The Mathematics of the Snellen Eyechart

In this video I attempt to explain the math(s) behind the Snellen Eyechart and why letters have to be a certain size.
You can also use the math explained in this video to calculate fun things like, how big would the letters be on an eyechart that's 5 km away or, what how far into the distance can a person be seen.

[Cubeux-rvjn]

Hi, cliff. I saw your video and I just started my journey getting rid of glasses. I am here to say HI.

[otiebrown]

Excellent review, Cliff.
I use it to measure my refractive state.
Now about +0.25 Diopters, with 20/20.

[priyanksharma]

This guy is so straightforward. I love it m especially his book

[rgd-xd]

Hi Cliff,

I hope everything is going well for you. I am writing this comment to know how are you doing now with your vision. Did you achieve 20/20?. I also noticed that yo have three books available on Amazon, one being the guide on vision improvement and the other two about the Internet and the future. Anyways, thank you for your videos and dedication on this matter that affects many of us. I hope one day I can achieve 20/20 myself.
Regards.

P. S: Excuse me if I have made a grammatical mistake as I am not a native English speaker.

[Ice-qyvf]

Hi Cliff,
Have you heard of Mark Warren from Myopia is Mental? What do you think about his method?

[endmyopia]

Nice one, Cliff. You're way better at me about getting into the math fun. ;) Christina had posted your video in our forum (Le Meow - finally off Facebook), and I reposted it in the Endmyopia Facebook group as well.

[otiebrown]

A Good presentation, Cliff.
The size of the 20/20 letter is 0.9 cm.
The size of 20/40 is 1.8 cm.
If you print your Snellen, you should check this yourself.
Also, always put a bright light on your Snellen, for consistently.
Also, mark 20/20, 20/40, 20/60, in big letters, so you know which line you are actually reading at 20 feet - if you are serious about this science.
I have yet to see any, " improvers ", take this essential step.

[okayUSAokayUSA]

Hi, Cliff

I'm not sure if you are reading these messages anymore but I have a question as I've read your book and looked at your content over the past few days.

I'm 19, and was prescribed to wear glasses around 2-3 years ago during lockdown when I was playing video games all day. It was something like -0.75 or so (I can't remember exactly honestly). My vision was just a little off, it wasn't that bad. I could only see the difference wearing glasses when I was looking at very far objects. I wore it a little bit but I absolutely hated and still hate wearing glasses so I rarely wore it (and worse it less and less over time). Over the past few years my vision has gotten worse due to staring at screens all day and now it's quite bad, for instance if I see someone in public I know I won't be sure if its them or not unless I'm super close to them, usually they will recognise me first. So although I don't know my eyesight prescription now as I haven't been to the eye doctor basically since I went there for the first time, I know its pretty bad now. I haven't even put on my glasses once for like 4 months now and I'm trying to fix my eyesight.

I've read your book and want to know if I should follow the instructions for Pseudomyopia or for Axial Myopia. I haven't been to the eye doctor for years and although wearing glasses helps me significantly at seeing I don't like wearing them and don't really wear them. I'd think based off the fact that my eyesight got worse due to staring at screens and that I don't wear glasses I should follow the instructions for pseudomyopia but maybe since my eyesight is now significantly worse I should follow the instructions in your book for axial myopia.

For context, my dad has been wearing glasses since he was young (he's nearsighted) and my mum has been wearing glasses when reading at night (far sighted) for maybe a year now. My younger brother is also nearsighted and has been wearing glasses sometimes for a few years.

Any help would be greatly appreciated

Kind regards,
David

[jean-philippeferron]

Are there apps that you can use to test your own myopia? I have a prescription from two years ago but my vision improved. Both eyes are different (-1.75, -1)

[baishuijing]

Hello, Cliff, I have just read your book and thank you. I have a question: I have two different eyes, -8.00, -6.25(really big difference), to get the prescriptions to be equal, what prescription should I go for? go for -7.1, -7.1?

[buttcreamz]

Hello Cliff, this might be a bit of a stupid question regarding the 20/20/20 rule: Does it matter if it is more than 20 ft? From where I work it's either more or less (looking out the window vs looking around the room)

Thanks!

[Alien]

Hi Cliff, People mention your book here. What is the title and where do I find it? Thank you

[NoodyAlhothaly]

Finally someone explained that a one degree is actually 60 min

[Chenobii]

Cliff, I read your guide and had a question. In the guide I don't think I saw a specific minimum time needed to spend looking at the chart 20 feet away. How much time should one allocate every day to looking at distance things? Also, couldn't find a private message button, is youtube comment the best way to ask questions?

[moist]

Hi cliff. You said in the ebook that we need a little bit of blur for the eye. I have -3, 5 diopters discription glasses. Like your ebook said, my less than 20/40 glasses should be -3, 00. But you also said that '' use the 20/40 glasses when is not critical to have distance vision like doing laundry, watching TV, work, do not where when driving at night or hunting vampire'''. I don't get this line . That some close-up work and wearing those 20/40 glasses can't have a blur in such close-up distance, only blur when look something far like reading sign or board. . Can you explain for me ? Thanks for your kindness

[cathieharrison]

This video is fantastic, thank you. I’m sitting an exam in a few days and I couldn’t get my head around calculating Snellen Acuity, but this has made it easy to understand.

[sanafawad]

Hi,
I forgot to mention in my last comments the fact that my daughter never complained about not being able to see nor did she have any head aches. I took her to the optometrist cause I noticed she takes longer to see things that are far and squints in order to make out what's written.
So should I get glasses made for her. If yes then what number would u suggest?
Thanks

[rejectmodernity]

Guys, please help me. What should i do to improve vision? Pls reply someone who did it.
God bless

[sanafawad]

Hi Cliff,
Thanks for all the information. My daughter was prescribed -3.25 glasses by the optometrist, she is 11 years old and has never worn glasses before. I haven't ordered them as yet. Should I order glasses cause it says on ur post THE NO BULLS*** GUIDE TO VISION IMPROVEMENT that if u have never worn glasses then don't go for them and get her to use distance vision and practice the 20/20/20 rule?
Or should I get glasses made?
what number should I go for if I need to get them made -3.00 or -2.75?
I would really really appreciate ur feedback and help.
Thanks.

[ccaterpillar]

Hello Cliff, in your book you mentioned that are not a fan of plus lenses, you have also mentioned some reasons, like convergence and presbiopia. Are there any other specific reasons to avoid plus lens therapy in cases of mild myopia, -1, 00?