- [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.
And, you definitely know that huge E.
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
hidden away in those block letters.
It doesn't just tell you if you need glasses,
it tells you something about the fundamental limits
of human vision.
- And, we wanted to put that math to the test
on a slightly ridiculous scale.
We've got the Coast Guard coming by.
Well, let's back up.
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,
but he eventually settled on letters.
And the designed hasn't changed much since then.
The chart is designed to measure one very particular aspect
of our eyesight, visual acuity,
or our ability to see very fine detail.
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.
Which is weird, but here's why.
- [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.
If you move the lights closer together,
they'll start hitting photoreceptors
that are closer to each other.
Especially because, as the light travels through the eye,
it spreads out a little bit.
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,
that's the resolution limit of your eye.
For healthy adults, it's on the scale
of just 1/60 of a degree, also known as one arcminute.
- Snellen's eye chart is designed
with this resolution limit in mind.
Take that little E in the middle.
It's built on a five by five grid,
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,
each little black bar or space,
is exactly one arcminute thick.
If you can read most of the letters in that row
at 20 feet away, you have 20/20 vision.
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,
most adults actually can.
In fact, 20/20 was never defined as perfect or even average,
it's just a baseline to measure everything against.
The row beneath that is the 20/15 line.
That means if you have 20/20 acuity,
you'd have to walk forward five feet to see it clearly.
- All in all, with corrective lenses,
there isn't actually a huge range
in acuity from one person to another.
Partly, because we have similar densities
of photoreceptors in our eyes.
It's like we're all taking photos
with the same camera sensor.
Compare that with other animals like eagles,
which have a very different optical setup than we do,
including many more photoreceptors.
Their eyes are camera sensors optimized
for spotting rabbits from midair.
- Whatever your resolution limit is,
you can use it to calculate your own acuity
at different distances.
Based on the Snellen chart, Rachel's acuity
is a bit better than 20/20, which makes
her maximum resolution just under one arcminute.
So, let's have a little fun with that.
All we're gonna do is plant you in front
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.
We'll start small, that pier
is about 1500 feet away from us.
Since Rachel's acuity is a little better
than one arcminute, she should comfortable see an E
if it's about 25 inches across.
Hello?
- [Rachel] Hi.
- Hi, I'm going to show you an E,
and you tell me if it's facing the right way,
if it's backwards.
- Okay.
- [William] Okay, ready?
Which way is the E facing?
- The open part of the E is facing towards the sky.
- [William] Perfect.
Now for the big finish.
Treasure Island out there in the bay,
is about two miles away, so to pull off the same eye exam,
we're gonna need a much bigger E, like 15 feet tall.
Here we go, alright, gently, gently.
Oh wind's catching it.
Did you see it?
- I see an E, and it's pointed to the right.
- [William] Tentative pointed to the right.
That is correct.
- (laughs) The last thing to mention here
is that acuity is important but it's not the be all end all,
your ability to sense movement, color, contrast,
depth, all of that factors into successful eyesight.
So acuity is just one tool of many
to help you make sense of the world.
- Now we've got the Coast Guard coming by.
We are happy to be here testing the Coast Guard
on their visual acuity.
Okay, I think we're done.
Hey everyone, thank you so much for watching.
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