Wiki: hyperopic defocus is the cause of myopia?

First caveat is that I’m new here. Have been reading stuff over on the wiki rather than here. (Kind of testing its role as the intro to all this stuff.) Apologies if it has been discussed here many times already, but a quick search found one mention - - perhaps it didn’t get much discussion there because it drifted onto it from the original topic.

The wiki says several times that hyperopic defocus causes myopia. Which seems fair. There’s plenty of animal studies cited (though I’ve not read them). With animal studies, you can put as strong a lens as you like on the eye, and there’s not a lot they can about it.

The wiki rather implies that it is the cause of myopia. A subtle difference, perhaps, but it’s confusing me. Actually, doesn’t just imply it. eg at it is definitely saying it is the cause.

First, I should make sure I understand the term: it is seeing a blurred image because the light is focused behind the retina. Now, is this properly blurred, like you’d have with myopic defocus due to uncorrected myopia. Or just a tiny bit out of focus - similar to the point you’re aiming for with active focus. Such as depicted at the bottom of ?

If (or rather, when) I’m wearing glasses and and can’t quite see something clearly up close, I take my glasses off. I don’t sit there with sustained blur doing nothing about it. (I suppose if I wore contacts that might be slightly harder.) Prior to covid, I’d meet friends to play boardgames, wearing (what I’m calling) normalised, but if I have to try to read tiny print on cards, I have to take my glasses off to help my poor probably-presbyopic eyes.

Otherwise, accommodation automatically deals with the hyperopic blur - it’s the whole reason the lens is there, to increase the overall focusing power of the eye when objects get closer.

So if we are talking about just a tiny bit of blur (because the feedback is imprecise or the ciliarly muscle is lazy and so the accommodation doesn’t go quite far enough to completely resolve the blur, and it basically acts like active focus in reverse and acts as a stimulus to elongate the eye. That makes sense.

The wiki (and the thread mentioned above) does touch on an alternative hypothesis :

" Even if hyperopic blur is not induced by the lenses, the accommodation system is being constantly stressed and this encourages eye axial lengthening"

To me, that sounds pretty plausible as the stimulus which could lead to induced myopia.

Am I missing something ?

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I think it’s an inconsistency of wording.

Wearing distance glasses and looking up close, your eye briefly detects hyperopic blur, then engages accommodation to resolve that blur, causing eye strain, causing axial lenthing.

The color/intensity of text/light comes into it because there is a half diopter difference in focus between red and green light waves, they’re never going to be both in focus at once, so the amount of stimulus you have that is in focus or blurred matters.

Hmmm… but then when you look away, you’d briefly get myopic blur, which would also engage accommodation to resolve. Which would be an equal and opposite stimulus. That would be happening all the time as you looked around the world, whether or not you were wearing glasses. Might have expected that you’d need a sustained stimulus to provoke a change in one particular direction …?

EDIT: From : on wearing distance glasses for close work… “While it does appear to work, due to the amazingly flexible nature of the human eye…” (ie you can see clearly => no hyperopic blur ?) “… this practice significantly worsens myopia in many cases (especially when using a full prescription to look at a close-up object for extended periods).” - implies it’s the extended period that matters, not a transient effect as you switch focus distance.

EDIT: sorry, I may have misinterpreted your reply. I read it that it was the instantaneous hyperopic defocus that was the stimulus for the elongation. But re-reading, I think you’re agreeing that it’s the accommodation that is the stimulus, not the transient blur. In which case mentioning the hyperopic defocus bit that happens briefly beforehand just seems unnecessary.

The relaxed eye is at maximum distance focus, active accommodation is only for near work.

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Yes, the body adapts to the task you put it to most.

I wonder if my problem is that I am misunderstanding the term “hyperopic defocus” here. What if it doesn’t mean “causing a blur by focusing the light behind the retina”, but instead it means “shifting the focusing system of the eye in the direction of hyperopia” (ie behind the retina, by means of minus lenses). ie introducing a hyperopic bias to the focussing system, forcing (or compensating for) accomodation (tensing of the ciliary muscle) to bring the image back into focus.

Okay, if that’s the case, now things suddenly start to make a lot more sense (well, to me, at least)

The one happens briefly before the other.

No, I still don’t think I’ve got it. This paper goes into quite a lot of detail of the procedure they were using :
So they are effectively preventing accommodation from resolving the blur, so that they really are measuring the effect the blur itself has. Which is all well and good, but brings me back to my starting question : what does this have to do with real life, where accommodation would prevent (or rather, almost instantaneously resolve) the blur in the first place.

OTOH, whatever the cause, it does all seem justify the endmyopia program, since AF really is about inducing myopic blur (that accommodation cannot resolve). If I understand correctly, they are saying it’s nothing to do with the ciliary relaxing even more, but the retina is moving closer to the lens, on a timeframe of the order of minutes. (= time to establish active focus …?)

Though they find it to be a temporary effect. (They are specifically looking at the recovery time after the effect is removed, rather than the effect itself. Which itself is interesting…)

? What ? How so ? Accommodation doesn’t prevent all blur.

That was (one of) the questions I asked in the first post. Does accommodation leave just a tiny bit of blur, barely perceptible, that acts as a stimulus for elongation. (If it was perceptible, I’d take my glasses off rather than enduring it.)

But this paper is talking about full-on 3 diopters worth of unresolved blur.

Accommodation only works in range. So if you look before 25cm you will be in massive hyperopic blur with emmetropic eye. The more the closer to the eye. Refractive state can change where this range starts. With uncorrected myopia hyperopic blur is closer to the eye. Blur is always present outside of accommodation range. For normal emmetropic eye this range is 25cm - 6/8m.

Well… okay, but first, the starting point of all this was the wiki’s assertion that " The primary stimulus for [axial elongation] of the eyeball is [hyperopic defocus] which occurs when someone:

  • Is engaged in close-up activity
  • Wears more correction than they need to when seeing the object that is close up.

So we’re not starting from an emmetropic eye. If you’re wearing you’re glasses, the near point is mapped closer to your eye. (I could probably calculate it.)

And second, my assertion is that if you’re “in massive hyperopic blur” doing close-up work with your distance glasses on, you take your distance glasses off. So while it’s all very well that these scientific studies observe interesting effects when the eye is in massive hyperopic blur, is it really what happens in real life ? I’d have thought that either (if you’re young) your eye can accommodate to clear the image (as I quoted Jake above) then there’s no blur (and maybe it’s the accommodation itself that’s the stimulus for elongation), or you can’t accommotate the blur, and you just take your glasses off.

Okay, I suppose there’s a third option, that you can nearly accommodate the blur and, for some activities at least, you can push the work (such as a book) away until it becomes readable. So in that case you probably have a little bit of hyperopic blur remaining. But if this is no longer the scenario that the papers are discussing, are they still relevant ?

Yes they are. Because there is something like peripheral defocus. So you can see sharp centrally but at the same time have distortion at periphery, which can lead to elongation.

I would also suggest to change that description to hyperopic defocus is causing axial elongation. Not myopia. Because myopia can be different things like spasm. This is not caused by defocus.

? I’d have thought that either (if you’re young) your eye can accommodate to clear the image

This is what happens with emmetropic eye. But putting glasses on doesn’t make you an emmetrope. You still have improper shape of the eyeball. Glasses just trick you to think you have normal eyesight because they remove some symptoms, but you aren’t emmetrope when you have your glasses on. Eyeball has wrong shape and full glasses from close-up are causing hyperopic defocus, even when you see sharpness.

Can you define “hyperopic defocus” for me please. In simple terms a physicist can understand (ie lenses and focal lengths and focal planes), preferably without using words like emmetrope, (since I’ll then have you ask for a definitons of those too, since I won’t know if my understanding of a term matches yours)

I thought it was blur, yet you see sharpness ?

If you look at sth, you look at point, right? If you see that point sharp, there is no defocus there. But there is periphery that you see around that point. Retina is not one point and if light is focusing in wrong places, then an eye can react to that.

Defocus is when light is not on the retina but before (myopic defocus) or behind (hyperopic defocus).

Correction is changing how the light is focusing, but it doesn’t change shape of the eyeball. So glasses can give you no defocus centrally but different that normal at periphery, because myopic eye has different shape and glasses are changing angles evenly on the whole retina.


This picture shows hyperopic defocus or myopic defocus at periphery. If you would take myopic eye with full corection up-close, this picture would look like this but hyperopic defocus would be all around retina except centrally where it would be in focus.

