Spherical Equivalent

I heard we can exchange 0.25 Sph for 0.5 CYL correction and it’s simpler than adding astigmatism correction. but we can only do this if we want to do cyl reduction, Is it true? if we do this for spherical reduction then we won’t get blur

That’s fundamentally true, in theory. There are limits to how much you can trade, in my experience. And you still end up working to have good “natural” vision with the cylinder correction coming down.

Basically what you’re doing is taking the average amount of max cylinder correction, and distributing it across the entire sphere.

You may do this and still have some residual blur (or worse). Not trying to talk you out of it, just know it’s not a sure-fire thing.

if I exchange 0.5Cyl for 0.25Sph will it make my sph overcorrection and hyperopic defocus? because I give SPH more than full prescription

Yes, don’t use this.

Cyl gives ideal correction? Drop Sph.
Cyl doesn’t give ideal correction? Drop it entirely first if it’s <= 2 D, or do this after eliminating ciliary spasm / after 2 successful Sph reductions.

@Ursa reckons it’s harder to gain in Sph if you have asymmetric Cyl.

In terms of spherical equivalence, it’s exactly the same.

So although it seems like you’re increasing the sphere, it’s basically redistributing it–you have a meridian which is that strong with the cylinder. The exchange regularizes it into spherical.

There is an entire section on Astigmatism and Cylinder, as well as several folks who’ve been dealt with it whose posts can be found by checking those keywords. There are a number of strategies for dealing with cylinder. The issue is that everyone seems to deal with astigmatic blur differently, so there’s no “one true way to rule them all” approach.

I don’t think that is what I said. I said that my symmetrical mirror image axes of astigmatism seem to cancel each other out, as in uncorrected binocular vision I do not have any astigmatic ghosting, but with monocular vision I do. I think I did not go as far as drawing any conclusions about asymmetrical axes of astigmatism and difficulty in reducing sph., but if I am mistaken in this, please refer me back to the relevant post…

this is of course a simplification.

the average power over all meridians (lines at different angles passing in front of the eye) is the same when you trade 0.5 cyl for 0.25 sph (imagine you have a vertical axis for cylinder, then you remove 0.25 D of power on the vertical and add 0.25 D of power on the horiztontal wher eyou don’t realy need it), so you are reducing prescription complexity but losing something of the accuracy of the prescription in terms of how well it fits your eye


Meridians with hyperopic defocus may cause eye to elongate.
As corneal and possibly lens SE might be not stable.

Do you believe that eye has a fixed SE?

well does anyone really know what happens if you have hyperopic defocus only on some meridians?
we only know that it over all meridians can cause eye to elongate…I guess it is not such a stretch that it could also for some meridians

so yeah pure guess but I’d probably also only do a cyl to sph swap on already-reduced-in-sph-power lenses to avoid putting too much sph power where you don’t need it

as for fixed SE? it depends, my axis hasn’t really shifted over 15 years and sph and cyl i.e. SE was also pretty much stable until I started EM…so yeah it can be fixed

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Not exactly. Some light rays will be myopic defocus, some hyperopic defocus. It’s not straightforward. This is why the focal area of astigmatism correction is called the “circle of least confusion”.

All the research related to EM is immature. Nobody really knows, we only speculate.

I’ve never heard of astigmatism caused by irregular retina shape, only cornea and lens. I’d assume then that this does not cause lengthening. The neurons that process focus information are going to get mixed signals. They may be able to translate that into uneven ciliary muscle contractions, which may result in long term changes to the attached tendons, but this is all speculation.

We could do with this reminder at regular intervals. :upside_down_face:

I don’t like wiggle words (should, may, might) but when the ground we are on is unstable we shouldn’t overstate it either.

I don’t consider them wiggle words in all cases

might can be quite an accurate word if referring to a statistically/mathematically calculated probability. I mean you could know that something has an exactly 40% chance of happening and then you could say, it might happen :wink:

I don’t see should, may or might on this ‘wiggle word’ list

I am very familiar with the words on the list, but that they are called wiggle words was news to me. Never too old to learn. :smiley:

Should, may and might are modal verbs, and have very specific functions.
‘A modal verb is a type of verb that is used to indicate modality – that is: likelihood, ability, permission, request, capacity, suggestions, order, obligation, or advice. Modal verbs always accompany the base form of another verb having semantic content.’ (Wikipedia)

They only wiggle for people who do not know how to use them correctly. And sometimes they do more than move slightly from side to side.:wink:


Don’t mess with Hannie, she likes the grammar rabbit hole almost as much as the vision ones :smiley:


Possibly more. :smirk: It was my bread and butter for a good many years (teacher, writer, editor, translator).

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