Confused newbie - how to Snellen?!

Dear fellow myops,

I am relatively new to EM and I am confused about my measurements. I started this (hopefully-) improvement journey at the beginning of last month wearing R: -2.00 D SPH and L: -1.75 D SPH corrective lenses. I immediately stopped wearing my glasses for all close-up and started implementing good eyeball habits - outdoor activities, close-up only for short increments, no phone in the dark etc… I also got my first normalized (R: -1.50 D and L: -1.25 D). I was actually disappointed to find I had absolutely no distance blur challenge. I had apparently been very overprescribed and wished I had dropped .75 D instead of only .5. But I figured any reduction is already a good first step.
My cm measurements weren’t very helpful from the start. They vary SO much according to lighting, time of the day etc. - edge of blur can be anywhere from 75 to 120cm! (although 120 was outdoor on a sunny day…) Always with a small difference between L and R eye. I have been working on close-up AF (at around a meter distance with no glasses) but no distance AF since I had no blur horizon to work with. So I was eager to get my second normalized to start practicing distance AF. After reading long and hard I decided to go ahead and equalise. Since I was already wearing my glasses so much less than before, I started really noticing the shift in focal plane difference every time I put on my glasses. Also my cm measurements always showed slightly less than .25 D difference between the two eyes. So I got my (current) normalized with R and L: -1.00 D. So here comes the tricky part. I can definitely tell a difference comparing to my previous normalized - yay for blur challenge! Mostly when it comes to reading subtitles (end of the day, dim lighting etc.), although outdoors the blur challenge is still pretty minimal. Still, it’s there. BUT I cannot figure out how to evaluate using my Snellen. Here are my current results:

  • No glasses: very blurry, can read 20/40 line.
  • Current norms (-1.00 D): Slightly blurry, can read 20/15 line with 1 mistake.
  • Previous norms: can read 20/13 line with 2 mistakes.

I do not know my Snellen by heart, I have repeated this 3 different times at different times of the day and had someone assist me to confirm I’m not crazy. I’m using a 6m Snellen, indoors with natural light. I must be doing it all wrong. Has anyone else been able to read the 20/15 line even though it’s blurry? I thought if you can read the 20/20 line there should be 0 blur.
I am currently using my glasses only outdoors and in the evening / dim light. But I know I still have a good chunk of improving to do before i hit 20/20 without glasses! Please share if you have had any similar experiences with Snellen and feel free to ask any questions if my post is too jumbled. Also, how often do you wear your -1.00s?
Thanks for taking the time to read this far!

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Yes, I can read a Snellen line even though it is very blurry. It is hard to measure vision on a Snellen for that very reason. You have to see each letter with crystal clarity before you can make any judgement on the Snellen. I can see the 20/20 line with an uncorrected -1 right eye, but it is very far from clear. This is a bit of a nuisance when cm measurements are no longer very practical on account of distance. You can keep on using the Snellen, but only consider yourself as having ‘passed’ a line when you can see it with absolute clarity.

There is a lot more to character recognition than refractive state. You can definitely comprehend what you are seeing without it being crystal clear. You can also have the opposite problem of not comprehending what is perfectly refracted.

I’m also wearing -1.00 D norms but I can definitely not read the 20/40 line, more like 20/70 without them. Use the Snellen to diopter table and if you think you are seeing 20/40 then you should see the 20/20 line clearly with -0.50 lenses.

Use the Snellen to diopter table to determine what line you should be seeing without glasses. In my case I now see 20/20 with -1.25D so using the table I should be seeing about 20/70 without the glasses. So if you are seeing the 20/20 clearly with the -1.00 D lenses than without them you should see about 20/50-60.

Now remember that amount of blur and that should be the blur that you should be using. I don’t know if this makes sense or is accurate, but as long as you stay consistent that’s the most important.

Also, read the snellen from the bottom up. When I go top to bottom, the AF kicks in by the time I get to the lower lines.

Snellen is about readability, cm measurement is about blur. Officially usually you considered as “being able to read a line” if you can read at least half of the letters correctly.

