Differentials have extra 0.25 CYL

Hey guys, I just found out that the differential lens I’m using has a -0.50 astigmatism. The differentials give me blur challenge but considering my astigmatism is -0.25, would I be doing damage to my eyes by using the differentials which have 0.25 more CYL than what I’m used to?

It’s certainly distortion in an undesirable direction.

I’d be inclined to drop it altogether if it were me.

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When starting this journey it is better to keep the same astigmatism correction in your normalized and differentials, so that your visual cortex isn’t jarred more than necessary. Nonetheless, further down the road, some EMers have found they can eliminate the astigmatism correction altogether from their differentials when it’s small like 0.25 diopters, I don’t think anyone ever adds more cylinder unless the prescription was wrong to begin with.

You have blur challenge, but some of that blur may be due to the extra cylinder. Your visual cortex may very well become used to it since the differentials are only off by 0.25, but the danger is getting used to it also means becoming dependent on it. You definitely don’t want your brain to rely on that extra cylinder to see clearly which you didn’t even need at the start of this journey. As Kem said it’s moving in the wrong direction for progress.

Just one more thing to note; from other EMers’ posts and my own experiences, the visual cortex seems much more sensitive to changes in cylinder than spherical power. You may not notice it now because the extra directional blur is mixed in with the spherical blur.


Thank you Myst and Kem. I certainly do not want to be dependent on more cylindrical correction and give myself extra challenge or confuse my visual cortex. I’ll stop wearing the differentials. I also wanted to ask, with a -1 lens I could see 20/50 on the snellen chart. I usually use my laptop from about 75cm away but things are a bit more blurry than I would like them to be without my glasses. Do you think I need differentials that let me see 20/50 or 20/70 for up close usage?

For the laptop it’s not about a specific snellen reading. If you use it at 75cm you want to get differentials that put the edge of Blur at that distance. In my experience it’s hard to decide the needed lens power based on snellen chart

@myst This comment certainly caught my attention. Tomorrow I’m expecting the arrival of a Zenni package containing my first two pair of glasses(es) - intended to serve as my normalized distance lenses.

Why two pair? Well, not being sure how much to mess with astigmatism from the outset, I got one pair with my current CYL Rx (-0.50/-0.75) and the other with 0/0 CYL. For context, the SPH will be -2.00/-2.00… a mere 0.25 less than my current distant Rx.

Based on your comment above, I will report back as to whether I sense a significant difference between the with and without CYL lenses.

It’s hard to say what Snellen line number you should see with your differentials. This is very subjective and depends on the individual. If you are going to use your differentials for the laptop, then your measurements should be how well you can see your laptop, not the Snellen chart from 20 feet away. Whether you use differentials or the naked eye, the text on the laptop should appear on the edge of blur. Too much blur will just cause eyestrain without improvement, even regression if your brain is overwhelmed. @halmadavid discovered that when he experimented and posted that his cm measurements increased when he moved his laptop closer. I can’t find his post for you but it’s somewhere in this forum. You may be able to AF way more, like me for whom AF is very easy, but there’s a fine line between enough blur and too much blur for consistent progress.

@SomeGuy, changing 2 things, spherical and cylinder at the same time is really challenging. It might be best to start with the glasses that still have the astigmatism correction and only challenge your brain with the 0.25 spherical reduction. When you’re seeing with perfect clarity then move to the glasses that reduce the cylinder. Although, even then, I would be careful to reduce both eyes by the same amount. The brain is also sensitive to changes in the power or cylinder ratio between the eyes. But that’s just me; in a vast population, there are sure to be exceptions.


Thanks for the additional comment.

Happily for me, I ordered “one of each” - both with and without the CYL adj. See what happens when you can buy Rx glasses @ about $15 a pair? :wink:

I’d be curious to know how significantly myopic you were at the time you had the less-than-ideal experience you’re referring to (i.e., being more sensitive to a CYL change than a comparable SPH change)?

The 0.25 SPH difference I’m embarking on might loom even larger (percentage-wise) going from -2.25 to -2.00 than perhaps something of a markedly larger initial diopter starting point. If so, this only serves to underscore your cautionary warning.

My myopia was around -9.25 and -10.25. (Why the big range is the subject of a long post that I will write one day, maybe a year from now, if my progress remains consistent enough for me to share what I’ve learned.)

For years, my optometrist-provided prescription gave my left eye 0.25 diopters more power than my right so that was the same power ratio between the eyes that I was wearing at the time. However, my left eye improved more quickly than my right eye and I decided to equalise the spherical power and eliminate the cylinder correction at the same time. After all, I rationalized, the power was equal anyway and I was wearing less than a diopter of cylinder:
OS: 0.75 cylinder
OD: 0.50 cylinder
Anyhoo, when I did that I couldn’t see anything without double vision everywhere. My head and eyes felt disoriented and my visual acuity regressed by 0.5 diopters. I eventually got used to it, but I had to crank up the spherical power and it was 2 months before I felt good enough to cautiously reduce the power by 0.25 diopters again.

