I’ve been researching eyewear before I blow ~$100 on a piece of plastic that goes over my eyes which supposedly 1) doesn’t let UV-A,B,C through 2) polarizes light so, for example, snow won’t be so reflective 3) distorts the imagery as little a possible (“optically perfect”) and 4) also doesn’t break easily or obstruct field of view. The first two are pretty obvious (but are all of the glasses that polarize doing it the same way when they don’t all seem to be the same color or transparency?).
My main question is how does one gauge or measure this “optical perfection” that all of these companies are claiming. For example, one of them makes the claim that a technology called “Polaric Ellipsoid” allows their eyewear to distort light less than any other company’s, though I could find no literature or objective testing even explaining what their term means. The claims by different manufacturers are never ending, and I couldn’t find any objective optics sources to scrutinize them scientifically. Some claim that their polycarbonate lenses are as good as it gets, while others claim that polycarbonate is not a good lens material. Are these just marketing techniques under the facade of science? How can one ‘benchmark’ these types of optics (for “optical perfection” and such)? Thanks for the input.
I’ve been wearing glasses for nearly twenty years now. One thing to think about - any optical distortion (and with modern lenses and a reputable maker these are nearly negligible) - your eyes and brain adjust to get used to any pair of lenses that are “close” to good within a few weeks.
Buy lenses based on the secondary features. Don’t sweat the basic optics.
Sorry I didn’t specify, but these aren’t for any sort of corrective purposes (I thought that was implied by “optically perfect”). These are for general use, and activities like biking (where 25mph winter air in your eyes does a number on them.
Lens thickness usually isn’t specified. I understand that optics problems are solved, but when buying optics from manufacturers who refuse to give meaningful specifications and benchmarks to avoid competition - it becomes more difficult.
Having studied and worked with optics a little, and worn glasses for about 20 years now, I’m pretty convinced that the companies that peddle glasses don’t give the consumer much to go on. There must be some people in there that understand optical quality, but they must not have much to do with creating sales literature, and they must be pretty outnumbered by the fashion people.
The point of polarizing is to make the glasses selective for what orientation of nonconducting smooth surfaces will be visible by their specular reflections at low angles. If you hold a flat windowpane closer to parallel than perpendicular to your line of sight, but still angled to it, you will easily see reflections in the glass of the scene more or less in front of you. The light by which you see those reflections will be polarized. In fact if you hold the window at Brewster’s angle (which depends on the glass) the light will be completely polarized. The polarization will be oriented in the plane of the glass and perpendicular to your line of sight. Polarizing glasses will block some particular polarization, generally horizontal. That means road glare and other light reflected from horizontal objects will be reduced. Metallic surface objects are not included in this, because they reflect by a different mechanism.
The degree of polarization of glasses could be tested by looking through two pairs in series, and orienting them at right angles to each other. Try holding two pair up so you can see through two lenses one after the other, perpendicular to both lenses, and rotating one of the pairs around your line of sight to see how dark you can make them. Excellent polarizers used in optics labs can become so dark when viewed this way, IIRC, that only 1 part in 100,000 of the light comes through. Eyeglasses won’t do this well, but you can compare two brands for the completeness of their polarization in this way.
If you like, there’s another test you could do that would be an objective measure of optical quality, though it would be an imperfect one.
You could use a camera with a good quality lens operating wide open to take a picture through the eyeglasses. I’m thinking of a fixed focal length (not zoom) lens on a 35 mm camera, or even a medium format camera if you have one or know somebody who does. A 50 mm f/1.4 lens on a 35 mm camera, or even better an 85 mm or so operating at f/1.4 or 1.8 or 2, as wide as it will go. Such a lens is expensive because it is hard to make it sharp when using the entire aperture. It is also harder to make that large of a region of a pair of sunglasses sharp, and that is why this would be a challenging test for them. You want to use a slow film, which will have finer resolution (and also make it easier to use the lens wide open). Focus very carefully and shoot a distant scene with lots of fine detail, like a big brick wall hundreds of feet away. Use a tripod and several times defocus the lens, carefully refocus, and take a shot, repeating this to improve the chances of getting just the right focus. Then look at the negatives under high magnification to see how good the detail is.
This needs to be done carefully and there are a few things that can go wrong. It is also unfair, perhaps, in the sense that your eye doesn’t need to use that much of the eyeglass lens at once. But, if there is an area of the lens where its focus is changing (more precisely if there are areas whose curvature is a little different), then those spots will work less well when you happen to look through them.
By the way, there are a few people here who seem to me to do more with optics and have more experience than I. Cal Meacham comes to mind, but I think there are others too. If you are lucky they may come along…
I searched for similar information the last time I was buying a pair of glasses, and I found a bit. Specifically, I was trying to see if there was anything that I could do to avoid chromatic aberration – basically my glasses act a bit like a prism when I look out of the side of the lenses. All lenses will do it to some degree, but it’s really distinct with a very high prescription. Anyways, wikipedia had a good article about corrective lenses. In particular, look at the section on the Abbe number (a measure of optical quality that determines chromatic aberration among other things). There’s also a useful description of lens materials.
