[timr]

A human eye with 20/20 vision can distinguish ~150ppi at 2 feet (a typical screen-to-eyeball distance). Beyond that, it's overkill. The apple thunderbolt display is at 109ppi, so there's a bit of room for improvement, but not 60% improvement.

Beyond this limit, I suppose there are people with 20/1 vision or something, but they're pretty rare.

[tbrownaw]

That is, you can distinguish lines that are about 1/150 inch apart.

But for a display, you want to be able to have diagonal (or curved) lines at 1/150 inch apart, with pixellation artifacts that are small relative to those lines.

That probably translates to something like 450ppi (line width of 3px, so the artifacts should probably be about 1/3 of the line size).

[rayiner]

Humans can distinguish a lot more than that. See: http://clarkvision.com/imagedetail/eye-resolution.html.

His calculation gives 530 ppi at 20 inches.

Explanation: "The eye is not a single frame snapshot camera. It is more like a video stream. The eye moves rapidly in small angular amounts and continually updates the image in one's brain to "paint" the detail. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina."

[introspectif]

I've never gotten this argument. I mean, I have totally normal (nearly 20/20, but not quite... thanks to aging) vision, yet I can see the blurred edges of diagonal and rounded images perfectly fine on an iphone 5s at 2 feet. I think people just aren't really looking. I tried a thunderbolt display recently, and it was painfully low resolution. I haven't seen the math recently, but I have hard time believing that any phone today, let alone any monitor or television, really reaches the resolution of print.

[barrkel]

I have my monitors mounted on arms, so they can be anywhere from 1.5 meters away with me leaning back, to almost right in my face. The ability to physically "zoom" the whole monitor adds another dimension to use, and it's one where you really appreciate higher pixel density.

Aliasing is another big deal. Jaggies are very visible until you get into quite high pixel densities.