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Reading Glasses - (ie Spectacles)


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#1 Dekan

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Posted 21 November 2012 - 05:35 PM

I wonder whether the use of "reading spectacles" can actually make eyesight deteriorate.

To put this question in context - I've unfortunately reached the age at which focussing on close things, gets difficult. So reading a book, requires holding it at quite a distance.
Not exactly "at arm's length", but further away than is physically comfortable.

To overcome this problem, I have for many years used a "reading-glass" - a single, non-achromatic, convex crown-glass lens, 4 inches in diameter.
This lens has enabled me to read in comfort even the finest book-print, for hours on end, without any eye-strain.

However the lens does have two drawbacks:

1. The 4" diameter lump of glass is quite heavy, and after a while, the effort of holding it, causes aching in the wrist;
2. It has to be held in one hand, so if the other hand is holding the book, there's no hand free to write notes etc.

So to try to get round these drawbacks, I recently bought a pair of reading-spectacles - 3.5 dioptre. These give about the same magnification as the single lens. And they're very light, perched on the nose. No wrist-ache - and they leave both hands free.

The only thing is - after wearing the spectacles for 20 minutes or so, my eyes start to ache. Which causes me to take the spectacles off. And after taking them off - my vision seems blurred for a while, which I never experienced with the single lens.

All this makes me fear that the spectacles - if used continually - may cause eyesight damage. Is this fear justified, do you think?
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#2 CharonY

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Posted 27 November 2012 - 09:01 AM

Using glasses with different dioptres can result in headaches until the brain adapts to it. In addition, the eyes (and long-term presumably also the brain) adapts to the different focus while using certain glass strength and after removing them they will focus wrongly. Continuous use of glasses of a wrong strength will result in your eyes adapting to them and may affect your normal eyesight. Ideally, you should have reading glasses that correct your eyesight as little as necessary (i.e. have it measured by an optician) to avoid larger problems.
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#3 Dekan

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Posted 30 November 2012 - 04:24 PM

Thanks CharonY - appreciate your advice. The spectacles I referred to aren't "prescription glasses" from an optician after an eyesight-test.
They're a very cheap pair, bought for 1.00 from a chainstore. So I can certainly understand why using them might cause eyestrain.

What still puzzles me though, is why similar eyestrain isn't caused by the big 4", hand-held, single lens. This lens too, wasn't prescribed by an optician - in fact I got it for 1.00 from a store, just like the specs. But whereas the specs make my eyes ache, the single 4" lens doesn't.

Tentative explanations that occur to me are:

1. With the big single lens, both eyes can look through the same optical medium. Which is what happens naturally, when we look at things through the air. And this might be easier, and produce less strain, than if each eye is forced to use a different medium - as it is with the two different lenses of the specs.

2. The big single lens is not constantly held in front of the eyes. Eg, suppose you're reading a book, with the lens held in your hand. When you turn a page, or put the book down, to reflect on some thought, or light a cigarette, or sip your drink, or whatever - it's easy to lay the lens aside for a moment. Thus allowing your eyes to obtain relief from the lens's influence.

So your eyes can relax periodically, and strain doesn't build up. This is not the case if you're wearing specs. Specs can hardly be continually whipped off and put back on again all the time - the lenses stay in place in front of the eyes. And so the strain does build up.

I'm aware that the above thoughts are a bit clumsily expressed! I'd be glad of any comments - thanks again.
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#4 StringJunky

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Posted 30 November 2012 - 07:09 PM

Best thing to do is actually go and get your eyes tested and then you know what corrections are required if you buy off-the shelf ones in future. Cheap reading glasses, I've found, aren't consistent in their optical configuration and can cause eyestrain. Foster Grants, which are £10 to £15, are much more consistent. The main thing is, know your optical requirements first by buying your first pair properly measured by an optician. If your eyes need +2 and you are using +3.5 (stronger) it can cause quite bad eyestrain, especially cheap ones.

