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View Full Version : Why are progressive lenses made with such a large amount of uncorrected real estate?


jayjay
05-30-2009, 12:35 AM
In this (http://boardstest.straightdope.com/sdmb/showthread.php?t=519328) GQ thread, CalMeacham links to this website (http://www.allaboutvision.com/lenses/progressives.htm), which explains how progressive lenses work. Since Cal states that he doesn't understand why the vision correction doesn't extend the full width of the lens, I wanted to ask the question to the Teeming Millions, because I'm damn curious now.

Apparently, progressive lenses have a relatively narrow vertical field of vision correction, with large sections of lens real estate on the sides of that which don't correct. Why is this? What's the reason for wasting so much lens on non-corrective real estate? Why not just make the correction field the full width of the lens?

Napier
05-30-2009, 07:30 AM
Because there is no shape that would be correct. I think the top half, which is all for distant viewing, is correct, and a central band down to the bottom is fairly correct for progressively closer viewing, but the bottom half on either side of this band can't be correct. Once you start making a lens in the middle, you have to keep following the same curve to make it bigger and bigger, if you catch my drift. If you study two regions of the central band and use them to decide how some spot off to the side has to be curved, you will get two different shapes the spot off to the side must be.

I think it is more correct to say that NONE of the lens can really be quite right, but it can be close enough for satisfactory use over much of the lens. These areas are where you can't even do that.

The older type "lined" bifocals don't have this problem and are for the sake of this discussion quite right everyplace, though of course the sharply viewed distance takes on a different value in the "adds" (the special regions).

Napier
05-30-2009, 07:40 AM
Actually, posting about this makes me a little uncomfortable, because if CalMeacham says he doesn't get it, as he works in the field, it's hard to believe I do get it. Maybe he refers to a higher standard of understanding.

But I thought of a better way of explaining it. The classic shape for a lens surface is spherical. So imagine the top half of a progressive lens is spherical in surface. Once you establish any four points on a sphere, you have constrained the entire thing (ignoring pathalogical cases). Clearly you would know how to keep going into the bottom half of the lens to establish its shape.
But instead you do a narrow band downward from the center, and you start making this curve more and more tightly, like a watchspring measured at the outer end and progressing inward. This is going to make a gradually more powerful lens. At any point along this band, since its curvature in the vertical plane is getting tighter, you have to curve it in the horizontal plane more tightly too. The band is going to be shaped, now, like the outermost face of a slice taken through the middle plane of a nautilus shell.
However, if you start in the center of the band and work your way out sideways, you have to keep following its lateral curvature, and if you do this at two different locations you will converge at the same point but at different altitudes. I mean, if two people on the Earth start on the Prime Meridian in Greenwich, and one goes north to the north pole and the other heads south to the equator, and both turn 90 degrees (one left and one right), they meet at the same point eventually, and this triangle has 270 degrees of internal angles. Similarly, two microscope lensemakers navigating around on this lens will disagree about how far in or out the surface should be in the lower corners depending on how far up or down on the band they were when they started.
So the lensmakers pick some sort of moderate surface height for those regions and call it a compromise.

cromulent
05-30-2009, 10:32 AM
To add to the what the previous poster said, the amount of "unusable real estate" varies from lens to lens. Some progressives have large distance/near areas, but a lot of unwanted astigmatism in the periphery, and a very sharp change in the prescription from near/far. Some progressives are what is called a "soft design," with a gradual change from the near to far prescriptions, but less area to look through for distance/near, although clearer vision at intermediate. Patients may have an easier time getting used to one or the other, or might have occupational reasons (such as a lot of computer use at an intermediate distance) to prefer one over the other.

yabob
05-30-2009, 12:31 PM
I don't notice the "unusable real estate" on mine, but I have a larger issue in that I don't really make use of the damned things at all. My optometrist has talked me into continuing to have them a couple times now ...

I don't look through the bottom of my glasses to read or look at a computer screen, and I can't seem to train myself to because I find it horribly irritating. I've been myopic all my life, and once the presbyopia started setting in, I just took off my glasses to read or work at a computer screen. I got away with that for a while, until the optometrist insisted that I should have progressives. Well, I still take the bloody things off, like I'm doing right now. You hand me something to read quickly while I have my glasses on, and likely as not I'll find myself looking over the TOP of the damned things. I DO NOT want to read through the BOTTOM of my visual field. In fact, if I'm concentrating on a computer screen or a printed page, I want to get close enough to it so that it fills my visual field and I don't have all this distracting nonsense around the periphery. I've been taking flak for years about sticking my nose in screens and hunching over whatever I'm reading to view the page at about 3 inches or holding it up completely in front of my face. I LIKE it that way, in spite of what it does to my posture.

I have always gotten glasses with very large lenses, too. When I'm actually looking at a distance, I hate seeing the edge of the glasses, and this unfocused area around the outside of my vision. The screwed up bottom of the progressive lenses would probably be really annoying in a smaller size.

I may train myself to use the things as intended someday, but they just don't work very well for me.

rocking chair
05-30-2009, 07:50 PM
i'm with yabob on this one. when you are very myopic you are very trained to bring things closer to your eyes. progressives have you going against nature. much easier to just look under your glasses, or take them off.

Napier
05-31-2009, 08:12 AM
Ditto yabob and rocking chair, if you are nearsighted but otherwise OK it makes no sense to use glasses for near sight, especially if they are also limiting or difficult in even the slightest way. They tell me progressives are very profitable and much oversold as a result.

BTW craining one's head weirdly to look at a computer screen can indeed be very hard on posture. I was doing it and getting neck pain, and this was the first sign of my journey through neck trouble, fusion surgery and other unpleasant bits. AFAIK the underlying problem is a lifelong accumulation of wear for many reasons, but I think the eyeglasses-driven posture accelerated it.

