# Differentials and High astigmatism - exact blur horizon or circle of least confusion or something else?

I am not sure i understand the approach with differentials and high astigmatism.

I saw jake’s video regarding the topic

So there, he suggests a rule of thumb of subtracting 1.5D from full prescription and add -0.25 SPH for every -0.5 CYL you reduce, generally start with swapping 1 cyl with 0.5 sph

Then there is this:
https://endmyopia.org/pro-topic-how-to-reduce-prescription-complexity/

Which says to deal with astigmatism first for close up and only after eliminating it start reducing it for distance (not to mention equalizing astigmatism - if i get rid of it for closeup, but leave it for distance, i will have a different L/R ratio on different glasses, and that is not recommended, according to my understanding.)

So if someone can outline the recommended strategy for dealing with high astigmatism and close up, please share your experience and advice, and let us have a discussion to clarify this area a bit.

4 Likes

@itamar you might want to re-watch that video. The title is a bit deceiving, because he is actually telling you that you can get rid of astigmatism on your diff if it is less than or equal to 1 diopter. There is also an issue of CYL to SPH ratio – which I am dealing with presently. Reducing my SPH by 1.50 for my 1st diff made my SPH correction less than my CYl, which is causing vision problems.

I have an opto of LE -4.50, -2.00 and RE -3.50, -2.50. My diff’s are LE -3.00, -2.00, and RE -2.00, -2.50, and it sucks. My cm measurements are saying I should be at LE -3.50, -1.00 and RE -3.50, -1.00. I borrowed my daughter’s glasses for diff’s, which are LE -3.00 and RE -3.25, w/no CYL, and the vision is much better, left eye slightly worse than right.

Am thinking about ordering diffs at -3.00, -3.00 w/no CYL for diffs to get make sure there’s blus challenge.

Keep looking at all the other astig blog posts and videos, like this: https://endmyopia.org/how-to-choose-a-differential-prescription-if-you-have-high-astigmatism/. I also recall reading a much older blog post by Alex that suggested reducing CYL first – before reducing SPH – to reduce complexity. Clearly, the thinking on this is evolving over time, based on the experiences of users, but I suspect this might also be a fairly subjective area, dependent on each user’s situation.

Good luck!

4 Likes

Thank you @Astigmatism_Assasin

I am currently at -4/-1.75 both eyes normalized
And -2.75/-0.75 diffs which kind of sucks, too weak…

I am thinking of ordering -3.5 sph both eyes, this seems confortable with the test lens kit lenses that i have.

When you talk about your cm measurements, how do you infer your diff prescription from your cm measurements? Arent the cm measurements measure your actual myopia? (Full prescription equivalent)

Thanks for all the info

Yes, the cm measurements are for the full SPH/CYL, which for me is now -3.50/-1.00 for both eyes. Diffs should be weak enough to give you a little blur challenge for close up, which I seem to be getting with a loaner pair of -3.00’s that have no CYL. My original diffs with reduced SPH and full CYL made my vision feel weird. That’s great you have a test lens kit, eliminates a lot of guesswork.

1 Like

Sounds good @Astigmatism_Assasin
What is your Normalized prescription currently?

I wonder what would @jakey have to say in this situation, considering diopter gap.

Because in your case, the diopter gap is 0.5D if you dont count for spherical equivalence, and 1D if you do. (Which is the minimum jake recommends)

Just ordered 3.75’s for normalized, no CYL. Still experimenting.

1 Like

@Astigmatism_Assasin
What is your visual acuity with these normalized?

I will let you know in ten days when I receive them!

1 Like

I wonder what should we aim for when choosing a differential in this case.

Should we aim for the same blur horizon for both spherical axis (no astigmatism correction) and the astigmatism axis?
Or should we aim to reduce the astigmatism as much as comfortably possible?

Example:
We determine that in order to achieve a blur horizon at 60cm we need x diopters of spherical, and to achieve blur horizon on the astigmatic axis we need additional y diopters of cyl.
Should we get differentials with x sph and y cyl, or should we get differentials which will put us in the circle of least confusion (i.e. the same correction error for sph (overcorrection) and cyl (undercorrection))
So in this scenario we will drop the astigmatism and add y/2 sph. (Maybe make sure y is not bigger than 1D)

Thoughts?

