# How to specify bifocal glasses?
**URL:** https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300
**Category:** Factual Questions
**Created:** [December 1, 2025, 8:37pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300 "2025-12-01T20:37:54Z")
**Posts on this page:** 13
**Page:** 2
### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [December 7, 2025, 1:57pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/21 "2025-12-07T13:57:20Z")
As I understand it, generally astigmatism is specifically having two different corneal curvatures when measured at two orthogonal angles around the optical axis, which can be corrected with lenses having the same feature in the opposite direction. This is the “cylinder” part of the prescription. When those of us with astigmatism get glasses, that’s what’s going on. There’s a good article here:
> **[Astigmatism](https://en.wikipedia.org/wiki/Astigmatism)**
>
> Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors present can be corrected with simple glasses. In an eye with astigmatism, either the lens or the cornea is slightly egg-shaped, with higher curvature in one direction than the other. This gives distorted or blurred vision at any distance and requires correcti...
I see references in this article to “irregular astigmatism” and also to less simple shape deviations. There’s a link there to “Corneal topography” which measures arbitrarily complex shape deviations. I don’t think corrective eyeglass lenses can compensate for these. I think it takes surgery or contact lenses (which provide a new front surface for the eye).
There’s a different use of the word “astigmatism” to describe an aberration in lenses having no rotational asymmetry (camera lenses for example). It concerns light rays traveling parallel to the lens axis but displaced from the axis. This isn’t what “astigmatism” in eyes and eyeglasses is about.
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### Author:  [@Joey\_P](https://boards.straightdope.com/u/Joey_P)
#### Post date: [December 7, 2025, 2:10pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/22 "2025-12-07T14:10:30Z")
> [@Marvin\_the\_Martian](#):
>
> I use these. Mine graduate from 1.25 to 2.5, they go up to 1.75 to 3.5. There may be other brands with stronger option:
I had no idea you could get something like that without a prescription. I’m in my mid 40’s and my close-up vision is starting to go. If I’m wearing my regular (distance) glasses, I have no problems reading close things, however, with my contacts in, I’m starting to struggle, especially in dim lighting and/or just being later in the day in general.
In any case, I picked up some OTC readers which I’m really struggling with. They work great for something in just the right spot, but everything around that ‘just the right spot’ is blurry and moving my head/eyes just makes me dizzy. Having been a glasses wearer my entire life, I assume I’ll get used to that in a few days if I force myself to wear them for a few hours a day. But at the moment, my arms are still long enough to avoid it.
My real issue is going back and forth between papers on my desk and my computer screen. When I’m doing, what amounts to, data entry, the extra second I need to refocus each time slows me down enough to not be worth it. At least not right now. I’m going to have to give these a try.
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### Author:  [@commasense](https://boards.straightdope.com/u/commasense)
#### Post date: [December 7, 2025, 2:22pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/23 "2025-12-07T14:22:09Z")
If you don’t want to pay the sky-high prices for glasses that your optician charges, you can buy them online. After the eye exam, just decline to buy them in the shop and ask for a copy of your prescription.
There are several online glasses stores. The only one I’ve used is [zennioptical.com](https://zennioptical.com), and I’ve been happily buying my glasses there for decades. I pay around $100 for glasses that the shop wanted to charge me $300+ for. They are not name designer frames, but the lenses are great, with all the features (anti-glare, oil-resistant, blue-blocker, etc.) that the shop glasses have, at a fraction of retail store costs.
I didn’t have a problem with ordering glasses without actually trying them on and looking at myself in a mirror. (I’m a man.) But IME women are more reluctant to do this. So Zenni (and I imagine the other online sellers, too) has a feature where you can upload pictures of yourself and they’ll show you what any pair you’re thinking about looks like on your face.
One measurement you’ll need that is not part of the prescription is your interocular distance in millimeters. Ask the eye doctor or the shop clerks to do this for you before you leave. The online stores give you instructions for how to measure it, but it’s easier and probably more accurate for someone else to do it.
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### Author:  [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)
#### Post date: [December 7, 2025, 2:32pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/24 "2025-12-07T14:32:28Z")
Glasses from my optician are better than glasses I’ve ordered from Zenni optical. But I’m extremely fussy about my vision. (And i get glass glasses because they are optically a little better than the plastic ones.) Zenni optical does a decent job at an extremely competitive price. And also, they don’t require you to prove you have a prescription, so you don’t technically need one.
Still, i stand by what i said above:
> [@puzzlegal](#):
>
> But i agree that it’s worth going to a professional, especially for your first pair. You want to get the distance between your pupils right, and there are probably other things. Try an optometrist. Their specialty is fitting you with suitable glasses.
