The human eye CAN tell the difference. What it can't do it distinguish individual pixels, just like you normally can't see individual frames when a movie or game is faster than 24fps. If your eye-sight was so poor that you coun't see better than 300dpi at one meter, you would not be allowed to get a drivers license in most countries. Road signs are designed to be read by the minimal allowed vision at a certain distance, that means you must be able to read half a meter high letters at 1km, which requires 1/5cm resolution, which is the same as 600dpi at 1m, and that is minimum.