Tech In Plain Sight: Eyeglasses

Glasses wearers, try a little experiment. Take off your glasses and look at this page or, at least, at something you can’t see well without your glasses. Now imagine if you lived in a time where there was nothing to be done about your vision. If you wear contacts or you have good vision — perhaps you had surgery — then congratulations. But for most of us, vision changes with age are a fact of life. Even many young people need glasses or some other intervention to get good eyesight. At first glance, you might think eyeglasses are an obvious invention, but it turns out we didn’t get real glasses for quite some time and modern glasses are truly a piece of high tech that hides — quite literally — right in front of your face.

What’s Wrong?

Human eye with parts labeled
Parts of the eye (CC=BY-SA 3.0 by [Holly Fischer]
Before we talk about correction vision, it helps to know what can go wrong with your eyes. To understand that, it helps to review how your eye works to start with.

First, light enters the eye through the cornea, a clear dome in the front. The light then passes through the pupil, the black dot in the center. The colored part of your eye, the iris, controls how much light goes through, sort of like a lens aperture on a camera.

Inside your eye is a transparent lens structure that focuses the light rays. They pass through a jelly-like substance that keeps your eye round and the focal point is at the retina which contains light-sensitive nerves. Unlike a camera, the retina isn’t flat like a piece of film but curves. However, like any camera, the image is now upside down, but your brain doesn’t mind. As an aside, though, if you flip your vision around so it is really upside down, your brain will eventually dutifully flip it back for you as you can see in the video below.

When Things Go Wrong

People generally think of vision problems as being far-sighted or near-sighted. That is, fuzzy objects up close or far away, respectively. However, you can also have astigmatism which just causes general fuzziness and what we think of as far-sight can be caused by two distinct problems with your eye.

Astigmatism is when the shape of the cornea is not perfect, so light coming in can wind up at more than one spot on the retina. If you have astigmatism, everything looks fuzzy and something like an LED will appear to be more than one LED from a distance.

Hyperopia, a type of far-sightedness, and myopia or near-sightedness happen when the length of the eye is not correct or the lens system has an incorrect focal length. For hyperopia, the image focuses behind the retina and myopia has the focus ahead of the retina. The other cause of far-sightedness is presbyopia which is where the center of the eye’s lens hardens with age. The end effect is the same as hyperopia and it is why as we get older we can’t read fine print.

The Optics

Lens and prism diagram
You can think of a lens as two prisms either base to base or apex to apex

You can think of a lens as two prisms. For a concave lens, the two prisms meet at their tips. For a convex lens, they meet at the base. If you aren’t used to thinking of a lens as a pair of prisms, you might enjoy the video below.

As the video mentions, light sort of bends around the base of a prism. Ok, it doesn’t really bend, but that’s a good way to think about it. So as light goes into a concave lens, it tends to spread out but through a convex lens it tends to converge on a spot some distance from the lens — the focal length.

That’s true, at least, of a spherical lens. You can also have a cylindrical lens that focuses to a line instead of a point. If you need both types of lenses at one time, you need to find a toric lens.

By making the image spread out or converge before it hits your eye, glasses can correct for common vision problems. With a cylindrical lens, you can fix astigmatism, too. Obviously, if you have multiple problems, you’ll need a toric lens.

Ancient History

While it seems simple to create a lens and hang it in front of your face, there are two parts to that. First, you have to know how to make a lens or find one that occurs naturally. Next, you have to have the idea of how to suspend them in front of your eyes.

Glass has been around for at least 4,000 years, but not high-quality glass. There are quite a few claims of ancient lenses used to either magnify or focus the sun to start fires, but they are either natural stones or very poor quality glass, and there is debate if they would have worked for either case.

The Romans became good at making glass in the first century and realized that a bean-shaped piece of glass — a convex lens — would make objects appear bigger. The word lens comes from the Latin word for lentil.

However, history is a little vague about the use of lenses to help people see. It looks like an Arab created reading stones in the 900s and they became relatively common by the year 1,000. If you’ve ever used one of those solid plastic bars that you put on something small like a phone book to make the text bigger, that’s the same idea. Of course, the material would be glass or crystal.

Glasses Arrive

El Greco portrait showing a cardinal wearing glasses
This portrait by El Greco from about 1600 shows Cardinal Fernando Niño de Guevara wearing glasses (Public domain)

It would be the 13th century before we started seeing what we would think of as simple glasses. Early glasses in Italy were crude blown glass held with a frame of leather or wood. Until around 1600, though, you’d hold the glasses in your hand or push them down on your nose. The early glasses were all convex, although by the 1400s concave lenses were known. It would be 1604 before Kepler would explain why they all worked.

