Burning Diopters

Burning Diopters

I’m not going to get technical or use math in this post but want to explain how a magnifying glass works but before I do that there’s a little history to magnifying lenses. I won’t go over what’s already on the internet about this but you might have wondered how people started fires hundreds of years ago or even thousands of years ago. It’s not the only way fires were created long ago, I’m sure the fire starter experts know all the other ways. Let’s just say that the modern world of optics started when someone found that piece of rock that when held at the right distance from some dried plant material caught it on fire. Can you imagine being the first person in the whole world to pick up a rock and see that it can start a fire! Who knows when that happened. I’m just guessing, maybe 5,000 or 10,000 thousand years ago (not exactly modern times I guess). A magnifier designed to do this is also called a burning glass or burning lens. I did read on wikipedia that people used vases filled with water to focus light and thus could start a fire with that. I’d like to see how those were used. Here’s a link to modern day fire starting with plastic bottles.

A magnifying lens makes something larger than it really is. Now this is magic! But not really because we all know what a magnifying lens does but maybe not exactly how it works. Let’s take this one example of how it works in reverse. If you hold the magnifying lens at the right distance and try to burn something using the suns rays you are making the big sun in the sky very small onto whatever you’re trying to burn. And what is really happening is you’re concentrating all the light from the sun into a very teeny small area and all those concentrated rays make a lot of concentrated energy. You might wonder how rays can be compacted (but really called focus). Well, that’s just something that is part of the laws of nature.

If we take that same experiment and instead of the suns rays going through to the piece of something you’re burning, put your head in front of the magnifying lens and look at that little piece of something you were burning. The magnifier works both ways you could say. Dual purpose. It really depends on how you use the magnifier. You probably use it to look through so you magnify what you’re looking at instead of making fires I hope.

I know I said I wasn’t going to do any math in this post and so far I haven’t. But let’s look at a couple of things about a magnifying lens that make them work. First, its thicker in the center and thinner on the edges. Some people’s glasses are made this way. I wrote about farsightedness and presbyopia recently. Both of those crowds use these kind of lenses to converge rays so they can see clearly. Yes, converging in this case means bring the rays closer together (as opposed to diverging rays in a nearsighted person’s glasses). This type of converging lens also has what we call plus power and is measured in diopters (that’s basically what your eyeglass prescription numbers are all about). I believe I read that the origin of the word diopters came from Johann Kepler who lived around the 1600’s. Yes, he’s mentioned in every astronomy book I’ve read (and, yes, I do love astronomy).

A magnifier usually isn’t measured in terms of it’s dioptric power though. It’s measured in how much larger it can make images appear. There are formulas to calculate this but I said I wasn’t going to do any math. Over the counter reading glasses do magnify a little somewhat. I’ve heard some people call those glasses, “magnifiers.” But true magnifying lenses have so much dioptric power than you’d have to put something just inches away from your face to read and to use them as reading glasses. And that’s not comfortable. This is why you put magnifiers inches from what you’re looking at to make them work instead of inches from your eyes.

Confused? Hopefully not too much. Wait until I write a post on virtual images.



Presbyopia is not farsightedness!

Presbyopia is not farsightedness!

I know not many people are going to really care what it’s called. I had a patient yesterday tell me that if anyone wants to know what the problem is with his eyes they should just call me. He said that’s what he pays me for. I get it.  No matter how much I try to find the right analogy to help explain something, I think most of the time the patient just wants to know if it’s normal or if it’s a problem and how it will be fixed.

But I’m going to try explaining this anyway (it’s all over the internet already….the explanation…..but not on my site).

We need to start with a person that does not have a prescription and is 28 yo (just a random young age I picked, could be 10 or 38 too). That person reads the smallest letters on my distance eye chart so I call that 20/20. That person reads the little letters on the little up close chart too that I stick in front of their face about 40 cm (16 inches). I call that 20/20 at near. There’s a word that describes the result of this situation and it’s called emmetropia (can’t sell them glasses darn it).

But that same person comes back to see me 20 years later and is now 48 years old. You know what they’re going to complain of right? They are having some problems reading up close but still see fine at distance. I’m sorry to tell the patient but they have reached that time in life where the good ‘ol drugstore reading glasses will solve their problems (probably). At least the presbyopia problem. Presbyopia is Greek and means old eyes. It sucks! But this person is NOT farsighted. Read on.

But lemme go back to the 28 yo person and change something about them. Let’s say they read the distance eye chart fine and they read the up close chart fine. Nothing different about that so far. But perhaps they have told me their eyes might tire quicker than they used to and go in and out of focus when they read. Ah hah! I already know what’s going on. Well, don’t tell me that patient has presbyopia….they aren’t old enough. I do my thing and spin my little lenses on my machine and find out the patient actually has a prescription for distance! And you might wonder, how could they read perfectly and have that? It’s a dirty little secret, that’s why. And it’s call farsightedness. They are farsighted.

Maybe we just should just get rid of that word. It doesn’t really make sense if a person can see far AND near and be called farsighted. Those eyes can deal with having to do some work all the time when we’re younger. But maybe not for a long time when reading. And eventually their distance, sometime years later, will also not be as clear. And eventually they will be 48 and develop presbyopia too. Yep, they’ll have both conditions.

Let me tell you about a real patient I had a few years ago. She was 35 (I think). She had a pair of drugstore glasses and used them for reading. She wasn’t 48 (or even in her early 40’s when presbyopia can rear it’s ugly head). She was using +2.50 power reading glasses! I did a little testing and found out her distance prescription was, guess what? +2.50! I asked her to put those reading glasses on, the ones she uses to read her books, and asked her to look at my distance chart. She read 20/20 with the glasses on and she also read 20/20 with them off. It’s that dirty little thing…..farsightedness. I think she was slightly confused and I didn’t help much trying to explain this occurrence that her reading glasses worked even when she looked far away. Oh my. I think she thought her glasses were just evil little instruments that people like me sold to unsuspecting shoppers in drugstores.

So why did I write this post? I sure would like to just give this to every patient that is farsighted and/or is just getting presbyopia and let them read this. Maybe they will get a sense of what I’ve said here so that when I try explaining it….however I do in the exam room, that they will somehow magically understand what they have.

Now I know what’s going to happen here. You are going to be thinking that you’re particular situation doesn’t quite match up to what I’ve said here. Well, ok. There’s a lot more that could factor in to all this. But I’m not going there. Let’s leave the 28 yo old patient happy that I just saw and you can come in and we’ll figure out exactly how your situation differs if this isn’t cuttin’ it for ya!

Are reading glasses ok to buy? Listen, if your mom (or whoever) says no, then come see me and I’ll clear that up. And if they work for you, then you probably found a solution to your problem….I hope. But buy multiple pairs, trust me on that.

Oh, farsightedness does have a real name in doctor speak….hyperopia. Does that word make sense either? Not to me. But, thems words for ya.

Here’s my favorite ophthalmologist talking about his reading glasses product line….