# Questioning the wisdom - cm measurement - diopter calculation - VERTEX DISTANCE

So, if you don’t have a glasses prescription to start with, we recommend taking cm measurements and converting that to diopters, and ordering glasses to match.

We measure distance to blur from the eye.
Isn’t the resulting diopter calculation the strength of lens at the eye?
For higher prescriptions, wouldn’t vertex distance come into it and raise that value for a glasses prescription?

This doesn’t matter for anyone under around -4D, the math nets out to about the same and we’re aiming for normalized to be a bit weak anyway, but above that we’re getting into being a full quarter diopter off. Could we improve the accuracy of this measurement by compensating for vertex distance when ordering glasses? We still run into issues with accuracy of small measurements above -10D…

2 Likes

My favorite vertex distance calculator:

2 Likes

of course it is a contact lens prescription that you get from cm measurement. You are only the second person (after me) to seem to notice this and post it. See I even included it in my "how to " guide on diopters for differentials

1 Like

Could you copy that content here: https://wiki.endmyopia.org/wiki/Differentials

1 Like

I just made another how to for cm measurement

3 Likes

@Lajos, mind if I move this thread to the public section?

1 Like

1 Like

Is the conversion to vertex distance mainly from contact rx to glasses rx? I havent seen a lot on the blog on vertex conversion for when we measure distance to blur from the eyes. Jake has the amazing edge of blur conversion to diopters, but do we need to also do the vertex conversion when buying glasses online? I wasnt aware of this conversion and I assume it doesnt really make a big difference in when getting glasses. How many of you have done this conversion. im a bit confused.

The weaker your glasses, the less vertex distance matters. If you’re 0 to -4, the difference is less than a quarter diopter and doesn’t matter. At my lens strength (-17ish), I can change effective power by a full diopter by changing where the lens sits on my nose, and switching frames can change the power. (Nevermind the 4D difference between contacts and glasses.)

In between, you may find your glasses a little weak if you don’t take vertex distance into account, but too weak is better than too strong, and some trial and error is expected in the process.

Odd, the other one I moved myself, can’t seem to move this one. @halmadavid can you move this thread to public?

Done

2 Likes