The other other objective measure for a lens material is its refractive index. A higher refractive index means a thinner lens. Low refractive index means you could have the old coke bottle glasses. But, if you don’t have a strong prescription, the thicker lens might not be all that thick.
Picking a lens material will be about a tradeoff between optical quality, refractive index, weight, and cost. For example, polycarbonate lenses will cost a bit more than standard plastics and will have a lower Abbe number, but they’ll be a lot thinner and lighter and they’re also pretty durable.
I ended up picking out the polycarbonate lenses, which are pretty standard these days, since I wasn’t willing to spend another couple hundred to get high-index polyurethane lenses (which would be thinner and have a higher Abbe number).
Non-prescription sunglasses are pretty varied in what you get for your money. And much depends upon taste too. Assume you are being ripped off. Because you are. I once had a very interesting conversation with some guys who worked for a very very large lens manufactuer. They told me about a very well know brand of sunglasses. This brand was in essense a phantom. They were an office somewhere. The design of the glasses was contracted to various fashion designer houses. The lenses were purchased from the company the guys I was talking to worked for. They cost $1 each. They were shipped directly to the frame manufacturer, who installed them into the frames. The frames cost $2 each. The glasses company had the boxes manufactured and directly shipped to the frame manufacturer, who for a tiny extra fee put the sunglasses into the boxes. The finished products were then directly shipped to the agents in the destination markets. At a cost of about $5 per pair. Advertising on TV, billboards, and glossy magazines acounted for more than the productin cost of the sunglasses, and was of course outsourced too. The glasses sold for about the $200 mark. The brandname company never laid a finger on them. If you saw a pair of “no-name” rip-off copies of the glasses at 1/4 the price you should buy them. They were not a rip-off. They were the same item, just without the name badge. Exactly the same product otherwise. The rule is simple. Don’t buy fashion brands. Ever.
That said, there are good sunglasses, with good and useful technical attributes. But it depends upon what your needs are. Even polarizing is not always the best thing to have. Depends upon what the activity is. A personal recomendation - which is worth what you paid for it. I have been wearing Maui-Jim glasses for a while, and they are good. The clarity of vision and support from the company beats anything else I have had. I sail, and I was simply stunned at how much better their sunglasses were than others I had tried. We have a rule sailing these boats. If you can’t afford to lose it, don’t wear it. However I have broken this rule with these glasses, the improved clarity is worth it. I really would not have believed it. On the other hand, they are expensive, and I’m sure the margins are eye watering.
I was just reading about lens material, which was very helpful. Thanks for the abbe number link lazybratsche. I learned that Polycarbonate lenses cannot compete with CR-39 or glass lenses. Apparently, glass is optimal - so I guess I’ll be looking around for the cheapest glass sunglasses (no prescription). UV protection and polarization would be nice too.
There is at least one brand of non-prescription sunglasses (Ray-Ban) that still uses glass, but that’s about all, as far as I know. That doesn’t help me, though, because I need prescription sunglasses.
It’s next to impossible to find glass lenses these days, and this is a pet peeve of mine. I wear glasses full time. I’ve been wearing glasses for at least 35 years. Can’t do without them. I have a clear pair, and a pair of sunglasses, and usually at least one spare pair of clear glasses around somewhere. And I have a pair that I wear for motorcycling and cycling, made out of polycarbonate for safety reasons. I’ve been wearing glasses for at least 35 years.
I like glass lenses. They just feel better to me. And the extra weight doesn’t bother me. These days, however, it is impossible to find glass lenses.
None of the opticians around here, at least none that I know of, will make glass lenses. At first they just would refuse to make glass lenses for rimless frames. I have worn the same frames since high school. Well, not the same frames, but the same style of rimless frame, from the same manufacturer. At some point, I ceased to be able to get anyone to make glass lenses for them. I could still get glass for my sunglasses, which are full wire frames. But then I couldn’t get glass lenses at all.
I’m well aware of the limitations of glass when it comes to safety. But for every other reason, glass is better, and I’m pissed off (well, mildly) that I can’t get glass lenses anymore.
To be clear. For non-prescription sunglasses Abbe number is vastly less important. Since they are not corrective they can be a thin as mechanically reasonable, and are not intended to be refractive. Chromatic abberation isn’t nearly as big an issue as for corrective optics.
Glass is always heavier. And easier to break or chip. But for the absolute ultimate in performance it may have a slight edge. But a cheap (non-anti-reflection coated, lesser glass) pair will not be as good as a good pair of plastic lenses. There are high performance glass sunglasses. The prices can be breathtaking.
