Notes about eyeglasses

From Amit’s Thoughts

These are my barely-organized notes about my need for eyeglasses to correct my nearsightedness. It is likely that I have some of this wrong because I don't fully understand it.

Correction

When an optometrist measures your "prescription" they're measuring the 0th and 1st order zernicke polynomials. The 0th order is "sphere" and is a single number, rounded to the closest 0.25. The 1st order is "cylinder" (astigmatism) and has two numbers: a direction axis and an amount. There are higher order aberrations too but the optometrist doesn't measure these. These higher order aberrations can cause halos and other effects; see this example image that shows a "coma" (point smeared out like a comet), "spherical aberration" (starbursts and halos), and "quadrefoil" (point spreads out multiple directions). Laser surgery can often correct these but sometimes the healing process generates new ones.

Focal range

The eye can adjust the focus between far and near. Being "nearsighted" means your focus range is too near, which means you can't reach the far distances, and some of your near range is wasted. Being "farsighted" means your focus range is too far, which means you can't reach the near distances, and some of your far range is wasted. All the corrections (glasses, contacts, laser surgery) move the range but don't expand it. As you get older the range shrinks; this is called "presbyopia".

If you are nearsighted and get laser surgery, you'll move into the Regular range, but as you get older, you'll move into Presbyopia-far. This will mean you will need reading glasses. If you are nearsighted and didn't get laser surgery, you'll move into Presbyopia-near. This will mean you won't need reading glasses, but you'll need to take your glasses off to read things. In my case, computer use is in-between. It's not great with my regular glasses, but it's also not great when I take glasses off. I ended up getting in-between prescription glasses that are sharp at medium distances.

Eyeglass materials

I chose to wear glasses instead of contacts or getting laser surgery. I collected some information about different lens materials.

• http://www.allaboutvision.com/lenses/how-to-choose.htm - table of properties
• https://web.archive.org/web/20090815102701/http://www.hbutz.com:80/wwwboard/messages/201.html - lots of ranting, but there's a data table near the end
• http://www.eyecarebusiness.com/articleviewer.aspx?articleID=107335 also has a table
• https://www.eyecarebusiness.com/issues/2012/august-2012/a-look-at-lens-materials
• https://en.wikipedia.org/wiki/Corrective_lens#Ophthalmic_material_property_tables has a table
• https://web.archive.org/web/20160322110111/http://www.opticampus.com/tools/materials.php had a table
• http://64.50.176.246/tools/materials.php
• http://ophthalmiclenses.blogspot.com/2011/07/lens-material-reference-chart.html

nd, ne = refractive index cvf = curve factor, lower is thinner density = heaviness, lower is lighter uv = wavelength, higher cuts out more uv I think abbe = chromatic abberation, higher is better r% = reflection, lower is better, higher needs anti-reflective coating, factor of 2 depending on how reported

I merged the tables from the sources above into:

Other notes:

• Trivex impact resistant, lighter but thicker so ends up being same weight as higher index, better abbe than most materials, very good uv blocking (don't need uv coating), scratches easily
• Seems to be some tradeoff between lightness and thinness
• This thread says polycarbonate is the standard but CR-39 (thicker and cheaper) and high index plastic (thinner but expensive) don't get scratches as much as the middle options
• High density leads to more reflectance
• Of these, Trivex and MR-8 seem most interesting, best Abbe number
• Blog post explaining why LED backlit monitors have worse chromatic aberration
• this page claims Trivex has minimal "material stress" which affects overall clarity
• this page claims MR-8 has minimal material stress
• also "Trivex has high resistance to circumferential deformation."
• this page says MR-8 easier to drill, for rimless
• MR-8 has an abbe number of 41 compared to 32 for most high index
• Trivex has abbe 45; glass has 59
• Trivex has much lower density than other materials, and for -3 to +3 it'll be the lightest lens, and outside that range the thinness of high index wins over the lower density
• Glass, cr-39, trivex are ground whereas polycarb and MR-8 are molded. Grounding produces sharper images. But MR-8 is glass-molded which is better than injection-molded.
• Trivex is slightly yellowish
• CR-39 can shrink over time, popping out of the frame

I tried to find MR-8 but couldn't, and chose Trivex instead. I have gotten some scratches on it, although I don't have a proper comparison with other lens materials. But otherwise it's been a good choice for me. [Update: 2024 - Zenni claims to have MR-8 as an option]

I am sensitive to chromatic abberation. I see the color fringes on my screen. Even if you don't see the fringes, it's making things blurrier. The Lateral Chromatic Abberation is the prescription value divided by Abbe number. For example, if your prescription was -5 and you were considering high index lenses with Abbe 30, the lateral chromatic abberation would be 0.16. This page has a table:

If you instead went with Trivex, -5 divided by 44 for trivex is 0.11. On the table, that's the difference between 20/24 vision and 20/22 vision. Not a big difference but it is a reminder that lens material affects how sharp you see. Thinner isn't necessarily better!

Other

• This article claims Abbe is overrated, and polycarbonate aspheric lenses are better than cr-39 spherical lenses.
• This article explains base curve but I haven't read it. At the end it suggests Sola Enimga lenses may be amazing (and I found a forum post from someone who had them, said they were amazing) but Sola's no longer making them
• This page gives the lens thickness for different types of materials.
• Free form lenses sound interesting.
• Blue blocking spectrum diagrams show which frequencies are blocked.

That's what I've collected so far. Any other resources I should know about?

Labels: health