# 1st diffs and norms - which principles to follow?

When I ordered my husband’s diffs, I followed what I thought Jake meant in the 7-day email course regarding lowering/dropping CYL. Here was the original “prescription”:

R = -5.25 CYL -0.25, L = -5.00 CYL -0.50.

I lowered the spherical by 1.5, then dropped the -0.25 CYL in the R eye and subbed 0.25 spherical for the 0.50 CYL in the L, resulting in

R = -3.75, L = -3.75

But now that we’re getting ready to order normalized, I’m wondering if I did that right. I understand the principles are:

1. Two spherical drops before astigmatism drops or equalizing.
2. Keep same focal planes (both eyes on the same ratio) between diffs and norms.

He hasn’t been complaining about his diffs, but when I asked, he did say he gets a headache when looking at our less clear resolution computer screen (which is also a little farther away). Books and iPad are OK. I’m wondering if it would be best to –

1. Redo the diffs with just a 1.5 spherical drop, leaving CYL and eye difference the same, or
2. Go ahead and order norms as an equalized, no CYL pair at probably -5.00 for both eyes.

In other words, is it better to drop CYL already when it’s so low, and make the 1st norms match the focal plane of the diffs already gotten, or to start over and keep the CYL in both diffs and norms until we’ve done two spherical drops?

A little more information that may pertain: he’s had contacts before, with this same glasses prescription, that were equalized with no CYL at -4.75 in both eyes. He seemed to do fine with that (though I don’t know why the optometrist put less diopter in the prescription without CYL; I though it would have had more SPH since CYL was dropped?).

I hope this isn’t “diopter specific.” Please treat it as a question of which principles are highest priority.

I realize also that this is a bit of a trial and error thing, but I thought it might be good to get the more expert advice of those who have already traveled this path! Thanks in advance.

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These are some really good questions. I don’t think there’s a right or wrong answer when it comes to cylinder and differentials. I do hope a few people with direct experience will comment on what worked for them, though.

That said, the question of whether to keep norms and diffs the same as far as astigmatism correction, or to back off more aggressively on the diffs (which your husband did) is a valid question. And I don’t think there’s a right answer.

What I do know, without having direct experience with this, is a given amount of cylinder correction, e.g. 0.25 D, ends up a lot more potent in a differential than in a normalized. This is just math and optics…that quarter diopter ends up being a larger percentage of a weaker lens. You can illustrate this by rotating the lens and seeing how much the image changes. With cyl value equal, the lens with the lower sph will exhibit more “astigmatism” as you rotate it.

Another way to put it is: the astigmatism is not constant at all distances. In compound myopic astigmatism (what you’re dealing with here), the astigmatism is nothing more than a component of the myopia in most cases. So, the person has more need for the cylinder at distance (where they have most of their problem seeing well).

Additionally, some types of astigmatism (certain axes) help with reading. So correcting it can be counterproductive in those cases. Moreover, the opposite axis, while it might make reading more difficult, can often be neutralized by accommodation at near, rendering it moot anyway. In those cases, it would probably be worthwhile to substitute more sphere for dropped minus cylinder. (If the differentials were made too weak, the person might might have more trouble because they aren’t using as much cyl-neutralizing accommodation while reading). At least that’s how I understand it to work.

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I used an online calculator. That is equal to about -5.00 in glasses. So, the optometrist essentially dropped the cyl and equalized both eyes to the value of the less myopic eye. Very similar to what you and he did in his diffs. If he’s used to that, maybe it’s worth being a bit more aggressive with normalized cyl reductions. But do remember that the tear film supposedly partly compensates for low astigmatism. So, maybe you don’t want to be as aggressive as the Optometrist was, when it comes to the glasses…especially the norms.

When I had contacts once upon a time, the technician (not so intelligently) did the opposite. I had -4.75 and -4.25 SPH in my glasses at the time. They gave me -4.75 in both eyes for contact lenses. Guess what I tested out as needing, a year or so later? You guessed it. -5.00 in both eyes (for glasses)…exactly as you’d predict from the math.

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Thanks so much for the reply! That is helpful to know that contacts are calculated differently from glasses. Yes, maybe since he’s done well with the contacts in the past, we could drop the CYL and equalize, but not drop the SPH much on the first normalized, just do the equalizing part (he hasn’t changed much in centimeters since starting the diffs either, so minimal reduction is probably best). And he doesn’t seem to see any directional blur when I have him look at the astigmatism wheel either, one eye at a time, no glasses.

That’s interesting that some axes of astigmatism cause more reading problems than others. Do you know which axes those are? I have astigmatism too (-1.25 axis 122, and -0.75 axis 57), and the double vision is quite apparent without glasses, mostly on the vertical. I kept all the CYL in my diffs, and then tried another pair the same SPH but 0.50 less of CYL in each eye. They are definitely more challenging and cause a bit of headache, so I’ve put them away for now. I don’t want to be making my astigmatism worse by using the full CYL in the diffs, though! Maybe awhile after getting my 1st norms I can try the lower CYL diffs again.

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Wow, that definitely sounds like a sign you’re on the right track with reducing cyl fairly aggressively. The fact that the cms aren’t moving means the myopia isn’t per se getting substantially better. And the lack of directional blur means the correction with the cut is being tolerated well. I think these are good signs.

This might be one of those cases in which equalizing and dropping (or dropping most of) the cyl is called for in one step, because they kind of compensate for one another…creating visual synergy and helping to reestablish a level of balance in the system.

The other nice thing about cyl reductions is they only impart an overall 0.125D sph reduction per 0.25D drop. Since lenses essentially only come in 0.25D, you can potentially use this fact to your advantage during some types of equalization steps. That said, I’ve read that cyl reductions can take a long time to adapt to fully…possibly a year or more sometimes.

