant price). My next machine might be 175dpi, and the one after that 245dpi. At present it looks like Moore's law is being used to reduce the price of screens rather than increase their dpi, but I anticipate the dpi growing again soon. I've already seen 200dpi screens on sale for about $3000.

Sooner or later it will get home to web designers not to design in pixels anymore, because each page will fit on a postage stamp if they do. But at what point will they realise they shouldn't be designing their sites with fixed sizes at all, pixels or points, but using percentages, and letting the user decide on the base size, so that users can increase the base size to make the fonts larger and therefore more readable? Probably when their own sight starts to fade, around retirement age I suppose. Pity.

First published in ACM/Interactions, September 2002

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