Name: Testing the limits of human vision
Uploaded: 2018-06-20T08:38:49.000Z
Duration: 5 min 34 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

- [William] You have seen this eye chart before.

It's at the DMV, it's at the optometrist,

it's in doctor's offices all over the place.

You probably know that font as the eye chart font.

This thing is a dead simple tool for testing eyesight

and it's helped millions of people see better.

- But, as simple as it is, there's some very precise math

It doesn't just tell you if you need glasses,

it tells you something about the fundamental limits

- And, we wanted to put that math to the test

The eye chart we all know and love was created back

in 1862 by a Dutch ophthalmologist named Herman Snellen.

His early prototypes used abstract shapes,

And the designed hasn't changed much since then.

The chart is designed to measure one very particular aspect

It's basically the resolution of the eye.

Now here resolution isn't measured in pixels,

it's measured in degrees, like of an angle.

- [Rachel] Say you're looking at two lights shining

at you from a distance, you can tell them apart

because the light from each source is traveling

through your eye and hitting a different set

of light sensitive sells, called photoreceptors.

Especially because, as the light travels through the eye,

Eventually, the two lights will blend together

and your brain will start to interpret them as one.

That angle, between the two paths of light right

as you lose the ability to pick them apart,

of just 1/60 of a degree, also known as one arcminute.

so that's the black lines and white spaces

are all the same width, about a 16th of an inch.

If you stand 20 feet away and look at that little E,

If you can read most of the letters in that row

Meaning, at 20 feet, the patterns of white and black

are just at the limit of your eye's resolving power.

Now, you might be able to read a row or two beneath 20/20,

In fact, 20/20 was never defined as perfect or even average,

it's just a baseline to measure everything against.

you'd have to walk forward five feet to see it clearly.

Partly, because we have similar densities

Compare that with other animals like eagles,

which have a very different optical setup than we do,

you can use it to calculate your own acuity

Based on the Snellen chart, Rachel's acuity

her maximum resolution just under one arcminute.

of this window right here, and we're gonna

go out there and show you some very large E's,

and all you have to do is tell us if you can identify the E.

than one arcminute, she should comfortable see an E

and you tell me if it's facing the right way,

Subtitles ListPlay Video

Testing the limits of human vision

eventually

ridiculous

aspect

slightly