Around 1600 someone worked out how to clip glasses to your ears although this is often attributed to Edward Scarlett in 1727. As you can see from the adjoining El Greco painting, though, some people had this style of eyewear as early as 1600.

Earpieces were a game-changer although you can still find the occasional monocle or pince-nez. There was still one major problem: glasses strong enough to help you see things close up made a mess of things that were far away.

Benjamin Franklin, who had poor vision, invented the bifocal where each lens had a near part and a far part. There have been some claims that Franklin didn’t invent them but, rather, popularized them. It would be 1825 before astronomer George Airy would figure out how to correct astigmatism.

Modern Glasses

Like everything else, glasses went high-tech at an accelerating rate through the 20th century. Frames are now made with memory alloys that return to their original shape. Lenses made of special material are lightweight and durable. They also can have coatings to reduce glare, block UV light, or turn dark in bright lights.

One of the biggest improvements stemmed from the bifocal. First, there were trifocals with three lenses. However, now sophisticated lens grinding techniques allow progressive lenses where the lens has different properties that vary continuously across the lens.

Computers can grind lenses in classic shapes or create aspheric or atoric lenses that can correct vision in more sophisticated ways than a normal lens. You can see a common eyeglass lab’s process in the video below.

RX and Other Solutions

It helps to know Latin if you want to read an eyeglass prescription. You’ll see numbers under the heading OS and OD and rarely OU0. The O is for oculus (eye), the S is for sinister (left) and D is for dextrus (right). The U means both eyes.

If you don’t have astigmatism, you’ll see diopter numbers for each eye. This is the amount of focusing change you need and lower numbers are better (a diopter is the reciprocal of the focal length in meters). A negative number means you are near-sighted and a positive means far-sighted. A zero would mean you didn’t need glasses, so you probably won’t see that unless you have only one bad eye.

For astigmatism, you’ll see three numbers. The first is the diopter, the same as above and is referred to as the SPHERE. The next number is a measure in diopters of how much astigmatism requires correction (CYLINDER). The final number is the axis between 0 and 180 degrees which gives you the rotation of the correction.

If you need bifocals or progressives, you’ll see an ADD number as well. This is the additional diopters needed for a bifocal. In the case of a progressive lens, of course, there will be a continuous slope of magnification across the lens. This number is nearly always the same for both eyes, but if one of them has PAL next to it, then it should be used for both eyes with progressive lenses and the other number is for proper bifocals. You’ll sometimes see a PD or pupillary distance. That’s for setting up the frames but doesn’t affect the optics.

You’d think with contacts and laser surgery for vision correction eyeglasses wouldn’t be very common. But they are. Why not? They are relatively cheap, don’t take much in the way of maintenance, and they work well. While the tech that goes into them today requires computer-controlled grinding machines and lots of math, even the simple glasses available centuries ago must have seemed like a miracle to people who could no longer read or see things clearly.

It is possible to build your own lenses, but it is hard to match the quality of a real lab. Glasses used to be nerdy, but now they are fashionable. If you want to look like a nerd again and you need progressives, maybe you’ll prefer a pair of these.

47 thoughts on “Tech In Plain Sight: Eyeglasses

  1. Two items that fall under more recent technology:
    1) There is a drug that has just been approved to temporarily treat presbyopia by changing the pupil response (it improves close distance viewing with a small worsening of far distance viewing)
    2) No word on LASIX? Laser reshaping of the lens is pretty cool, but has its limits: you have to have enough lens to carve away. Some profoundly nearsighted folks (e.g. me) may have an elongated eye such that you can’t change enough of the lens to fix the focal point.

        1. I’ve heard that people who have had their natural lenses replaced with artificial ones can see UV light (because the blue cones in the retina are UV sensitive but the lens of the eye block UV) so it that true? Can you see UV through your new lens? Have you tried? Does only one eye have a fake lens and the other a real one? Can you do a comparison? What’s it like having superpowers?

          1. One thing I immediately noticed after they took off the bandages the next day was how everything looked strongly blue compared to my other eye. The surgeon said that it’s because the other eye has a 66 year old yellowing biological lens it it. He said that my brain would adjust and sure enough, after 3 months there is no difference (except for some floaters in the operated eye).

            But seeing UV? I have a UV flashlight and the light does look somewhat brighter than the other eye.

          2. When I got my lens replacement I had about a month between the two surgeries. At that time I noticed that I can no longer see the blue glow from UV “blacklight” lights with the artificial lens. It actually is advertized as a feature: the artificial lenses protect the retina from uv radiation.

            Of course this has basically no noticeable impact on the appearence of fluorescing objects. Neon colors are still fine :)

            It is however a loss I noticed. But easily compensated by the gain in color perception: apart from the natural yellowing of the lens also the cataract makes the world more greyish, so on that front it was a clear (pun intended) improvement.

        2. Well Michael maybe, since you have a UV flashlight, try a few tests to see if you really can see UV. Maybe get one of those filters they use for UV photography, see if on a bright day you can see a difference in brightness through it with your “new” eye vs. “old” on a bright day. I’d be fascinated in knowing if it’s really true you can see UV.

      1. In ancient times they would stick a needle in your eye and push the cataract down so you could see over it. They also discovered how to insert a hollow needle and suck it out. You wonder who first tried that.

        1. Yeah, well that’s the first part of the surgery. Liquefy the old lens and suck it out before inserting the new lens. But if you do that without replacing the lens you can’ see anything but a blur of light I think so how could it be effective back then?

          1. I understand that in the old days (without lens replacement after removal) you would receive eyeglasses that actually took the place of the eye’s lens. Look for a picture of Israeli PM Menachem Begin, who I believe had such a procedure.

          2. Anthony maybe I’m wrong but I thought seeing UV occured for people who had a gap in time between having the bad natural lens extracted and having a replacement. But not many types of materials used for glasses allow much UV to pass. So I dunno.

  2. I’ve been very confused by two prescriptions I got in different countries.
    The new was similar to the old, except the sphere value was equal to the sphere + cylinder value of the old one and the axis was rotated by 90°.

    Is this related to a way things are preferred in some countries or in an industry (medical vs optical process)?
    I don’t know if those notations have a specific name, but things could have gone wrong if I would have just come with my old prescription.

    1. Prescriptions written in plus cylinder format are a throwback to the days when lenses were made with the cylinder power ground into the front surface of the lens. It is (in the US at least) mostly Ophthalmologists (opticial MDs) who write this way. Mostly because that is how they were written when ophthalmology started. Minus cylinder format is more correct as lenses are made in that format; with the cylinder being ground into the backside of the lens with the spherical power. OD’s write in this format because it came into being at around the same time they did. The two formats can look very different and have the same power. For example 0.00 +0.50 x090 = +0.50 -0.50 x180.

    2. The phoropter (the vision measuring machine) readings depend on your subjective perceptions. No two readings will be exactly alike. The readings may also be influenced by things like what you did with your eyes in the hours prior to the exam, image contrast (did one of the machines produce brighter light), etc. Overall correction (sphere + cylinder) is the easiest for you to perceive, but when it comes to determining how much of your error is due to each one, you may have a harder time telling which of the two images presented looks better. And cylinder axis is the hardest for you to tell. If you are like me and have trouble telling which of the images looks better you may start producing inconsistent axis answers and the examiner may decide that they have better things to do then spend any more time on you and declare that you’re done. I recommend going to a place with a phoropter that shows the A vs B images side by side instead of sequentially. Makes a huge difference. And don’t let the examiner stop the exam if you don’t feel you are done.

  3. On a related note, are there any notes on when sunglasses were invented? The question has sometimes been in the back of my mind ever since I went to visit a historical exhibit and the tour guide was wearing sunglasses with an eighteenth century costume. This looked a little jarring and my first thought was this couldn’t possibly be period correct, but it wasn’t long before I remembered “They could make bifocal glasses then; nothing should have prevented them from making tinted glasses too.” But when did opticians start offering glasses with tinted lenses.

    1. I don’t know about sunglasses specifically, but people regularly smoked glass for viewing things like the sun or very hot items (wrought iron puddling, glassworking) once they realized that you’d go blind from cataracts quickly if you didn’t have some sort of filter. I believe that happened well before glasses themselves were invented.

  4. I wish “cheaters” (reading glasses) came in more diopter numbers.
    Then I might be able to buy two pairs (one for each of my eyes Rx), pop the right or left lens out of each, swap into the other frame, keep one pair and donate the other pair to the Lions.

    1. >Then I might be able to buy two pairs
      Never thought of the idea of swapping lens for L & R eye! Great Idea!!! Of course, given that I have readers stashed EVERYWHERE due to distraction-induced short term memory failures (e.g. I just set them down, but where oh where….), I’d need significantly more than two pair…

  5. There is a study of stats that finds kids that don’t get enough sun and brightness develop myopia because the muscle that works the iris doesn’t get enough use. I see those mini blinds in so many SUV’s toting children.

    With my myopia though I can inspect closeup and don’t need or can use a magnifier. I am always complaining that others don’t see details in finished work. I tend to use a tablet without glasses. Cold solder work won’t hide from me.

  6. Before the tsunami there was a Thai tribe, the Moken, who lived in houses built on stilts in the shallows. Children grew up spending a huge amount of time in the water. Usually people have bad vision underwater because the air-cornea interface refracts much more than a water-cornea interface. With this tribe though, the kids’ eyes’ lenses learned to accommodate it and they had great underwater vision. In effect, their lenses changed shape to become more like a dolphin’s. Testing revealed that it wasn’t genetic, nor a consciously learned behaviour, but that other (European) kids could after several sessions also start adapting in the same manner.

  7. I have not had Lasix because I was told that the resulting scarring causes stars viewed through a telescope to have diffraction “spikes” similar to those seen from the secondary mirror supports in an ordinary reflector scope. I like my astronomy. :-) Can anyone confirm this?

  8. Last time I ordered glasses (progressive bifocals) I also ordered a separate pair of readers that included the cylinder and axis for each eye. The result seems (to me) to be superior to ordinary simple magnifiers.

  9. For decades I suffered from double vision, because my eyes couldn’t relax to facing forward in parallel for very distant objects, thus my lenses include a prism element. My previous pair of glasses had a prism value of 3.5° in each lens to create a combined angular shift of 7°, making the two images overlap enough that my brain/eyes could correctly ‘lock’ them for stereoscopic vision. I say previous pair because over Covid lockdown I hardly ever wore my glasses as I usually only wore them for driving, and during lockdown I drove very rarely, and I was astonished to realise one day in early March this year that my double vision had corrected itself! I could converge my eyes correctly on distant TV masts on the horizon without my glasses.

    I am assuming that going for long periods without wearing my glasses had allowed my brain sufficient time and incentive to adapt its ocular control, in a way that it hadn’t previously found necessary.

    My optician however wouldn’t accept such a large change in prism in one step, so we compromised on 1.5° per lens, and next March the prisms will gone.

  10. I always wondered how they make prescription lenses to fit differently shaped frames.
    Is there a set of standard lens shapes? Surely they don’t send exact fame measurements to the lens maker.

    1. Measurements are taken for the position and distance between your pupils.
      It might be possible to trim down a bigger lens for a smaller frame, assuming the distance between lenses doesn’t change much, but it’s unlikely to be worth it

    2. “Is there a set of standard lens shapes?”

      Yes, a lot of eyeglasses are cut from round blanks to fit the frame.
      I think there are also blanks that have astigmatism factored in, the blank is rotated within the cutter to match the degrees rotation needed for the Rx.

  11. …and even with all todays’ advances, your lenses still get scratched up after a few months if you even think about not cleaning them in a certain way.

    Yes, I know that you can get several pairs of glasses for a bargain these days; yes I know that you shouldn’t wipe them with whatever is at hand; I know that you can pay more and get “better anti-scratch” coatings, but it’s still unnerving.

    Why don’t we have (as close to completely as possible) scratch-proof lenses yet at an affordable price?

    If price was not a problem, are there lenses that wouldn’t get small scratches after a wipe-down with a basic paper tissue after a few hours spent in an indoor construction site or a DIY workshop, for example?

    1. Back in the 1970s, there was a product called Glaze Coat (IIRC) that could be wiped on scuffed lenses to fill in the scratches. But, an Optometrist would charge $70 (1970 $) for the coating.

      Back then they would charge $80 (1970 $) for a set of contact lenses (glass), but they only paid about 15 cents each for them!
      (my sister worked for an Ophthalmologist back then).

  12. I thought each eye had its own retina. And that astigmatism was an error between the two eyes such that you see two separate images instead of one. If one eye saw two images on its retina then when you add that to one or two images from the other eye wouldn’t we see three or four images? Unless one of the images overlaps one from the other eye. But if we close one eye wouldn’t we still see two images if indeed two images were focused on that eye’s retina?

    I think your definition of astigmatism is incorrect or improperly stated. Unless I’m misinterpretating what was written.

    1. Astigmatism is caused by the cornea. With astigmatism if you look at a point of light, it gets projected onto the retina as a smear or a line of light. Problem of one eye alone (although if one eye has it usually the other does too).

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