I think the reason you don't get eyestrain from your lens is because you are only using it over a very short range of distance and, more importantly, manually adapting the focus with your hand so your eyes don't have to, hence the lack of discomfort. Also, with long-sighted glasses on you will look around from reading and your eyes will try to bring things into focus that are maybe a bit too far for you to do so easily...with a lens you just look away.

Edited by StringJunky, 30 November 2012 - 07:11 PM.

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#5 CharonY

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Posted 3 December 2012 - 02:49 PM

As StringJuy said, the hand lens is not held at fixed distance from your eyes,which would force your eyes to change focus. Instead, you normally keep your eyes relatively relaxed and adjust focus by moving your hand.
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#6 StringJunky

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Posted 3 December 2012 - 04:20 PM

As StringJuy said, the hand lens is not held at fixed distance from your eyes,which would force your eyes to change focus. Instead, you normally keep your eyes relatively relaxed and adjust focus by moving your hand.


Re-reading this it gave me a thought for people that have a severely reduced focal range because there's problems with their eye muscles. A flexiible lens with adjusters around its perimeter that pull or release equally and these are connected, via a microprocessor, to an infra-red lght/sensor which is embedded in the frame. The information is sent to the microprocessor that calculates the distance and activates the adjusters to create the correct focal length.

I've just done a quick Google for flexible polymer lenses and apparently scientists have developed the most important part of my idea which is the lens...this is for lens implants though:

"Researchers at the Case Western Reserve University in Ohio, working with spin-off lab PolymerPlus, created the lens by stacking up layers of laminated plastic. Weighing a tenth of a traditional lens, the polymer version is up to three times more powerful and, crucially, had the capability to be flexible enough to incrementally change its refraction of light." http://www.theregist...nolayer_lenses/

More practically with respect to the OP, and more expensively, I suppose one could get some varifocal glasses that are ground within a limited range just for reading so you just peer through the optimal part at any given time; this would probably reduce eyestrain.

Edited by StringJunky, 3 December 2012 - 04:32 PM.

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#7 billoutt

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Posted 31 December 2012 - 09:39 PM

I have a number of reading glasses that don't have the diopter (strength) on them.  Does anyone know of a chart online that I could print out to use to ascertain the missing strength number ?  Or suggest some other prodedure to help me.  thanks, Bill


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#8 ewmon

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Posted 1 January 2013 - 06:52 AM

I have two ideas — 

 

#1 — When using the single 4-inch lens, are you reading the same text equally with both eyes; that is, does one eye read through it more or less along its optical axis while the other eye relaxes far off axis (I'm guessing about 0.2 rad), then when you stop using the lens, the relaxed eye can see "properly" and the "reading" eye is out of focus (but you don't notice it)? 

 

#2 — If both eyes (set the nominal 62 mm {aka 2.5 inches} apart) are reading equally through the 4-inch lens, I'm assuming they are both equally offset from the lens's optical axis (I'm guessing about 0.1 rad each, that is, for simplicity of calculations, 1.25 inches offset over 12.5 inches distance), and I'm assuming this offset provides some sort of cylindrical component to the eyesight correction (that is, the correction is not just spherical). Sorry, I'm not good enough at optics to know if I'm talking fact or fiction. So the question to the optical experts is: Does tilting the lens introduce a cylindrical component that might help his eyes to relax? (I know in photography, tilt shift produces a unique effect.) 

 

You could test my hypothesis in #1 by alternately closing one eye and then the other both while reading through the lens and without it, but someone knowledgeable in optics will probably need to give #2 a yea or nay. Does anyone have an opinion? (What a silly question! tongue.png)


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#9 Bill Angel

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Posted 1 January 2013 - 04:51 PM

I have a pair of prescription reading glasses, and as one of my eyes is noticeably weaker than the other, the lenses are of different strengths. I don't think that drug store reading glasses, with both lenses of the same strengh, would work for me either.
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