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Comment by AdrianB1 | Hacker News Reader

For more than 10 years I am going to the same optician for glasses. As a sport pilot, I need to have 2 pairs at all times (one is kept as reserve in the cockpit); changing one to another, with identical "everything", produces a serious discomfort that I cannot explain. I learned to live with the idea that eye sight is complicated.

Measurements at different points in time can differ slightly: in the morning just after I wake up I get some values, after staring at this monitor for 12 hours I get slightly different results. I always make the glasses from the morning measurement, but luckily the values rarely differ from one year to another (I change the lenses every year on the pair I wear on the motorcycle, riding with the visor up I get the lenses scratched in about one year from the dust in the air).

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> Measurements at different points in time can differ slightly...

Indeed! The best thing you can do before an eye exam is be rested, eat an apple, don't stare at your phone in the waiting room, look out the window if there is one.

Tired eyes will give differing results. Low (or high) blood sugar (even in healthy people) with give differing results. Straining your eyes by looking at a screen in the waiting area can give differing results.

Note for all these I say can and not do.

I suspect the discomfort when switching glasses is that the inter-ocular distance is different between the two glasses (one is likely right or "more right"). You have the right correction, but it is off-axis WRT your pupils with one pair.

When I go to my $$$ optician, the technician marks my pupils on the blanks in the frame that I've selected using a sharpie (before the glasses are made) and adds the inter-ocular measurement to the glasses information. When I put on the resulting glasses, I don't have any eye strain symptoms and my vision is good (instantly).

When I went to cheap storefront glasses places (I don't do that any more) and I put on the resulting glasses, I get a "stress" signal from my eyes and then they adjust to work with the suboptimal glasses but the eye strain is always there.

My optician friend is doing the inter-ocular distance right, he showed me the process while he was cutting the lenses on a high-end computerized machine. But the only difference between my glasses are the frames, there are some differences in the frames that makes a difference in the final product. I guess it is the angle between the lenses, even a small difference can be creating the discomfort. It takes me a few hours to adjust.

Interesting! For what it's worth, I also learnt that there are quite a few lens manufacturers out there and they all differ slightly regarding lens thickness and refraction properties. That may be another explanation.

And, if your optician is close by and shares your belief that your values vary over time, getting your values checked in different "states" (morning / evening / tired / etc.) can be helpful for deciding on what the "right" values for your glasses may be.

Slightly different pupillary distance I would bet.

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