There is always blur. If there would be absolutely no blur you could able to distinct infinitely small details. Even if you can see 20/10 there are still some blur. Eagles can see 20/4 - 20/5. The best possible human vision would be blurry for them.

People who were always emmetrope say “I see no blur”, because that’s what they used to, and that’s their normal. Give them glasses which corrects them 20/10, make them use it for a few days and then they will say their previous 20/20 vision that it’s blurry.

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Not true. At some point you don’t have enough rods/cones in the back of your eye to see more sharply.

The average person with 20/20 can’t be corrected to 20/10, that requires either more rods/cones or better visual cortex processing than average.

Not true. At some point you don’t have enough rods/cones in the back of your eye to see more sharply.

Well yeah, it’s true. What I said is only true theoretically, if I disregard that the light also has to be detected :slight_smile: Though it’s not the absolute number of rods/cones what matters, but the density of cones/rods in your fovea. And that’s higher than what’s needed to 20/20.

The average human eye have 1 arc minute resolution (some sources says even lower: 0.6). In 6 m distance that’s 0.17 cm. 20/20 Snellen is way bigger than that. I’m pretty sure an average emmetrope human can be corrected to 20/10, but at least to 20/15.

So I still think that there is blur at 20/20. Just emmetrope people are “blur adapted” to that.

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@halmadavid, your knowledge and responses always give me hope that is not out of the question, even for a high myope like me, to reach 20/20 again, with no glasses. Thank you for all you do for this forum!

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at your low level of myopia maybe this is not useful, but who knows:

https://community.endmyopia.org/t/how-to-measure-cm-to-blur-for-sph-and-cyl-especially-for-higher-myopia/11186/13

the main point for you I guess is that you can eliminate the variability by measuring blur on a computer monitor which is of course back-lit.

you would probably need something bigger at that distance to see the blur - like a large displayed letter where you can run your eye up and down its edge - always use the same monitor and same letter and same edge of it to check cm to blur - so I guess for you in the 100cm range?

Thanks for all your responses!

Hm. I’m not sure this makes sense to me yet. Not even the 20/200 “E” is clear. I thought everything passed one’s individual edge of blur (around a meter for me) is to some extent blurry. I can only see the “E” and the next few lines because they’re big enough, but not because they’re crystal clear and suddenly the next line becomes blurry…

^ this exactly! So does being able to “recognize” the characters in the lines, blurry or not, “count” so to speak? Jake talks about aiming for norms that give you a correction to about 20/50 (which translates to around 20/30 outdoors) in order to have a decent amount of blur challenge. Does that mean you can read the 20/50 line on Snellen but no further?

Thanks, this is good advice. I’m still figuring it all out, it’s hard to pinpoint an amount of blur and remember it when it’s no longer there… I’ll keep practicing this art.

I know what you mean. Still, Jake said in one of the videos that there should eventually be no blur. Things only get smaller, but never blurry. I’m paraphrasing but I think that was pretty much it.

@Lajos Thanks for the tip. I’ll look into the link. Unfortunately, I don’t have so much as a tablet - all my screen time happens on a smartphone… :see_no_evil::hear_no_evil::speak_no_evil:

this is true hence why most people say they are “at” a line on the snellen chart when they can easily read the letters (but not necessarily without blur) . recognition of the letters is enough.

but as someone else posted, just be consistent - if you say one day I’m at 20/25 because you can recognise each letter then always use this measure and not, on another day, because you can read more than 50% of the letters.

of course don’t rely on memory - be honest with yourself - can you REALLY recognise that letter or are you just remembering it based on its basic outline?

Any time I’m at a doc with a digital Snellen chart, and I see OFLCT I make him change the letters, because I saw that slide way too many times as a kid.

From my current knowledge, and correct me if I’m wrong, the Snellen chart is for visual acuity, or to be able to discern characters at a given distance. Since all lines are at the same distance, you will have the same amount of blur for each line, but it is just hardly seen on larger characters since you do not need a lot of clarity to be able to discern them. This test is important for when driving as you will have a little of time to read moving signs. If clarity is concerned, go for the cm distance to determine your visual bubble.

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