CalMeacham
05-31-2009, 11:29 AM
I don't want to scare anybody off -- I'm in optcs but not in eyeglass optics, and reading books on the subject makes me realize that those eyeglass guys frequently speak a very different language. But I've worked on contact lenses and for a former eyeglass company, so I got exposed to the Other Side, without learning enough to be an expert.


That said, I can easily image a "progressive" lens made by taking one of your Ben-Franklin-style hard-edged bifocals and gradually blending between the two sections. It's not at all clear to me why there is that large "useless" area to either side.

I'm not saying that it's pointless -- there have been large numbers of extremely talented folks working in the field (like "Doc" Tillyer) who no doubt contemplated precisely these issues. Perhaps it's easier to reliably fabricate, or something. Maybe doing what I suggest really looks weird when you actually wear it, or the "blending" takes up too much of the lens to be useful. I'm just saying that the answer isn't obvious to me, and I've never seen it explained anywhere.

cromulent
05-31-2009, 12:43 PM
That said, I can easily image a "progressive" lens made by taking one of your Ben-Franklin-style hard-edged bifocals and gradually blending between the two sections. It's not at all clear to me why there is that large "useless" area to either side.


They do have these; they're called "blended bifocals." And you can usually see the "blended" area; it's not invisible as it is in a progressive. Most patients don't like the glasses to look as if they have any sort of add at all, so that's an immediate negative for these. Moreover, there is no intermediate power whatsoever, which is a big problem for some patients.

As for Yabob and the other posters with problems with progressives, obviously low myopes (less than 4 diopters or so) are going to have an easier time simply taking off their glasses to read. It's the higher myopes, patients with astigmatism, and hyperopes that progressives benefit. If you find you are in front of a computer a lot and progressives are giving you posture issues due to the limited area that you need to view through the lens, you might consider a pair of glasses simply for computer use, with a lower add power to compensate for the fact that most people use a computer at a distance farther than their reading distance. But if you see the computer just fine with your glasses off, then of course that will probably feel most comfortable. If you don't mind taking a pair of glasses off and on, and have no symptoms related to eyestrain, then why bother with progressives?

jayjay
05-31-2009, 01:03 PM
They do have these; they're called "blended bifocals." And you can usually see the "blended" area; it's not invisible as it is in a progressive. Most patients don't like the glasses to look as if they have any sort of add at all, so that's an immediate negative for these.

Vanity's the proximate cause of an awful lot of silliness, isn't it?

Saintly Loser
05-31-2009, 01:13 PM
Vanity's the proximate cause of an awful lot of silliness, isn't it?

It is. I can't deal with progressives, or "blended" bifocals. But my regular bifocals, with a line, cause me no problems at all. I'm not so vain that the thought of people seeing the line bothers me.

Looks like I'm due for trifocals now. My glasses work well for distance, and for reading, but not so well in the middle. Hopefully I'll find them as comfortable as I find my bifocals.

cromulent
05-31-2009, 01:47 PM
Vanity's the proximate cause of an awful lot of silliness, isn't it?

Definitely. Imagine if presbyopia happened in reverse -- if you needed a reading add as a a young adult but "grew out of it" in your 40s. If that visible line made people look younger, they might put it on just for appearance's sake. :)

jayjay
05-31-2009, 01:55 PM
Definitely. Imagine if presbyopia happened in reverse -- if you needed a reading add as a a young adult but "grew out of it" in your 40s. If that visible line made people look younger, they might put it on just for appearance's sake. :)

I had bifocals when I was in 2nd grade. I just don't associate them with "old people".

yabob
05-31-2009, 02:23 PM
...
As for Yabob and the other posters with problems with progressives, obviously low myopes (less than 4 diopters or so) are going to have an easier time simply taking off their glasses to read. It's the higher myopes, patients with astigmatism, and hyperopes that progressives benefit. If you find you are in front of a computer a lot and progressives are giving you posture issues due to the limited area that you need to view through the lens, you might consider a pair of glasses simply for computer use, with a lower add power to compensate for the fact that most people use a computer at a distance farther than their reading distance. But if you see the computer just fine with your glasses off, then of course that will probably feel most comfortable. If you don't mind taking a pair of glasses off and on, and have no symptoms related to eyestrain, then why bother with progressives?
Heh - the posture problems are from my reading / computer habits with or without glasses of all types, and I've had the issue basically forever. I don't feel like I'm working on the computer unless my nose is stuck into the screen, and I don't feel like I'm concentrating on reading something unless it's up against my face. I don't want to have other junk in my visual field while I'm doing those. Using glasses which forced me to look at the screen through a smaller area would probably IMPROVE my posture, but it also annoys the hell out of me. I pay the penalty in lower back pain.

Napier
05-31-2009, 05:28 PM
Heh - the posture problems are from my reading / computer habits with or without glasses of all types, and I've had the issue basically forever. I don't feel like I'm working on the computer unless my nose is stuck into the screen, and I don't feel like I'm concentrating on reading something unless it's up against my face. I don't want to have other junk in my visual field while I'm doing those. Using glasses which forced me to look at the screen through a smaller area would probably IMPROVE my posture, but it also annoys the hell out of me. I pay the penalty in lower back pain.

Well, actually, I'm with you there - I much prefer to be closer to the screen too. I would like to have my monitor mounted much closer than its base and my keyboard allow, maybe 8" closer, but haven't found a cheap consumer-priced monitor to do that and don't want to shell out $1000 for the same size industrial model with machinelike mounting that would work.

I am wearing computer glasses now, and they are nice. Best, I think, would be glasses optimized for a 12" viewing distance and a monitor that could be mounted there. It is a nice test case for the advantages of doing without a multidistance correction in a particular situation.

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