CM Measurements And Normalized Rx

Hello Endmyopians!

I’m a high myope - current Rx is OD -6.00, -0.5; OS -7.25 and I can read the 20/20 or 20/15 line on a 20ft Snellan chart depending on the LUX. I’ve been wearing this Rx for the last 1.5 months and the Rx change was just a drop in 0.5 CYL and adding 0.5 SPH to OD.

When I do CM measurements though I get on average, according to the EM calculator, OD -7.5; OS -8.0. My daily outdoor AF practice time is ~3 hours a day and I do my CM measurements after my 30 min to 45 min outdoor walks so I know I have no ciliary spasm.

Does anyone else experience their CM measurements not matching up with their normalized Rx? And when I make a reduction in the future should I rely on CM or the Snellan measurements?

I’m totally confused because I’m getting conflicting information on my measurements. I’m hoping someone can help clarify this situation. Thank you!

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Hi Candace,

I wonder if there’s an issue with the lighting or set-up you’re using for your cm distance to blur? Are you measuring to a screen or an actual eye chart? Maybe you’re using a very small font size?

I’m thinking maybe you’re doing CM measurements to small type so you’re really measuring to 20/10 rather than 20/20. (???) At high myopia the distance in centimeters gets pretty small -8D is 12.5 cm and -7.5D is 13.3 cm which is awfully close to whatever you’re looking at.

Somebody here will have a lot more knowledge than me, but maybe it makes sense to calculate your cm distance to blur with some correction (say -5D) so that you’re not having to decide if you’re 12cm away or 13cm away and having that sway the results by a large amount.

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@nycmao thanks for your feedback. I’m using natural lighting with size 12 font on paper.

In my opinion the CM measurements are best used as a gauge of whether your vision is improving or not, rather than an estimate of your RX because it is a very rough estimate of your Rx, rougher the higher your Rx is. This is because at lower levels of myope the difference between -2 and -2.25 is 50 cm vs 45 cm. But -6 and -6.25 is 16.66 vs 16 cm. Measuring is a lot finickier at higher levels of myopia.

Sometimes what people may do is use differentials to do their CM measurements.

The way those would work is that let’s say you’re wearing a pair of -4
And your CM measurement is 50 CM
Normally a CM measurement would be 1/.5=-2 Diopters
But since you’re wearing a -4 while doing it you do -4±2 = -6 diopters


Hello, I have been experiencing something similar. I have tried hard to get accurate cm measurements but they do not match up to my current prescription. They match up to the original 20/20 prescription that I had been wearing before starting on endmyopia/myopia-improvement journey. therefore the gauge I am using to assess my eyesight improvement is to wear lower prescription glasses and determining whether I see everything clearly with those. The ones I see clear with and I don’t feel strained in my eyes, are the ones that I take as my current prescription. (as a note I originally had -3.5 and now see perfectly with -3 :slight_smile: