If I put on reading glasses, everything is crystal clear as long as it’s 1-2 feet from my eyes. Anything else is blurry. Why is that? Why can’t the lens in my eye work with the glasses so that everything is clearer at all distances? Does the lens in my eye bend a fixed amount for certain distances (5 feet = bend5, 10 feet = bend10, etc). Or do the lenses in the glasses mean it’s only possible to be focused at that distance and that the eye lens can’t do anything about it?
Counter-example that we can also get an explanation of, maybe: For pretty much my entire life, I’ve worn glasses for distance. But they’ve never had any negative impact on my near vision. Admittedly, my vision is pretty spectacularly bad (-5.50 and -6.00, O.D. and O.S., respectively, with just a tinge of astigmatism that isn’t worth mentioning), particularly now, so by the time I get something close enough that I can see it clearly, I’m on the verge of having trouble getting both eyes to focus on it. 
But my eyes haven’t always been that bad, and I’ve never had to take my glasses off for anything other than to sleep or take a shower.
Although… more recently, I’m starting to have trouble seeing “closer” with my contacts in. When I say closer, I mean not really all that close in all–my laptop in my lap is getting to be a bit of a bother if I’m wearing contacts. Doc says this is because I’m starting to get the farsightedness of old age, but didn’t explain why it only happened w/ contacts (other than to say it was complicated).
Reading glasses have lenses shaped like () and cause the light rays to converge more so that they make a clear image on the retina, because they’re bending too slow.
Near-sighted glasses have lenses shaped like )( and cause the light rays to diverge more so that they make a clear image on the retina, because they’re bending too fast.
Many people who can’t read aren’t farsighted though, they have presbyopia. This happens to everyone over a certain age. If this is your problem, then “it’s only possible to be focused at that distance and that the eye lens can’t do anything about it” is true because your lens not focusing properly is the cause of needing reading glasses. When you’re younger, your lens can adjust (accommodate) better.
Presbyopia happens because you’re 40+ and your lens is hardening and getting less stretchy. Not much you can do without it besides reading glasses/bifocal-type glasses/dual vision.
Why isn’t there some sort of feedback in the eye to take this into account? When I look across the room with reading glasses and everything is blurry, why doesn’t the eye figure out that it the lens needs to bend a little more or less? It’s like the lens bends to a fixed amount and doesn’t attempt to compensate to focus. Or is it impossible to focus in that situation with the glass lens and eye lens?
That’s just what happens… for young people. But as you get older, the lens gets less flexible and the muscles weaker, and the muscles in the eye aren’t physically able to bend (stretch, really) it any more. This stretching is called “accommodation”, and a reduction in accommodation (called presbyopia) is something that inevitably happens to just about everybody as they age.
Corrective lenses can shift the focusable range to something more useful. Your eyes are still focusing differently between 1 and 2 feet. They just can’t go much outside of that range.
Due to the peculiarities of optics, for a given level of accommodation, the actual size of the range increases with distance. So you might normally focus from 6 feet to “infinity” (meaning you can see the stars clearly), but with glasses the range is just a few feet from minimum to maximum.
Wish mine was that good.
I’m sure part of it is that soft contacts offer a limited choice of focal lengths. By the time you’re approaching the double digits, they typically only come in half-diopter increments, which is quite noticeable. Moreover, there’s a good chance you’re a tad over-prescribed (something that tends to take care of itself, BTW, as you lose the ability to focus and get retested).
Spectacles, on the other hand, are pretty much bespoke. They’re also adjustable: you can slide them down to help you see up close…until you run out of nose.
Welcome to the exciting world of accelerating decrepitude.
That sucks.
Looks like I’m gonna need 2 pairs of glasses
Or get bifocals. Many modern ones have a small magnifying part, so it isn’t so dorky. Or you can get one lens for distance and one for close, but most people have trouble getting used to that.
Reading glasses are dirt cheap, non-prescription, at your drugstore. The main drawback is just having to carry another pair.
Or LASIK, LASEK, etc. for distance vision, if you wanted to.
Another alternative is no-line bifocals, but some do not like adjusting your head position for best vision.
There are also variable glasses involving “inflatable” lenses filled with oil. You increase the pressure on the oil, the lenses change shape slightly, and the focus is changed.
Right now, they’re damn expensive, and they only come with circular frames. But the technology shows promise! Just turn a little dial and go from reading glasses to driving glasses. Yay technology!
If you could have a couple presets and change back and forth on the fly that would be brilliant!
If the lenses in your eyes were working properly, you probably would not need glasses at all.
Actually, that is not true for everyone who needs glasses, sometimes it is purely a matter of the eyeball becoming distorted, and such people may indeed need only one pair for all distances, but very commonly you need glasses because your natural lenses are no longer adjusting to distance as they should.
If a young person with perfect, healthy, fit eyes put on glasses, would their eyes eventually adjust? Would the muscles in their eyes be strong enough to compensate or would it always be blurry?
It probably depends on how strong the prescription is, and it’s not just the muscles, but the whole visual system that adapts.
Yup. For instance, myopic (near-sighted, have trouble seeing things at distances) people tend to love “stronger” (more negative) lenses when they are having their vision tested, because for that short time period it makes everything look so sharp and clear, and may be disappointed when the person doing the refraction doesn’t give them that stronger lens. The problem is that unless the stronger lens is actually improving your ability to read more letters (instead of just making them look better), it creates too much eyestrain to deal with for extended wear and can produce headaches.
Light coming in from the periphery requires more refraction.
Reading glasses create positive refraction beyond the eye’s ability to correct for at distance. The native refraction of the eye (created by the cornea and lens) is all positive (convexly curved), so there is a limit to how much negative refraction the eye can add to a positively correcting lens. As you get older and the native lens stiffens, the ability to accommodate–particularly for near vision–worsens, because the lens does not bulge to more convexity when the ciliary ligaments relax the way it did when you were younger.
Anyway, if you wanted a longer focal distance with reading glasses, you could construct some with a more limited field of vision, all the way down to a pinhole. However the limitation of the amount of visual field would likely be too annoying.