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At 4 inches, the maximum the human eye needs is 2190 ppi/dpi.

The link seems to confirm the 400+ range is getting useless fast for anyone who isn't a baby holding the screen to their face.

The link provided by avree says, "...But the legally accepted norm of 20/20 vision only asks for 876 ppi/dpi at 4 inches!"

A baby's vision is around 20/800 [1] (extremely poor).

Thanks, that link says 300 ppi (magazines) and 720 ppi (fine arts prints). Very informative!