(I fixed my own typo in that quote. So it’s not exactly what i typed. Oops, should have caught that sooner.)
You want someone who isn’t you and who knows what they are doing to measure your interocular distance. And really, it’s worth paying to get the right prescription, rather than buying a bunch of glasses and hoping to hit it.
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [December 7, 2025, 3:19pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/25 "2025-12-07T15:19:38Z")
Agree with all of @puzzlegal’s wisdom.
But for folks on a budget, things like intraocular distance, appropriate fraction for bifocals, lens centering vertically in the frame are generally parameters that are permanent. So pay the optometrist once to get all those parameters nailed down.
Then get budget glasses and self-prescribe your adjustments for aging as necessary.
I’ll strongly second that the difference between getting those other parameters right vs almost right vs wrong is huge. Much huger than is obvious from reading about this stuff.
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### Author:  [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)
#### Post date: [December 7, 2025, 4:29pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/26 "2025-12-07T16:29:42Z")
To continue my defense of dime-store readers and bifocal readers, let me add a couple of more points:
Convenience: Since I don’t wear the glasses all the time and in all places, having multiple pairs around the house, at work, and in my backpack is convenient. Cheap glasses make that possible.
Wear and tear: I did not grow up wearing glasses, I did not need readers until I was in my fifties. I do not have the ingrained “eyeglass discipline” that long-time glasses wearers do. The few times I bought prescription glasses they were toast within a few weeks. Scratched, lost, even snapped in half.
I did order a pair of prescription safety glasses (with a prescription that matches my dime-store bifocals) to use in the workshop, but (1) I leave them there so they don’t get lost and (2) they are much, much tougher that everyday glasses so I haven’t been able to destroy them (yet).
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### Author:  [@needscoffee](https://boards.straightdope.com/u/needscoffee)
#### Post date: [December 7, 2025, 4:50pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/27 "2025-12-07T16:50:17Z")
Ideally the distance measurement between your pupils should also be measured starting from the center of the bridge of your nose. Eyeballs are usually positioned slightly asymmetrically. My pupillary distance is 66mm, but it’s 32.5 to the right pupil, and 33.5 to the left. In some people, those distances are much more off-center.
You get that measurement from the PD measuring device at the optometrist/optician/eyeglasses store.
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### Author:  [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)
#### Post date: [December 7, 2025, 4:55pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/28 "2025-12-07T16:55:36Z")
> [@Marvin\_the\_Martian](#):
>
> To continue my defense of dime-store readers and bifocal readers,
They are absolutely the best choice for many people. My husband has cheap readers scattered all over the house. They aren’t the best choice for me. I pay a whole lot more, but i wear my glasses every waking hour, and i get more from the more expensive lens my optician sells me.
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### Author:  [@filmore](https://boards.straightdope.com/u/filmore)
#### Post date: [December 7, 2025, 4:59pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/29 "2025-12-07T16:59:40Z")
You’ll need to get two pupil distances. One is for distance and one is for reading. Reading will be smaller than distance to account for the fact that your eyes are facing straight ahead when looking forward and angled together when looking at something close.
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### Author:  [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)
#### Post date: [December 7, 2025, 7:15pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/30 "2025-12-07T19:15:48Z")
I agree that for anything beyond reading/computer distance a proper prescription is necessary.
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### Author:  [@needscoffee](https://boards.straightdope.com/u/needscoffee)
#### Post date: [December 7, 2025, 8:51pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/31 "2025-12-07T20:51:42Z")
> [@filmore](#):
>
> You’ll need to get two pupil distances. One is for distance and one is for reading.
Strange, they’ve never taken my reading PD measurement, including just last week when I got a new Rx. I’ll give them a call tomorrow and ask.
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### Author:  [@filmore](https://boards.straightdope.com/u/filmore)
#### Post date: [December 7, 2025, 8:55pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/32 "2025-12-07T20:55:30Z")
If you get your glasses at local store, they’ll measure your pupil distance for you when you’re getting your glasses. If you get glasses online, you’ll need to know the number when you’re entering your prescription details.
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### Author:  [@needscoffee](https://boards.straightdope.com/u/needscoffee)
#### Post date: [December 7, 2025, 11:22pm UTC](https://boards.straightdope.com/t/how-to-specify-bifocal-glasses/1025300/33 "2025-12-07T23:22:03Z")
> [@filmore](#):
>
> If you get your glasses at local store, they’ll measure your pupil distance for you when you’re getting your glasses
Yes, they did the distance measurement at that time. The glasses are still being made. I have a complicated Rx.
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