Artemy Lebedev

§ 69. Image resolution

June 16, 2001

A bit of drilling for today. We are going to debunk one of the greatest human fallacies in the entire history of screen graphics. I’m talking about the resolution of computer images.

The point is a computer image does not have resolution at all. Resolution (or more precisely resolving power) can only be a property of an input/output device—a monitor, a printer, a scanner or a photo camera.

Scientists measure the resolving power as a number of distinguishable parallel lines or pixels per millimeter. An image cannot have any resolution, since it is only through a display device that it can be viewed. As far as a monitor or a printer are concerned, it makes sense to talk about counting the number of lines per millimeter or pixels per inch.

The number of pixels per inch indicated in the image size dialogue window is small potatoes for an image in Photoshop. This parameter doesn’t affect the way the image is displayed on screen, because the image is this many pixels wide and that many pixels high.

Many authors of manuals and books on website building either suggest using 72 or 96 pixels per inch, or sidetrack the issue. Using Photoshop’s “save for Web” option, you will get all your images invariably saved with a resolution of 72 pixels per inch. You can only get around this by using other tools for saving files (for example, “Save as GIF”), but nothing will come of it after all, since the resolution embedded in the GIF or JPEG formats will be ignored both by the browser and the monitor.

The mysterious 72 pixels per inch date back to the time when the WYSIWYG phrase (what you see [on screen] is what you get [in print]) made sense, i.e. 72 successive pixels on a Macintosh display were printed as a 1-inch long line. At the dawn of desktop publishing revolutions a pixel was defined as exactly equal to a typographical point which never was 1/72 of an inch, but pretty close to that value (this proximity varies in different countries, though). This ratio is also a basis for the PostScript language that governs the vast majority of printers worldwide along with all Adobe software.

See also: § 81. The life and extraordinary adventures of a typographical point

If you indicate a resolution of 1 instead of 72 pixels per inch in a GIF file, nothing extraordinary will happen. The image will be just as tall and wide in pixels as before. If you decide to boost the resolution, some programs will respectively try to diminish the image size on a print-out along with making up for the discrepancy between the size of a pixel and the point of a printer.

Obiter dictum

The size of a pixel is like the size of an angel: the exact data is missing.

But if you still try to print only one pixel, different programs will yield different results. One pixel printed on paper is approximately 0.35 mm on average.

For a monitor, resolution doesn’t play a part. The browser doesn’t interpret this figure at all (although different browsers have diverse ingrained notions of how many millimeters a pixel should be in the print-out). Photoshop uses resolution for purposes it was intended for—namely, as a logical value—in printing only.

For a more detailed account on the origin of 72 and 96 pixels and screen resolution, see the next section.

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