Interesting question. Above you mentioned trying -3.50 SPH both eyes using your test lenses (I am assuming that was with no CYL). Did you try using those for close up for a reasonable length of time? How was your blur? Any discomfort, headaches, etc.?

1 Like

@Astigmatism_Assasin not yet, i still have 1 more -0.25 step down pair from my current diffs, after that i was thinking of getting rid of the astigmatism entirely for closeup (in favor of some more sph) and odder cr-39 lenses.
But then there is diopter gap which would be less than the recommended minimum of 1D, so i am unsure what would be ideal, naybe leave a bit of astigmatism in?
Aldo, i equalized too early so if i remove astigmatism from diffs i can transfer the astigmatism difference to sph and that would allow me to maybe use them in the far future as my normalized…
So i am at least a few months from ordering my next diffs

@FMR @Varakari @Laurens
Guys what do you think about the above? Do you think it is better to go for ‘circle of least confusion’ (less astigmatism correction) or go for the exact correction needed for the given distance (same blur horizon for sph and cyl axes)

thanks

1 Like

Since you have a test lens kit at your disposal you should be able determine this yourself. If it were me I’d go with a bit less astigmatism correction for close-up. But you with that test-lens kit will have to determine whether or not you can drop it completely. Just keep in mind that for distance you want a nice 20/40 challenge and for up close you can look at some text at a little beyond arms length and see if that gives you workable active focus. Don’t make it any harder than this and don’t overthink it.

A little less correction for astigmatism up close is also useful to nudge your eyes into the proper shape they need to get.

4 Likes

I don’t know what’s optimal exactly, but I’m going by general principles:

• Show the eye a slight, but not excessive, undercorrection (goes for both sph or cyl)
• Be careful with highly asymmetric changes, especially to spherical equivalent
• Try to reduce complexity over time

So, if your cylinder correction hits zero on one eye, I’d have a look at the spherical equivalent. If the better eye in SE got rid of the cylinder on top, maybe it’ll have to wait a bit to see if the more problematic eye can catch up. However, if the eye with less cyl has more spherical error, it might be worth a try to challenge one on myopia while challenging the other on astigmatism.

The latter is what it’s like for me, so I generally have a bit more directional (cyl) blur on the right eye and a bit more isotropic (sph) blur on the left eye. That doesn’t feel uneven the way equalizing feels uneven, because the average error is in the same ballpark. In my measurements since winter, both eyes changed synchronously, so it looks like the overall stimulus they get is similar.

Spherical equivalent is really the most important point for keeping things equal. Unless you are equalizing, I would avoid changing the difference in SE between the eyes by more than 1/8 dpt. (Which is the unavoidable bit on cylinder reductions with an odd amount of 0.25 steps of asymmetry.)

2 Likes

With both a spherical and a cylinder diopter ratio challenge ( LE -6, -2: RE -3, -1 ) it seems it will be very tricky to carefully work my way down in both spherical and cylinder reductions. I am beginning to think I should have started out with a test lens kit, but as the budget is tight, I feared it might be an unecessary expense. If I end up with too many unusable glasses, I might decide to stop throwing good money after bad, and break the piggy bank for a test lens kit.

1 Like

I bought a test lens kit but i dont use it much
If you decide to buy one, i’d buy a good one (not the cheap chinese 100\$ kits.)

1 Like

I got my first self-prescribed Zenni glasses just about spot on - the left eye a little bit better than the right, obviously greatly relieved to be free of the way-off astigmatism axis. So I am spared the need of a test lens kit, which will pay for the next 4 pairs of glasses. I can read (but not super clearly) 20/20 with both eyes, so I have already ordered a reduced pair. By the time it arrives in 2 weeks, I hope to see 20/20 even more clearly and will be ready for a reduced pair.

1 Like

I got good results from this one for about \$200. It’s been a great help for me to specify the two Zenni orders I’ve made.

1 Like