Oh for heaven's sake,<p>[locate individual high contrast pixel] != [ability to distinguish individual pixels]<p>Vision and visual acuity is not this simple. See, e.g., <a href="http://www.michaelbach.de/ot/lum_hyperacuity/" rel="nofollow">http://www.michaelbach.de/ot/lum_hyperacuity/</a><p>I don't actually know, but perhaps seeing something as pixelated requires being able to resolve the edges of an individual pixel.<p>A better test would be to see if people can detect a difference in the pixel "phase" (for lack of a better word) of some pixel-drawn object (that is, show identically shaped objects that have a different internal pixel organizations because of their position on the screen). Do an ABX-style test, see if you can tell the difference.<p>I'll bet you can tell the difference on a classic display. I'll bet you can't on the iPhone 4+.<p>[edit: made the comment slightly nicer ... :/]
This is not the right test. Retina doesn't mean a single pixel would disappear.<p>I think the real test would be to draw a curve of one pixel thickness and be able to see a smooth curve and not a pixelated one.
I love my retina displays, but Apple's marketing annoys me as I can still see pixellation at times. This is a quick game to see if you can dispute the marketing claim.<p>Looking forward to a real retina display soon :-)
This is why @2x is silly. The only way to stay ahead of the constant march of technology is with vector graphics. Otherwise, your @2x image is going to look silly on an @3x display (or zoomed in).
The "Retina" display is just a marketing term which means that a person with 20/20 vision cannot distinguish individual pixels <i>at a certain distance</i>. Movie screens do not need to have a high ppi ratio because you view the screen from a greater distance. If you have better than 20/20 vision, or press your face up against the screen, you will be able to resolve individual pixels quite easily.
While I appreciate that you're specifically referencing Retina claims and therefore you really do need a "Retina" display for it to matter, I'd suggest against actively locking out everybody else. It's generally an interesting question about a given display and one's personal eyesight even without the hook of testing a marketing claim.