If you would take an emmetropic eye up-close this would show focus centrally and ! on periphery (or rather more normal, some defocus is not bad, but shape is increasing it) There would be no abnormal defocus there because shape of the eyeball is normal. The shape is crucial here. Glasses aren’t changing the shape.

That’s why you need differentials because otherwise you’re in peripheral hyperopic defocus.

Defocus doesn’t mean automatically it’s bad. But hyperopic defocus is much stronger usually because it changes on distance. Myopic defocus doesn’t really change. It’s the same if you look at 10m or 1km. And an eye reacts more strongly to hyperopic defocus. It doesn’t really matter if this defocus is only on central part of the retina or only on periphery, or both. An eye reacts to single defocus. Otherwise it wouldn’t get larger and larger. Or this theory is nor right and something else is causing elongation. That is also a possibilty.

To be pedantic, the light always stops on the retina. It’s the focus that’s in front or behind, both of which mean the image on the retina is blurred. But you knew that… :wink:

As always, there are more questions. How does the periphery of the eye distinguish between myopic and hyperopic blur. All it knows is the light on the retina is blurred. (Actually it doesn’t “know” anything - it’s just reporting light levels to the brain…) Eg suppose I’m using a magnifying glass to focus the sun onto a piece of paper. I haven’t quite got the focus right, so I see a blur. Without moving the lens a little in or out to see whether the circle gets bigger or smaller, I don’t think I can tell. (Can I ?)

I thought it was the macula that did most of the hi-res stuff. That’s pretty tiny, isn’t it ? How sharp does the rest of the retina see images ? Seems like a “design” flaw to shift the whole of the retina around because, despite the macula being sharply in focus, the peripheral stuff is unhappy.

Doh. Sorry, just thought of another question… suppose we are just at the very start of the process. We have a good (“emmetropic” ?) eye, but get ciliary spasm and pseudomyopia, and we get prescribed our first set of glasses. We are not yet myopic, so the eye is not mis-shapen. Yet progressive myopia still sets sail.

That seems like a bit of a jump. I’ll have to ponder that one. Differentials just happen to be just the right strength to make both the central vision and the peripheral vision just right, neither myopic nor hyperopic …? The only significant feature of differentials is that the eye’s lens itself is close to or at minumum strength. There’s still a huge amount of focusing going on with cornea + corrective lens.

And how does a plant know to grow in direction of the light? It’s magic but it happens. Have any pot plants in the room? See how they grow depending where the light is? An eye doesn’t grow like plants but it’s reasonable to say it can detect it to adjust.

Doh. Sorry, just thought of another question… suppose we are just at the very start of the process. We have a good (“emmetropic” ?) eye, but get ciliary spasm and pseudomyopia, and we get prescribed our first set of glasses. We are not yet myopic, so the eye is not mis-shapen. Yet progressive myopia still sets sail.

Hold on. You get hyperopic defocus here not from shape but from glasses. That’s why you get massive headaches when you put glasses on with only a spasm. From central defocus also. Like if you take emmetropic eye and look farther than 8m you’re in myopic defocus. Everything being quite sharp doesn’t mean there is no defocus. It’s just very close to the retina and it looks like it’s 100% sharp. The more from the retina the more blurry it wil be.

Differentials just happen to be just the right strength to make both the central vision and the peripheral vision just right, neither myopic nor hyperopic …?

Peripheral defocus or just defocus theory isn’t about AF or EM. It says that hyperopic defocus causes elongation and myopic defocus causes shrinkage. It’s animal study not even humans.

hang on… if I’ve got a clear (main) image, it means that the combination of corrective lens, cornea, and eye lens are all working together to focus the light onto the retina. If I make the corrective lens more negative, the eye lens accommodates to become more positive, giving the same overall focusing power.

Why does this affect the peripheral vision ?

Because the more myopic you are the more weird shape eyeball has. Glasses change light evenly. rsz_adobestock_68434458_650x301

See the difference in retina?

You put glasses on but this weird shape is still there. Your taking light differently than person with emmetropic eye shape. Glasses only correct central part of retina. They don’t adjust light unvenly on periphery to correct this weird shape so you get defocus there from up-close. But they don’t care about this because you see and in glasses business long-term isn’t important. They will make more money if they don’t try to adjust for that so who cares.

I think it that’s where active focus come in: if you just stare at blur, nothing happens. But when you clear the blur with active focus, the body knows which way the ciliary muscle was moved and then it knows which type of blur it have currently.