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Digital Image Basics for non-Photographers • Blaise Tobia
When looking at images
on a website, or looking at images directly in an application like Photoshop or Preview,
the only significant dimensions are pixel dimensions.
Print - or document - dimensions have no effect for these means of display.
The images will display at R pixels per inch of screen, where R is the display resolution of the screen.
Traditionally, R=72, although in practice R has been getting higher and higher.
On high resolution mini displays, R can now exceed 300.