I’m not sure which axes help or hinder reading, but I’m sure that someone here has a good handle on that. I haven’t delved too into astigmatism yet in my own research, because it hasn’t been a big issue for me (I haven’t worn cylinder in a very long time. In fact it was the first thing I tried to get rid of a long time ago, by asking my Ophthalmologist if he could get rid of it for me when I was a high school student).

I do know that my own very low astigmatism is most evident at lower correction levels. It adds a bit more myopia to the view, overall, and becomes evident during a SPH drop or equalization step. Otherwise, I don’t really ever notice it. Most people have some, but it can be detrimental to a person’s function to correct for it in a lot of cases…or at least unnecessary.

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Can you not invest in a new screen with decent resolution? Old screens with bad resolution are not good and may also flicker.

If staying within the options you presented:

1. redo the diffs - I’d sooner consider dropping less from sph than introducing cyl back. Especially if no issues with wearing contacts with no cyl. If it was me I’d test how the headache changes if wearing e.g. -4.25D no cyl for both eyes. After all the +1.5D is only the rule of thumb to shortcut on the first decision. (And have I mentioned I’d change the screen? )
Note1: The opto gave less correction taking into consideration that the glasses are a few cms away from the eyes while the contacts are on the eyes - it is a usual practice when myopia reaches -5D. (Official name: vertex distance. If you scroll down you’ll see -4.75 vs. 5). Below that they keep the glasses and contacts the same because they cannot reduce by less than 0.25D. E.g. -4 should be reduced to -3.87 but you can only have -4 or -3.75
Note2: With contacts the eyes benefit a lot from the peripheral vision and less correction may be enough.
2. normalised - figure out the diffs first, give time to feel the gains by wearing the correct differentials for a couple of weeks after which I’d order the same ratio as you suggest
Note3: -0.25 and -0.5 cyl is negligible. Optos often struggle to measure it even.
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Thanks! I’m thinking the CYL/SPH substitution may come in handy for me also when I reduce CYL and equalize in the future! I am sure I’ll have a few more questions then.

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Thanks for the reply! These are great details to know about why contact prescriptions may differ from glasses. I’ll check with him also on how the headaches are…maybe we will add some SPH in the diffs if it’s still a problem. And maybe he can use the laptop we have, since it’s better resolution!

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By this do you mean using differentials in close-up? Differentials in mid-range?

@Astigmatism_Assasin, I think what I meant was that if I undercorrect too much, especially at the 60cm distance, there can be some blur that causes problems. Whatever it is–uncorrected myopia, or astigmatism, or both, it causes a problem. Probably because accommodation is lower at lower correction levels. I’m not a fan of differentials that reduce accommodation to around nothing. I like to drop it down, but not eliminate it totally. I’m probably using about 0.50 to 1.50 D of accommodation for my closeup activities with differentials most of the time. Why? because I’m looking mostly at things that are no closer than 40cm, and not looking at things further than 65cm that often. That range of accommodation seems to be a good compromise between comfort and visual quality.

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It would be helpful to know how much cyl you’re dealing with and the type. The axes seem to play an integral role in determining whether one is sensitive or not to adjustments.

My most recent optometrist prescription was

R = -2.25, CYL -1.25 axis 122
L = -2.75, CYL -0.75 axis 57

I’ve read about different types of astigmatism (corneal, lens, etc.) but have no idea which mine is. I imagine you’d need to have a particular test done to find out? I do find that when I look at the world without glasses, images have definite doubles and sort of swim in and out of phase with each other as I try to focus.

As I mentioned, my husband’s astigmatism seems negligible. We’ve gone ahead and ordered his first normalized without CYL; we’ll see how it goes. So far for mine, I’ve dropped the SPH 0.50 diopters (cm measurements seemed right for that),and left CYL as is. The glasses should arrive today or tomorrow.

I am thinking ahead…with the next normalized, would dropping my CYL 0.25 in both eyes be advisable, with what you know about astigmatism? I was considering lowering SPH again first, but since it’s not all that high, I’m thinking maybe it’s best to start alternating CYL reductions already. Maybe a binocular CYL drop of 0.25, then a binocular SPH drop of 0.25, then try getting close to equalizing by reducing 0.50 of CYL in one eye and 0.25 SPH in the other?

And of course, I can try using the diffs with less CYL first, and see if I can get used to those.

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Cylinder reduction is a tricky thing… Some people can drop up to .75 in one go (though .5 is most common) and never miss it, probably much of the time it’s over correction. I had a friend that was given OD: 2.5 OS: 2.75 with 1.5 cylinder on both sides and she was miserably over corrected. She tried on 2.25 with .5 cylinder and could see a perfect 20/20. Other people struggle with a .25 drop in cylinder (they clearly weren’t over corrected). This is where a supportive opto or a test lens kit comes very in handy… however short of either, online glasses are fairly price friendly so probably order your best two guesses (a conservative one and a brave one) and see how it goes. There is a good chance since you haven’t taken any off the cylinder yet you can afford to lose more than the .25 though.

This has potential to backfire… but you can always backpedal if it does. Cashing in cylinder for spherical equivalent might work better since the heavier cylinder is on the side with less sphere… either way make sure you know what is up before trying to deal with equalizing because you want to be sure the ratio between your eyes is the same in diffs and normz.

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Thanks for the input! Good to know that dropping SPH in one eye and CYL in the other could backfire. It does sound like a headache when I think about it!

So you’re saying that it could work to (almost) equalize by dropping 0.50 CYL and adding 0.25 SPH in the right eye, leaving the left alone? That does sound safer…

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