How thick would bifocal eyeglasses made of actual glass need to be for a patient who’s both nearsighted and farsighted with vision between 20/200 and 20/220?
Hmm, I’ll have to look up my prescription, but i might be able to answer that. I have a fairly string prescription, progressive lenses, in glass, on my nose right now.
New opticians are always surprised that it’s not super thick.
I don’t think bifocals/trifocals with a line(s) a thing anymore. It’s progressive lenses without lines. Much easier to get used to.
My prescription is ~20/250-260 each eye and my high index plastic progressive lenses are ~3/16" thick. My non-high index pair are ~5/16" thick.
More important than thickness is weight. I used to have glass lenses and they would constantly work down my nose. They were also in a heavier frame. My current pair is heavier in the earpieces than the front. and well balanced.
I wouldn’t go with glass unless absolutely necessary. Say if I was working around things that my hit my glasses. But if I were in that type of workplace, I’d also have on safety goggles.
My prescription is -6.25 diopters, with +2.5 for the near vision part. Maybe someone can translate that into 20/???
At the very widest point, they are 6mm thick. Most of the lense is thinner than that.
They are made of glass. I’ve gotten glass because when i tried other stuff, i was annoyed by distortions and weird color interactions. So i went back to glass. Perhaps there are plastics with better optical properties today. My lenses in “ordinary” plastic would be a lot thicker, and they’d be a little thinner in high index plastic.
They weigh more than plastic, but it generally doesn’t bother me. I’ve been wearing them for years, and I’m used to it. They do have a higher specific heat than plastic lenses, which means they are more likely to fog up when i walk into a hot muggy place from a cooler place
This says that 20/400 is about -4 diopters, and my eyes are significantly worse than that. Also, 6mm is just a little less than a quarter inch.
I can’t find a cite, but as I recall, glass used to be required above a certain level until plastic technology improved. I’ve been wearing glasses for over 50 years and remember a time when glass was recommended over plastic as my eyes worsened.
Sounds like you may have had polycarbonate lenses: And they’re probably what I’m remembering as having a limitation as to how high power they could go.
#### Lower Abbe value
Abbe value measures the dispersion of light for different eyeglass lens materials.
Polycarbonate rates are lower, meaning they have the potential to cause slight chromatic aberrations (visual distortions) in your peripheral vision. However, many people do not notice this.
#### Not Suitable for All Prescriptions
Polycarbonate lenses are a popular choice, but they’re not for everyone. People with eyeglass prescriptions stronger than +/- 4.0 may need high-index lenses.
It wasn’t a slight chromatic aberration, it was striking and drove me nuts. But i also think i get better acuity with glass.
I think i started with polycarbonate, and then tried high index plastic and then went back to glass.
I’m much fussier about my vision than most people. I’m shocked at how many people go through the day with dirty glasses, or with coatings that are wearing off, and don’t even notice.