so having 20/20 vision sounds like an achievement excellence perfection
clarity but it really doesn't mean any of those things in fact it's one of the
most easily confused and misunderstood aspects of all of eye and vision care
and in this video I'm gonna be explaining 20/20 vision and what it
really means let's take a look hello and welcome this is dr. Allen here from the
doctor I health show helped me learn all about the eyes and vision and I'm really
excited today to talk to you about 20/20 vision and what that actually means now
the last time you went in to have your eye exam a vision check of vision
screening a doctor a nurse or an ophthalmic technician likely started off
testing your vision by you looking at a board or some sort of a chart on the
wall about 20 feet away asking you to read as far down as you possibly could
and that chart was likely a Snellen chart and the Snellen chart was
originally developed in 1862 by an ophthalmologist named Herman Snellen now
it's certainly not the only chart that's out there it's maybe the most common but
we have a lot of different vision charts out there we have the ETDRS chart we
use for scientific studies we've got cool shapes and different symbols that
we use for little children and we even have something like the tumbling II
which you've ever seen this it's for people who can't otherwise tell us what
letters they're seeing or maybe they just can't read at all maybe they just
don't know the alphabet but instead they can just hold up their fingers and tell
us which way the e is pointing is that left is it right is it going up is it
going down there's a lot of creative ways to test people's vision but as
people read down this chart getting to the smaller and smaller lines you will
start to see one of the smaller lines being the 20/20 letters but did you
notice how I didn't say that the 20/20 line was the very bottom yes in fact
there are smaller letters that people can see and some people do see better
than 20/20 but when is the last time you and your vision checked were you seeing
20/20 better or worse let me know in the comments
20/20 is known as what is called a Snellen fraction which is the ability to
see or identify a letter or some image of a certain height at a certain
distance and in fact did you know that outside
the United States it's not even called 2020 it's called six over six do to the
metric system where six meters is roughly equivalent to 20 feet ultimately
the 20 over 20 or 6 over 6 describes the visual angle of an object or an image
that enters into the eye for example if you are looking at a tree in the far
distance and you walk toward that tree the tree starts to look larger and it's
not that the tree actually grew larger in front of you right away it's that the
tree actually just starts to take up a larger visual angle inside the eye and
more of your visual field and therefore it appears larger now the textbook
definition of 20/20 vision that I memorized when I was in optometry school
was that 20/20 vision is when a person was able to identify a spatial pattern
that subtends 5 minutes of Arc at 20 feet wait what
again 20/20 describes the visual angle and at a 20 foot distance the letter
creates an angle of 5 minutes of Arc and if you already know what 5 minutes of
Arc means then I am super impressed because when I first heard this in a
pump to school I had no idea what my professor was talking about basically
there's 360 degrees in a circle take one of those degrees divide it by 60 and
that gives you a minute of Arc and that's a very small angle and the 20/20
letters substan 5 minutes of Arc and all the letters on the 20/20 line are
actually designed with that in mind being that a letter e will have three
arms and each arm will contain one minute of Arc including the spaces
between those arms to give a total of five minutes of Arc but largely 20/20
vision is what is known as a standard or what is a common visual acuity that is
obtained by the human eye and the entire visual system under normal development
being that 20/20 is just what is common or known as the standard there are
people who have vision above that standard and below that standard in the
clinic we denote the acuity as 20 over another number such as somebody who sees
worse than 20/20 and they are seeing something like 20 over 40
in this case the 20 on the top of the fraction describes the testing distance
of you the person that we are measuring versus the lower number which is
describing the testing distance of a fictitious person with the 2020 standard
vision and in this case that person would be standing 40 feet away to just
barely identify the 40 the 20/40 line which is what you the person with 20/40
vision would be being able to read at 20 feet so let's do one more example if you
were only able to see the 2/200 letter that's the largest letter e at the top
of most Snellen charts then that means that you at 20 feet could only see that
large letter and that would be to note the 20 at the top of the fraction where
a person who has normal standard 20/20 vision would have to stand 200 feet away
10 times the distance away to just barely see that letter at the top of the
chart if you are somebody that can only read that 2/200 letter even with
corrective lenses then you would technically qualify to have legal
blindness according to the United States but there's actually a lot more to legal
blindness and that's something for a whole other video by the way legal
blindness does anyone else think that sounds funny if there's legal blindness
then what's illegal blindness let me know in the comments what you think
illegal blindness would be and as I said earlier some people can see better than
20/20 in fact a lot of people can in this case if someone was seeing 20/15 it
would be compared to somebody as mr. Jones 20/20 vision could see only if he
stepped forward 5 feet the important message of this video is not to get hung
up or anxious about seeing 20/20 every time you have your vision checked it's
really just a benchmark or a standard and there are people who have worse than
20/20 vision seeing 20/30 20/40 walking around and are very happy and content
with their vision they're still able to do all the activities they want to do in
their life and go where they need to go the times when it is super important to
be seeing 20/20 is whenever somebody is operating a motor vehicle going to
school or if the young children and that's because if
you're not seeing well and operating a more vehicle you could be putting
people's lives at danger it's also been found that poor school performance is
directly linked to poor visual acuity and reading skills and young children
who are not seeing very well despite them not complaining about it can
actually not develop the proper neural connections within their brain to ever
appreciate 20/20 and they could be permanently robbed of ever being able to
see 20/20 in their lifetime there's actually a medical term for that that's
called amblyopia and we'll cover that in another video so if you a child or
another family member have not had their vision checked in a long time and not
sure if you're seeing 20/20 call up your local eye care professional and schedule
an exam tell them doctor eye health sent you