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Comment by kps | Hacker News Reader

> I also avoid […] high-index lenses for computer work.

Yes! You're the first to mention this.

It's not refractive index itself that's the problem, it's dispersion (roughly, the degree to which refractive index varies across the visual spectrum, described by ‘Abbe number’). We've all seen pictures of a prism splitting a beam of white light into a rainbow — for visual purposes, the less split the better.

Higher-index materials tend to have poorer dispersion, but especially in the mid-range 1.6ish, there are wide variations in quality at the same index. Glass tends to be best, if your prescription is light enough that you can handle the weight. Polycarbonate and acrylic are awful. MR-8 is in the middle, and what I've settled on for recent computer glasses.

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Hear, hear!

Here's a good way to test your glass's refraction index. On your desktop find a small red icon with something white in the center. Stare directly at it. Now turn your head until the icon is at the edge of your vision. If your lenses are cheap polycarbonate, the white part of the icon will appear to move towards the edge of the icon or even out of it.

Most non-cheapo glasses today in the US use Trivex. It's a polymer, not glass, but its Abbe number is 43, which is perfectly adequate.

Crown glass, with its Abbe number of 59, is superior, but the eyeball can discern differences only up to 45-50, so most of Crown glass's improvement over Trivex is imperceptible.

This is partly why it's not offered in glasses (again, in the US, at least according to my optometrist). It is also twice as heavy, shatters (polymers like Trivex don't), and scratches more easily.

Among non-glass options, CR-39 is a good choice for computer glasses with prescriptions weaker than -5:

  - high (58) Abbe number
  - thick (1.5 index)
  - cheapest

Thickness and weight can be mitigated with a smaller frame, e.g. vintage options.

Actually buying CR-39 lens might require solving a dark pattern maze of online or offline options, since the cost is so low.

https://www.selectspecs.com/ if you are in Europe. I've been using them for years.

My indoor (non Transitions™) multifocus² glasses are CR-39¹.

I'd used the same material for my outdoor/sports/driving frames but it's higher density caused them to slide down my nose during activity so the next pair I opted for Trivex.

¹–CR-39 requires full frame spectacles, as drilling holes is verboten.

²–Shamir Autograph III, awesome & highly recommended.

I don't know which type of glass my glasses have, but I know I had to pay a ton for it, because the cheaper sort can't handle my prescription without being really thick and/or heavy. But as someone that actually wears my glasses all the time and not just randomly, it's worth the extra expense.

Interesting, is the lens ultra high-index? How's refraction? As far as I'm aware, high-index, as the index becomes higher, generally will have a progressively worse Abbe number.

A fun fact I learned recently, after years of casually skimming color science, is that our eyes cannot focus the entire visual spectrum at once.

That’s why our cone response to the spectrum looks like https://en.m.wikipedia.org/wiki/Cone_cell#/media/File%3ACone... instead of having cleanly segregated red vs. green responses. If it was segregated, we could only focus on red or green but not both. By having a heavy overlap, we can get a sharp focus on yellow. And, the visual system makes the full spectrum work by deriving the red vs. green concepts from the difference between the two cone responses. Blue focus is accepted as a necessary sacrifice.

>instead of having cleanly segregated red vs. green responses.

I suppose that's why r/g colorblindness is so common.

A few years ago an optician convinced me to get some fancy 1.67 index lenses, but I couldn't deal with the chromatic abrasion. The text at the peripheral parts of my monitor were clearly split into their RGB components. For frontend work it was impossible, as I couldn't tell if something was not aligned or it was just the chromatic abrasion.

Now I always go for the thickest lenses (which are also usually the cheapest) for this reason. My prescription is -3.75, and there isn't any noticeable difference with thinner lenses.

chromatic aberration

https://en.wiktionary.org/wiki/aberration#Noun

All the high index lens materials suffer from chromatic aberration.

The lens material you've now chosen actually has the best optical properties out of all the plastic lens materials. Its only downside is that the refractive index is only 1.5 which does mean they will be a little bit thicker than the high index ones.

You could also try a material called Trivex which also has low chromatic aberration while being a little thinner than the material you are using which is called CR-39

It's not just about the material. It's also about how the lens is made. For me, choosing "freeform" over "aspheric" was night and day.

Where are you ordering glasses from that you can make selections like this?

Firmoo for example. Probably a local optician should also be able to make this choice.

One problem I have (well, one of many these days, including some double vision, sigh) is that I'm very far sighted in one eye, and near sighted in the other, so one lens is barely anything, and the other is a coke bottle.

You could also try Trivex instead of MR-8. Lens thickness should be similar but it has a higher ABBE value.

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