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Author Topic: Are normal sunglasses 20:20?  (Read 4664 times)

Offline Airthumbs

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Are normal sunglasses 20:20?
« on: 03/07/2011 09:08:43 »
Having had an eye test about two years ago I was informed by a somewhat surprised optician that my eyesight is better then 20:20.  He said my vision was 7:6, (UK).  My question is related to sunglasses that you would buy without being prescription lenses and would they be putting strain on my eyes?

Most people have tried putting on someones glasses and will often find that after you take them off your eyes feel a bit strained having tried to focus through the lenses.  I often get this feeling when I remove my sunglasses.  Would it be better for me to have some sunglasses made with lenses that match my eyesight instead of using standard off the shelf ones?


 

Offline CliffordK

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Are normal sunglasses 20:20?
« Reply #1 on: 03/07/2011 09:50:29 »
The "stock" sunglasses should be just fine.

I believe the way prescription glasses work is that the curvature at the two sides of the lens is different, which allows the focus of different parts of the visual field differently.

Your sunglasses should be made with essentially parallel sides of the lens. Which actually has the benefit that they can curve the lenses more with minimal distortion.

Anyway, if you don't need a prescription without the sunglasses, you would not need it with them.

I'd encourage going to a real glasses and frame shop and talking to them.  Perhaps the $100+ sunglasses are fundamentally different than the $5 drugstore glasses.  I don't know if there are any benefits of polarized lenses and etc.

Also, you said that you went to an optician???  Here in the USA we have Optometrists and Ophthalmologists.  The Ophthalmologists are medical doctors with an eye specialty.  Optometrists aren't.

Sometime around 50 to 60, your lens will start loosing flexibility, and you may find that you would need reading glasses when you would never have needed them in the past.  You certainly will need to have both close, and distance vision checked.  Diabetes, is also a disease that some people acquire which has the possibility of damaging the eyes, and those individuals should be closely monitored.

Anyway, if this condition is new, or your eyes are changing, then it wouldn't hurt to just get your eyes checked out.  Those people who wear glasses get their eyes checked every couple of years with glaucoma checking and peripheral vision tests and etc.  But, exams may not be as common for those who don't wear glasses.
 

Offline Bored chemist

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Are normal sunglasses 20:20?
« Reply #2 on: 03/07/2011 10:31:19 »
Lot's of people have eyesight that's better than 20:20 (or 6:6 in SI units).
It means that you can read, at 20 feet (or 6 metres), the same size of letters as the average person can see at that distance. Since there's some variation in eyesight there are plenty who turn out to be a bit above average.
In the UK an optician doesn't just prescribe glasses, they also are qualified to deal with other eye problems so I think they come somewhere between the two groups you mention.
If a lens has the same curvature on both sides then it's not a lens, its a window.
If you don't normally wear glasses then you don't need prescription sunglasses. If, like me, you can barely see without glasses then if you get sunglasses they need to be prescription ones.
 

Offline Airthumbs

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Are normal sunglasses 20:20?
« Reply #3 on: 03/07/2011 18:50:17 »
Thanks all, just a note, I did not have the old school eye exam using a snellen chart with the letters getting smaller and smaller but was sat in a dark room with some kind of red lasers on the wall opposite me.  The person who ran the test then placed different lenses in front of my eye and I had to tell him when the two red lines both horizontal and vertical met in the centre.  I believe this is much more accurate then using a snellen chart.

By the way, I just listened to a podcast about people who are able to see underwater in the oceans.  I have to say that having grown up in the med. we would go to the beach every weekend where my brother and I would spend hour after hour just snorkeling around.  One day after doing this for many years I wondered if the salt water would sting my eyes if I did not use the mask, to my surprise I was able to see underwater without any blurring whatsoever and had no stinging sensation.  I put the stinging sensation down to the amount of urine in the swimming pools we also used to frequent.
 

Offline CliffordK

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Are normal sunglasses 20:20?
« Reply #4 on: 03/07/2011 18:54:11 »
I think it is the chlorine in the swimming pools that bothers the eyes.  But, there is also a discussion about having to carefully balance the pH of the system.
 

Offline Airthumbs

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Are normal sunglasses 20:20?
« Reply #5 on: 03/07/2011 19:19:31 »
I worked as a Lifegaurd at a pool in the UK and I can tell you right now that it is the urine in the pool that makes your eyes sting.

Think about it this way, or try this out if you like.......  goto a swimming pool that has a baby pool as we call it that is separate from the adults pool.  Open your eyes underwater in the adult pool and make a mental note of how your eyes feel, unfortunately adults do pee in the pool so there will be a little bit or soreness.  Then repeat this in the baby pool, I warn you that you may feel like your eyes have just been rubbed by some course sandpaper!!

One day at the pool the automatic chlorine dispenser went on the blink and was continuously adding chlorine to the pool, a pretty dangerous situation.  Anyone that had any white in their swimming costume was now wearing something that can only be described a transparent, we had no complaints about eyes stinging but did evacuate the pool as the chlorine was evaporating from the surface and creating a pretty toxic cloud!! 
 

Offline Bored chemist

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Are normal sunglasses 20:20?
« Reply #6 on: 03/07/2011 19:52:16 »
You both have half the answer. The combination of urine and chlorine gives rise to chloramine which stings your eyes.
Meanwhile, back at the topic...
 

Offline Airthumbs

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Are normal sunglasses 20:20?
« Reply #7 on: 03/07/2011 19:57:08 »
You both have half the answer. The combination of urine and chlorine gives rise to chloramine which stings your eyes.
Meanwhile, back at the topic...

I was thinking the same thing in writing my response but could not think of a way of linking them in unless you try to wear sunglasses in the pool that is!  8)
 

Offline SeanB

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Are normal sunglasses 20:20?
« Reply #8 on: 03/09/2011 12:24:08 »
Getting prescription sunglasses gives the benefit of getting quality coatings on the lens, as well as an optically correct lens. Cheap sunglasses suffer from both distortion and poor coating. A custom lens will be able to have both an optically correct coating that absorbs UV, plus a proper polarising layer.
 

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Are normal sunglasses 20:20?
« Reply #8 on: 03/09/2011 12:24:08 »

 

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