PD measurement FAIL!


When I first started EM, I looked at my two pairs of expensive glasses from fancy optometry shops (where the PD number is of course as secret as the Nuclear Code), and neither of them had the PD number on them. So I did a quick measurement with a ruler and ordered my first normalized. I’ve now run through a whole bunch of glasses (since I have ordered multiple pairs for each diopter that I’ve gone through) and merrily used the PD measurement that I came up with that first time.

In the back of my mind, though, I’ve kept wondering if maybe I should have a more accurate less home-made measurement.

Today I learned about this dandy Android app (since I have a new Android phone after a series of eight iPhones—and, yes, I feel like a heretic after years of Apple orthodoxy in all my devices), and it tells me I’ve been WAY off: it gives me a PD of 68.5mm. I’ve been ordering glasses with 74mm PD!

My current normalized (two pairs) are pretty new. Is it important for me to ditch them and get ones with the 68.5 PD?


You might be OK actually, if your differentials are minus lenses the too-wide PD might be giving you some base-in prism effect which reduces convergence. It seems like you’ve been making good progress, I wouldn’t worry about it in your glasses, especially for close up right now.


I dont know if i would trust a phone more than my own measurement.

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Thanks, but I have never worn differentials. These are -1.00 normalized. And I haven’t worn any glasses for closeup for years, even before I started EM.

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Did you see the video? This optometrist says that the app gives as exact results as in an optometry office.

Hey, veterans! I was hoping for some advice as definitive as possible about this (maybe especially among successful low-myopes). I’ve experimented with looking through different regions of my lenses, thinking that might give me some idea of whether the PD difference is vital in this instance. Frankly, I don’t notice much, if any, variation in acuity in different areas of the plastic. (I’d rather not order yet more glasses at this point if it’s unnecessary.)

I measured my PD with ruler and mirror. You go as close as possible to the mirror. I looked only with left eye to position the ruler to my left pupil and then I looked only with right eye to see where my right pupil is. Later when I ordered glasses, they measured my PD by a tablet in the store “Fielmann” and it came out, that I measured quite exact. I measured L30 and R31 and their tablet measured L30 and R30,5. So since then I trusted their values.

By the way I can tell you another story: I ordered glasses online and was not aware of the exact center height of the lenses, this is a thing, which only real stores will check before ordering. So I got glasses with complete wrong centered height and I got headaches really quick. So that was a big and painful money wasting experience. The center height of the lenses must be right for your eyes and I think that is a thing, which online stores can not do. I think they just assume it (that is all they can do, since you can not give them these values). But if you are out of luck, the center height will be not right (especially if the glasses are too big and you want to make custom cut glasses etc.)

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With goggles4u and EyeBuyDirect in particular (might be the same with other online vendors) their online chat customer service is excellent and you can pass on additional information or instructions like pupil height (tends to be a big deal for progressive lenses) and they’ll send it on to the people who cut the lenses (it’s easy for them to put the number in the machine when they make it). If you have a frame you like you from one of the online vendors can just measure the info and pass it along. If you don’t specify anything they’ll use defaults which usually work.

One other trick you can use when choosing frames is to choose a frame PD that is close to your actual PD. Frame PD is the first two numbers added together in the three numbers that describe the frame: so 53-18-142 means the lens width is 53 mm, the nose bridge is 18mm, and the arms are 142mm, and the frame’s PD is 71mm (53+18), so someone with a PD of 71mm would have eyes centered horizontally in that frame, and they can put the center of the lens in the center of the frame (horizontally).

This is something I’ve been thinking about in the past months. I’ve never worn glasses.
Last spring I figured that when I get back to uncorrected vision I may want to have glasses to support when I feel it’s needed. So I ordered a pair of -0.75D from Zenni with 60mm PD as that was the measurement with the card method on their page. I didn’t wear those glasses.
Now that I officially don’t need corrections I decided to order glasses with -0.5D from Firmoo to keep in my bag. The high street opto measured my PD so I used that for this pair which was 62mm. I was a bit surprised that it was so much more but thought that was the official measurement so should be correct. (Measured by the opto assistant holding a ruler above my eyes. Measured twice at my request after I expressed my disbelief.) Didn’t like those glasses either.
So I ordered another pair of rimless glasses with -0.5D from selectspecs and went for 59mm for PD.
For all 3 pairs I selected “for distance vision”, so I don’t know if they then add the same additional sph for sharper vision or different ones… (for distance they add sph, for computer use they deduct sph and for “all distances” they may add some progressive features).
Also, there are optos who mention on their sites that for close up the PD is 2mm less than for distance. So the other thing I don’t know is how the 3 different companies adjust PD when you select “distance vision”.
All I know that I have 3 distance vision glasses with 2 different diopters (sph only) and 3 different PDs.
I can see best with the 59mm -0.5D. Second best with the 60mm -0.75D. And with the 62mm everything is off. It is most obvious if I change from the 59mm to the 62mm.
Does it help?

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Yes, it does, by confirming my suspicion that PD is not an exact science. And, after all, it measures only the horizontal placement of the center. I can remember, as a teenager, moving the temple-pieces of my glasses up well above my ears and finding that I could see better through my glasses thus slanted. I also saw other kids doing this. That is not about PD but about the vertical position of the lense. (In those days, we were dealing with ground glass rather than molded plastic.)

For several days now, I’ve been experimenting with looking through different areas of my current -1.00 lenses and can see no noticeable variation in acuity. So, unless someone alerts me to some convincing reason not to do so, I’ll just soldier on with the two latest pairs of normalized that I have.

Thanks, folks, for sharing your thoughts. I hope this will be useful to others down the line.

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Thanks for all the insight shared above. I’ve now had several weeks to compare my newest glasses that have the exact PD as measured by the app with my other pairs using my more approximate tape-measurement. I can detect no difference at all (as some predicted above might be the case). So I’m just wearing the ones that seem to look best on me!

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