Questions regarding first differential glasses

First question: I have been doing cm measurements off and on for the last week or two. My cm logs show fairly consistent measurements so far (within 0.5 to 1 cm day-to-day per eye). After adjusting for vertex distance, it looks like my base Rx is overprescribed by 0.5 D in one eye and 0.75 D in the other (comparing spherical equivalent values). So, when looking at ordering my first differentials, should I be using what the ophthalmologist measured or my own measurements for the full correction baseline (before reducing by 1.25 to 1.50 D for differentials)?

This seems like it could be important since the diopter gap from my opthalmologist is 0.25 D different than what I’m finding from cm measurements and I know we shouldn’t be messing with the diopter gap until we’re ready to start equalizing. Then again, that 0.25 may just be the difference in astigmatism correction.

Basically, should the cm measurement really be trusted for ordering lenses or just for tracking gains? I know it’s also used when determining the proper time to equalize, etc.

Second question: The EM site suggests that people with low astigmatism (< 1 diopter) can just drop the CYL from their differentials (potentially adding -0.25 SPH for every 0.50 elimination of CYL). However, in other places, the site says to reduce CYL in 0.25 increments in lieu of dropping SPH (for normalized and differentials).

I realize that at closer range, CYL is less necessary, so it is easier to drop it or reduce in the differentials than in the normalized lenses.

So I’m wondering whether it makes sense to just drop CYL altogether (differentials and normalized) when first starting out if it’s low enough? Otherwise, maybe just drop CYL from differentials and keep it in the normalized and drop a bit at a time over a couple of cycles?

I’m starting with -0.50 CYL in my right eye and -0.25 CYL in the left, so it’s almost negligible in my case. I know each person is different, and I can read (close up) without my glasses (no blurring or double vision), so I’m leaning towards dropping it completely (differentials a least, but maybe normalized too) and just adding -0.25 SPH to my right eye to compensate. I just wanted to validate that it makes sense before I shell out for glasses.

I’ve ordered a trial lens kit to help me validate some of these questions on my own, but I’m interested in the best practices. Plus the recommendations may be helpful to others too.


You seem to have measured carefully, and thought things through carefully as well, so your own conclusions are your best advice. We should not be giving diopter advice and recommendations on the forum. I know it is tempting to ask for and even to give this, but anyone who decides to do EM does so at their own ‘risks and perils’. This sounds gloomy, I know, but I also think that getting things wrong initially is likely to do little harm physically, although financially it may do so.

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Oh, I realize that. I have tried to word my question specifically without including too many prescription details to avoid asking for diopter specific advice. I am really just asking for clarifications around best practices from other students and practitioners of the EM system since there are some things that I found were confusing or vague.

The first question boils down to: Should the cm measurement be trusted for ordering lenses or just for tracking gains? Particularly if it doesn’t match up against optometric results. I feel that it is probably the latter, but it’s hard to tell.

The second question is really: Considering the website suggests dropping CYL at low diopters, does it make sense to do that as part of the initial “reduction” from a full prescription (especially when overprescribed), or is it better to wait until having completed a few cycles of reduction? For this, I imagine the answer is: maybe, test it and see. Which is what I intend to do. But I wanted to make sure I’m not shooting myself in the foot by trying to do too much too early.

Neither of those is really asking for anything specific to diopters. Just best practices.

In any case, I’ll test the various scenarios on my trial lens kit before I place an order to validate that they work for me.

I’ve been researching the EM system for a couple of months at this point. I’ve watched most of the public YT videos (as well as many of the students’ videos) have read through a good chunk of the blog, wiki, and Facebook. However, there are still a few aspects of the EM system that seemed a bit unclear. Some of these questions may have been answered here on the forum, but a quick search didn’t turn up any answers.

I am a researcher and analyst at heart, so I don’t have a problem with answering my own questions. But considering that I know that poor choices can lead to problems with the visual cortex and such, I am trying to err on the side of caution and get advice… at least to start. :slight_smile:


I have found this to be the case as well. Best practices did not work out for me at all, as I am an outlier, and I paid for following best practices to the cost of 10 pairs of unusable Zenni glasses. I did not have to replace these, as I am outlier in the sense that I went back to being uncorrected other than for driving, as I had been for the past 30 years, and still made gains. This has made it virtually impossible for me to make suggestions on the basis my own practices, other than to similar outliers, although I can recite the EM catechism fairly well. :smile:

I hope you did not read my previous comment as a reprimand. It is difficult to tell, from initial posts, how much people have read and searched, but you seem to have done so thoroughly. I hope you get some useful replies from other forum members. :smile:

I actually used my original prescription as the guide to buy my first differentials. My glasses were -10 and I reduced down by 2 diopters. I didn’t have any issues. The most important thing was a comfortable and ergonomic distance from my screen to edge of blur.

I cant give advice on astigmatism really, never had it. But general rule is do as little change as possible. 0.25 at a time and avoid doing CYL and SPH at the same time.

I know that you’re not supposed to drop CYL and SPH at the same time, but I wonder if that also includes overcorrection? I plan to remove the overcorrection as part of my first differentials and work back from there to my first normalized. But does that mean I shouldn’t be dropping CYL at the same time? Initially, I didn’t think overcorrection counted, but now I’m not sure.

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Are you sure your overprescribed and by how much?