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Whew … sounds like a miserable experience. Glad to hear you’ve gotten yourself back on a solid path forward.

I will certainly proceed with due caution. In all likelihood, the -0.25 reduction in SPH with the same carryover astigmatic corrections will at first probably be best - we’ll see [literally].

The thing that makes me wonder is that the Rx I had immediately prior to my current prescription only had -0.25 CYL (about 90 axis) in each eye while SPH was -2.50/-2.50 back then (apparently my Rx improved a bit as I’d aged?). So my current lower SPH but higher CYL might just be a screwy Rx.

Details to follow. :dizzy_face:

Do you mean that Halmadavid’s eyesight improved when he was closer to his laptop without glasses at the edge of blur?

I was considering getting a new normalised. With a -1 lens I can see 20/50 on the Snellen chart (3m A4). With my -1.75s I can almost see 20/20 now on the Snellen chart after having used them for 5 weeks (they were my first normalised and I had used them for up close use and distance use too). I have -0.25 CYL correction. I know Jake recommends a quarter diopter reduction every 3-4 months but If I went down by 0.5 diopters for my new normalised do you think it will be too much? Will I have to compensate for the removal of the CYL as well?

Also, I was wondering how you determine the correct amount of ‘edge of blur’. To what extent does the content you’re looking at have to be blurred in order to see the most improvement when active focusing?

That’s a great idea.

Regarding @halmadavid, I meant he confirmed that when his laptop was closer, i.e. less blur, he made more progress. I don’t remember whether there were differentials. What matters is the amount of blur. How much is too much is very subjective, so I’m sorry I can’t answer that. That’s where some experimentation becomes inevitable. Personally, I have had blur adaptation problems in the past, so I need to be very careful about when I am truly seeing well enough to reduce and how much I reduce by.
MathewEly has posted very interesting threads and videos about how he had to move more slowly and his prescription at that time was about where yours is.

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Cool. I will check his video out. By the way, could you explain a bit more about what blur adaptation is and how it happens?

Blur adaptation is your visual cortex unable to bring distance images into perfect clarity because it has become so accustomed to blurriness that blurriness becomes the new normal. After a while you may not remember what true clarity looks like.

This typically happens to people who haven’t worn a strong enough prescription to see clearly for a long time. Even before discovering EM, progressively myopic people know from experience that glasses have something to do with their increasing myopia, just not exactly how, so we fearfully wear a prescription that is too weak.

It doesn’t mean you can’t AF like a demon. I’ve always been very good at it, but even though I was able to turn double vision into “clarity”, I’ve since realized my idea of clarity wasn’t the kind of clarity a normal-sighted person sees. For example, I could read the 20/20 line, but that really wasn’t good enough because the letters appeared thin. True clarity means the lines of each letter are thick, black and crisp. The contrast between black and white should be bright, shiny, and jump out at you. For CM measurements, I realized true clarity for me means not just being able to read the text, but seeing the pixelated imperfections of each letter, especially more intricate letters like “e” and “a”. By the way, my myopia is high which means my CM distance to blur is short enough for me to see this; I don’t think most people will be able to discern these details. That’s why your mileage may vary. Of course the reason blur adaptation has been bad for me is if I reduce too much too quickly, progress stalls, even though I can continue to see the smaller text lines on the Snellen chart.


That’s a great explanation.

You have some great replies. I’ll just add: probably not the best thing to be wearing extra cyl you don’t need, especially up close.

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I also can’t find my comment, but what happened is that I reduced my differential, but did not moved the my screen closer to accommodate the closer edge of blur. This (too much blur and/or eye strain) resulted in that my cm measurement started to decrease. After moved everything closer (to be at the edge of blur) it started to increase again.

In my considerations for dropping cyl, I’m thinking of considering 0.50D of cyl to be a drop element equivalent to 0.25D of sphere, in other words, I’m thinking when I do a cylinder drop, it will be 0.5D. When I do a sphere drop, it will be 0.25D. Will try that at least once when I’m focusing on cylinder reduction and modify if necessary.

This balances my head knowledge that this is a long game and my heart’s desire to work my way through the process back to 20/20.

Will post in my Road to Emmetropia thread to publicize how that goes… more to follow.

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Sounds like a prudently measured approach. The good news is that if trying out a decrease in rx turns out to be too aggressive, those overly-aggressive glasses can be used (next) after an interim step (with perhaps a halfway rx point) has been reached.

Of course the complications of blur-adaptation and possible acute symptoms (headache, etc.) are not to be trifled with along the way :man_shrugging: