There are some experiences that eye doctors can explain that you don’t normally expect to happen with eyeglasses. Everyone seems to think that when you order up a pair of glasses to replace the ones you have that it should be simply an experience of seeing ‘better’ than before. Actually it’s not quite that simple.
First of all single vision glasses are going to correct for a specific type of vision. For the vast majority of people that’s distance vision. We all want to be able to see the dust on the hairs of the back of a fly at 5 miles, but without some serious additional hardware, it’s not going to happen. So the usual goal is to give people the ability to see at about 20 feet what the average person can see at 20 feet. This is the 20/20 vision thing that people keep talking about. If I have ‘20/15′ vision I can see at 20 feet, what the average person has to get to 15 feet to see. If my vision is 20/500 (which is not on the charts technically) I have to get to within 20 feet to see something that the average person can see, or more accurately ‘read’ at 500 feet. At some level before this you cross a boundary that says that you are ‘legally blind’ although that’s not usually described for most people.
Where ‘legally blind’ comes in is if the corrected vision is beyond some threshold, then you are considered legally blind. You may be able to see things, even recognize people, but you are unlikely to ever be able to see well enough to consider yourself safe as a driver.
The thing is that if you correct someone’s distance vision to 20/20, that may not help them with near vision. Or for that matter with vision at arms length, say a computer screen. This is where bi-focal and tri-focal lenses come in. In general these are called multi-vision lenses. And while the 20/20 description may be used, it really is a bit misleading. What 20/20 near vision means is not ‘I can see at 20 something what the average person can see at 20 something’ as 20/20 distance vision means. It means that I can see at normal reading distances what the average person can see at normal reading distances. 20/40 here would mean that I can see at ‘normal’ distances, what someone with normal vision could probably see at about twice that distance.
For most people the reason you go with multi-vision glasses is that the correction applied for distance vision is either too strong for near vision, or not strong enough. The amazing thing about the human eye is that the lens in your eye is adjustable. You have muscles all around your lens that will pull it to allow you to correct for near or far vision. If you take a piece of paper with printing to the top edge, you can hold that piece of paper up, and see across the room just above the piece of paper. Focus on the far wall of the room. Without changing your focus, the text on the page is within your vision, but you are unlikely to be able to read it. Now if you focus on the page, it takes a brief period of time to re-focus so you can see clearly at that distance. That brief period of time is your eye and brain adjusting the amount of strain that your eye’s lens is under to bring the piece of paper into focus. And with it in focus, the things that you were just looking at across the room, are still there, but are no longer in focus.
As you get older the lens in your eye has gone through millions or potentially billions of these adjustments. If you are familiar with materials sciences you will recognize that the fact that the eye’s lens holds up to these changes for tens of years is pretty amazing. There are not a lot of materials that we know of that are transparent to wavelengths of light in our visual spectrum that can withstand the flexing and relaxing that the lens of your eye goes through. For that matter there are not all that many substances that can withstand that level of change that are opaque. To put it in context, the method that science has created for changing the focus of a lens construction is to move the parts of the lens assembly closer and further away from the surface that the image will be focused onto until they get a correctly focused image. No one in their right mind would normally consider squeezing or stretching a flexible piece of material to change the focus for a camera.
As amazing as the lens in your eye is, it does break down after a while. The first sign of this is often that you have more and more trouble reading things at normal reading distances. If you find you are holding a newspaper at arms length to read the articles, that you can’t seem to focus on the page when it is nearer to you, you are beginning to suffer from this problem. Longer term you end up suffering from the lens itself significantly beginning to break down, with a condition called Cataracts. Essentially parts of the lens become opaque to light, or at best translucent.
With all of that in consideration, the usual stopgap solution is to use multi-focus lenses for glasses. American tradition has it that Ben Franklin invented bi-focal glasses by taking two pairs of single vision glasses, cutting the lenses in half, and pairing up the near vision glasses as the bottom half of each lens. This leads to some obvious ‘lines’ where the change in correction happens. Over the past 30 or 40 years, that ‘line’ has been being erased. First by grinding the edges of the lenses till they don’t have a change in the surface, then by changing the way that the second focus area was created. Instead of cutting a hole in the lens and fitting a different lens into the space, the primary lens is ground differently in the secondary focus area to provide for that zone’s correction. At that point the area where there would be a line is now a continuous piece of plastic that provides a correction that is different from either near or far distances. This is really where tri-focal lenses started becoming possible and not all that uncommon. A fairly well known brand name for these are ‘Transitions’ lenses.
But that isn’t the only thing that provides an issue when changing from one pair of glasses to another. People often think of lenses as being either convex, looking like two parentheses markers like () or concave, like )(. The reality is that there are a wide variety of combinations of surfaces that are created when making lenses. You can have lenses with one side being flat |( or |), or even with one side being concave and the other convex. (( or )). How lenses work is related to the difference in the angle of incidence that the light entering the lens encounters as it enters the lens, compared to the angle of incidence that the light encounters as it exits the lens. Different lens materials also affect how light ‘bends’ as it goes through these material changes. Glass has one variety of change, water another, various plastics all have differences as well.
I know, you’re wondering what does all this have to do with why changing from one pair of glasses to another would cause any problems. If I give you two different pairs of glasses, and they have the same lens centers in front of your pupils, and the same correction, you should see the same with each of them. Right? Well, almost, but not quite. It turns out that one of the things that affects how two different lenses function when the eye looks through them is the first angle of incidence that the light goes through. If I give you a pair of glasses that are ground so that the correction is right, with a concave outer surface and a convex inner surface, then give you a pair of glasses with the exact same correction but the outer surface is convex and the inner surface concave, you will perceive the difference. You won’t be able to define quite exactly the issue is between the two, They both will correct your distance, or near vision equally, but you will experience the world around you differently. In fact your likely to experience some form of nausea or even headaches for a brief period of time after you change from one pair to the other. The world will seem to function differently as you move your head around. Again it is hard to define the difference, but it is most definitely different. For most people there is a period of between a couple of hours, and a couple of days after you get a new pair of glasses before things seem ‘normal’ again.
One of the services you can have done, and this does depend on the company you get your glasses from, is to have them cut the new lenses so that the outside angle of incidence is the same as your old glasses. You there, I see your hand waving. Yes you could get both inside and outside surfaces cut to the same angle as your old glasses. Unfortunately if you need the added correction that your prescription asks for, that doesn’t help, unless the material is changed as well so that the new material provides a sufficient change in the correction of those cuts. Unfortunately that’s a lot less likely than you would think.
So why do I bring this up, and what does it have to do with the title of this post?
If you’ve been following my posts for a while, you should be aware that I’m having my vision corrected at my cornea. I’m getting a procedure called PRK done that ablates the surface of my cornea, sculpting it so that my cornea provides a correction to the path of my vision in place of glasses. About 4 weeks ago I had my right eye done, and in just over 2 weeks I get my left eye done. For the past 4 weeks after the procedure was done to my right eye, I wore what was left of my contact lens prescription so that I could interact with the world in a somewhat ‘normal’ way. However because contact lenses do change the surface of the eye in subtle ways, eye doctors do not want to have to deal with those subtle changes rebounding in addition to the correction they apply to the cornea. It can leave the eye with a correction that is vastly incorrect, and may result in needing to repeat the procedure again later on. Besides being expensive, it means that you spend a lot longer recovering. So they ask that depending on the variety of contact lens you wear, that you don’t wear the contacts for between two and three weeks before your exam, or your procedure. Tomorrow is my 2 week before mark, so when I woke up today and realized that I had a choice between taking out my contact lens, cleaning it and putting it back in, or taking it out and switching to my modified glasses that I’ll be wearing for the next couple of weeks, I decided that now was as good a point in time as any to switch to the glasses.
At some level I am glad I did. Wearing one glasses lens with a correction, and the other eye not having any corrective lens in front of it is leading to some interesting experiences. Once you have been wearing glasses for a while, you stop noticing that there is a portion of your vision that either has a ‘gap’ or an ‘overlap’ at the edge of your glasses. In my case there is a bit of an overlap. Meaning that if I look at something near the edge of my glasses, I can see a second image of it just outside of my glasses. Obviously the focus is different, and I’m not even counting the fact that the correction the glasses provides at that point is not intended to be looked directly through. This is often called distortion, although that is not always the correct term. Distortion in my opinion is changing the appearance of something in a way that makes it difficult or impossible to identify what it was originally, or to give a view different from that expected. What I can see is reasonably clear, even if the focus is not exact. In any case the place this makes things ‘interesting’ at the moment is that without a lens in front of my right eye, I can look at the frame of the glasses, and I don’t see the effect that I see out of my left eye.
At the moment my brain is learning to compensate for the differences between the way that my left eye and right eye are receiving the image of the world. There is a difference in how things track as my head shifts left and right, as my eyes move left and right in their sockets. and so on. I’m kind of expecting to notice this effect for a couple of days, then as my eye recovers from the procedure beign done to it in two weeks, I expect that I will spend part of the recovery timealso recovering from the change in how my prescription does not have this distortion again.
I’m not about to throw up. Headaches are a possibility, but I’m not really expecting them. If you wear contacts you can see what this experience is like yourself by taking an old pair of glasses in your prescription, taking a lens out, then wearing one contact on that side, and no contact on the side with the glasses lens in it. I’m not recommending that you go a day or a couple of weeks like that, but I’ll admit that it’s an interesting experiment and experience.
It’s one of the last experiments I expect to go through with glasses that I need for distance vision.
Once I’m done wearing these glasses, when I get PRK done to the left lens, my plan is to put the lens back in the right eye position on the glasses. I will then go around and collect the various glasses that I have about the apartment, and I am going to be going to a local Lions club drop off spot. And I will donate them to the Lions. I am not nostalgic about my glasses. They have worked well for me over the years. But I’m not expecting to need them ever again, and those I will need are not going to be these. There are millions of people who need glasses. Tens and hundreds of thousands of them can not afford the glasses they need. While my prescription may not be exactly what they need, the frames themselves are not a trivial expense, and new lenses can be cut for them as needed.
I have not always been a fan of organizations like the Lions. However I really think that what they do for people and vision more than compensates for any concerns I have over such a club. I may not join, but I will support, and donate the glasses I no longer need. I happen to think that you should also, but whether you think that or not is up to your own sets of criteria.
In any case, the next couple of weeks look to be a bit interesting.