Well, my cm measurements over the last couple of weeks suggest that I’m overprescribed by ~0.5 D in one eye and ~0.75 D in the other (after compensating for vertex distance and astigmatism) over my most recent prescription. But that is indoors under artificial light.

The funny thing is that my latest prescription (which I only got last month) is actually 0.25 less myopic in both eyes than the previous one. So my eyes actually improved over the last few years. This means the overprescription vs my most recent glasses would be 0.25 D more, assuming the measurements are accurate, of course.

I’ll be using a test lens kit to validate these measurements, so I’ll soon know for sure one way or another.

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I wrote about the 3 options with cyl here

But the most important questions to you to ask yourself:

How long have you had the cyl correction? If it’s fresh, don’t introduce it, don’t train yourself to get used to it, or you are just adding one more thing to reverse later. If it is not a new addition to your prescription how much are you used to the cyl correction by now? The latter will determine if you half it and add it back to sph, or just drop it.
0.25D is very small, 0.5D is not much either, but you won’t understand how much you relied on them until you let go of the extra “crutches”. And there is also the blur tolerance and AF-ability that are personal and add to the experience.
Dropping cyl involves a lot of image processing adjustment in your brain. That is the area of tolerance that you will have to figure out for yourself…

Same goes for your 0.25D difference in sph. If it is a new addition to your prescription, don’t introduce it. If it is an existing one, with 0.25D difference only it is more the eye dominance being corrected in the weaker eye to bring closer to the dominant one - which is against nature. But I guess this is an existing difference you got used to, so I would not drop the 0.25D difference in the first step, unless you find it very easy to do.

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Yeh thats good if you know.
The other thing to keep in mind that, depending on your astigmatism, your differentials can become your normalised. So if you buy incorrectly you haven’t wasted a pair.

Well, to risk getting more into diopter specific information. I have been wearing the same full prescription lenses for the last 5 years or so. I go to the opthalmologist every year, but they haven’t changed my prescription in quite some time. A couple of months back, I went to a new doctor who reduced CYL by 0.25 D in my right eye (non-dominant, higher correction) from +0.75 to +0.50 and reduced SPH by 0.25 in my left eye (dominant, lower correction) from -5.25 to -5.00.

So the new prescription is a reduction in power in both eyes, though in different ways. He did make a change in both SPH and CYL in the same prescription though which didn’t occur to me until I just looked back on it.

Since he also recommended computer and reading glasses based on the new prescription, I ended up picking them up from Zenni on a Black Friday sale. The computer glasses being +0.75 SPH added to both eyes and the reading glasses being +1.50 SPH added to both eyes. No changes in CYL.

I started wearing those glasses a bit over a week ago, but I found that I can use the new reading glasses as differentials (since the blur horizon is pretty close to the right place, though ~0.25 D overcorrected). The computer glasses get me to about 20/50 or 20/60 at 20 feet according to my Snellen chart (which feels like ~0.25 to 0.50 D undercorrected), but I’ve been using them as normalized until I can work out proper ones since I can see the TV (~15 ft away) clear enough to watch TV and get a bit of blur challenge for reading text.

So far no issues in either case, but it’s only been a week or so.

Yeah, I understand that part. I’m still trying to figure out AF, so I don’t know how that factors in yet.

Though now that I think about it, my eyes are technically used to 0.75 cyl in my right eye and 0.25 cyl in my left, since the 0.50 cyl is for my most recent prescription. I didn’t think about that. My visual cortex is likely still getting used to that shift. I can probably drop the 0.25 cyl in my left eye without too much issue (I didn’t have cyl in that eye at all two glasses ago), but dropping more cyl in my right eye might be too much. Though I would be adding back the -0.25 sph to compensate. :thinking:

I may need to test that on my trial lens frame to see how my eyes handle it.

Well, my new reading glasses seem to be well tolerated with the current prescription. Though they are just a bit too strong since my blur horizon is a bit further out than I’d like. Closer to 80 cm or 1m, where I’d prefer more like 67 cm.

I normally don’t use glasses to read close up (phone, books, etc.) and I don’t see any blurring or doubling of text without my glasses. So it doesn’t seem like I really need astigmatism correction for close up. It is possible that may be a bit overprescribed still.

Well, that is actually one of the reasons I’m investigating whether I can drop my astigmatism correction at the start, so I can lower the complexity of my prescription and allow more reuse of glasses earlier on.

Either way, two new pair of glasses (via Zenni) is like $20-$30 every 3 months, so I’m not super worried about it yet. Even if I can’t drop CYL at the onset, I’ll be able to drop it in small increments over the next couple of years, so it will work out.

Hmmmm. I think all things considered the golden rule is to keep reductions as simple, simple as possible. You may think you can do that little bit extra but from my experience it always doesn’t work out. Its better to reduce a little and then your eyes improve quickly to that small amount than reducing too much and then stalling your progress. If money isn’t an issue I’d go for the most simple reduction.

I dropped .5 and .75 cyd starting out from both diffs and normz. No regrets and would do the same again.

In my opinion a simple subscription wins out over a simple reduction.

Like you, I didn’t feel a need for cylinder correction when I was lens free. That was enough for me to spend a few bucks to try glasses without astigmatism correction. It took a few weeks to adapt, but it was the right choice for me.