Human visual acuity of 20/20 is being able to discern 1 arcmin in bright light. With trig, you get that, if you're holding the screen a foot away, that amounts to 0.000349 inches, which is 1432 dpi. If you're holding the device 2 ft away, it's 716 dpi. And against your face at 6 inches, you get 2865 dpi.
Could I impose for a citation for this? A quick check indicates that in the industrialized world, between 50% and 70% of individuals require corrective lenses.[1][2] That seems to indicate that the _average_ human does NOT have 20/20. It's possible in developing countries less than half the population does not need vision correction, but I'm not terribly convinced.
[1] National Centre for Social Research and University College London. Department of Epidemiology and Public Health, Health Survey for England, 2001. 2nd Edition. Colchester, Essex: UK Data Archive, June 2004. SN: 4628.
[2] Global Eyeglasses Market (2005). Eyeglasses MCP-2532: A global strategic business report. Global Industry Analyst, April 2008.
> “Normal” visual acuity for healthy eyes is one or two lines better than 20/20. In population samples the average acuity does not drop to the 20/20 level until age 60 or 70.
Thank you. I think I see where you're coming from.
That article has some interesting phrasing which I think is causing confusion. (For me at least.) It looks like they're using 'normal' to mean 'average in the range' some of the time and 'mean population visual acuity' other times. Mostly they're using the former. So 'normal vision' or 'average eyesight' refers to the range of 20/20 eyesight and not 'mean eyesight'. That doesn't explain that second sentence, though, which seems to go back to 'mean human visual acuity'.
At the very least, they could stand to revise the writing in that sentence. If they did mean to imply that the average person has 20/20 vision, I think they're going to have to provide a source. I did slightly more digging and I couldn't find any journal publications that list less than 60% of adults as having corrected vision. Either the remaining 40 have strongly superhuman vision or the number is off. I think there is at least one blind person (legally worse than 20/200) for every person with super human (20/10, the physical limit for humans) vision.
A simple numerical test based on that last point seems to illustrate the unlikeliness of a 20/20 average. To state again, 20/10 is the physical limit for human vision. Any higher and it's not physically possible to pack more photoreceptors into the retina or focus the pupil tighter. Bad (legally blind) vision is in the realm of 20/200. The National Federation for the Blind[1] shows there are roughly 6,700,000 blind Americans of all age groups. We will assume that they're exactly at the legal bound for blindness and not worse. To find how many superhumans we need to make the average 20/20, I think we can use (6700000200 + x10)/2 < 20 We need x to be less then -133999996. So it's simply impossible for the average vision to be 20/20.
It's still possible that number is correct and that the average American has 20/20, but at this point the majority of the scientific publications and formal surveys do not support the idea. If you can find additional sources (I'm so sorry to keep asking), please let me know.
No, the thing you are missing is that corrective lenses are a thing that exists. We're really quite good at taking someone with 20/100 vision and turning that into 20/20 or better.
Oh! Oooh! I see what you're saying. I thought you were asserting the mean _UNCORRECTED_ vision of an American Adult was 20/20. It's certainly much more plausible that the corrected vision is beyond 20/20.
I would recommend, for the sake of clarity, adding '20/20 corrected' or '20/20 rectified' to your original post. If I hear someone say, "I have 20/20 vision," my first thought is, "this person has perfect vision", not, "this person wears glasses."
I'll go back and add that clarification on my replies.
EDIT: It's too late to go back and edit my topmost comment. :(
Most of those people (IME) need the lenses to correct for short-sightedness, which if anything would make them better rather than worse at discerning phone pixels.
