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Exit pupil and vision with spectacles and contacts

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#1 stendec

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Posted 10 May 2005 - 02:48 PM

I visit my optician yesterday for a yearly examination of my vision. I also measure my exit pupils. The results surprise me a bit.

Pupils with spectacles:
Left 6,85 right 6,8

Pupils without spectacles:
Left 8,2 right 7,9

No drops used.
I am near-sighted, -4,25 on left and -3,0 on right eye. I always use contacts when I am observing the nightsky.

But how about observing without spectacles or contacts? Would I gain a bit light or will the exit pupil get smaller when observing through the binocular with diopter set to same value as my contacts/spectacles?


I also wondering if contacts and spectacles affect/reduce the light reaching the eye when observing with binoculars(or other instruments)? When i look into my eyes with contacts using a mirror I see much more glare and reflections than without contacts.
I dont know if there is something like multicoating or fully multicoating or equal in the spectacles/contact industry? :)

/Norrman

#2 btschumy

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Posted 10 May 2005 - 03:19 PM

That's a pretty interesting fact that I'd never thought about. If you have a strong prescription and if when looking through the front of your glasses you see things demagnified, then your effective exit pupil with glasses is less than the exit pupil without.

The corollary (I guess) is that if you are far sighted, your effective exit pupil will be larger when wearing glasses. Hmmm... I wonder if you could get some glasses corrected for far sightedness (even if you didn't need them) and enlarge your exit pupil that way. You would just need to focus your binoculars a bit differently.

Thoughts?


P.S. I know you can get some very nice Zeiss IR coatings on your glasses that do make a difference.

#3 Guest_**DONOTDELETE**_*

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Posted 10 May 2005 - 03:38 PM

Anything you place beween your eyes and the object of interest will reduce the amount of light you'll receive. Of course with contacts or glasses, being between your eyes and the object, they will (and should) absorb very little...but not zero. Heck, even some light is lost at the contact-free eye! Albeit very little, but some.

Now, why your eyes dilate without glasses is puzzling. Does this also happen when you're not wearing contacts? You didn't explicitly state this. Let's assume it does. In principle you're in better shape I believe because 1) less light is falling outside the pupil and 2) there is no longer any material that light must travel through to reach your eyes. But then again in this state, your vision would be poor (no contacts or glasses!) and if you're not in a dark sight you'll be gathering unwanted stray light.

#4 Erik D

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Posted 10 May 2005 - 03:43 PM

If you have simple myopia without astigmatism in either eye I can't think of any need for spectacles or contacts while observing.....as long your focuser/right diopter adjustment has sufficient adjustment range to bring images to focus in both eyes.

I have -4.0 vision in the right eye and 20/20 in my left eye. I do not wear spectacles when I am using binos but I do have special shooting glasses for target shooting....some of the rifle scopes do not have sufficient focus range to bring the X hair and target to clear focus at the same time.

I did find that some of the low cost Chinese binos have insufficient RDA for my -4.0 right eye. I checked out a pair of Swarovski 10X42 EL a few month ago and I was surprised to find the RDA range was only marked for -3.0 dipoter.

I think there could be 4-5% transmission loss thru un coated lens. I'd think the same applied to uncoated contact lens. I usually request anti-glare coating on my glasses and they do work well for me.

Erik D

#5 btschumy

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Posted 10 May 2005 - 04:42 PM

Now, why your eyes dilate without glasses is puzzling.


I don't think it's the dilation that is different with and without glasses. Instead it is the magnification/demagnification of the exit pupil that the glasses produce. My optical theory is not the best, but it would seem this would have a real affect on how much of the exit pupil reaches the eye's intrance pupil.

#6 btschumy

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Posted 10 May 2005 - 04:45 PM

If you have simple myopia without astigmatism in either eye I can't think of any need for spectacles or contacts while observing.....as long your focuser/right diopter adjustment has sufficient adjustment range to bring images to focus in both eyes.


What about the fact that you can't see the stars naked eye without your glasses and you get really tired of taking them on and off as you switch from naked eye to binocular viewing?

#7 MaritimeSky

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Posted 10 May 2005 - 04:52 PM

As a myopic eyeglass wearer, what I believe is happening to the entrance pupil size when wearing eyeglasses is what you see when you look in a mirror wearing eyeglasses which correct for myopia - reduced image scale.

If you look at a myopic eyeglass wearer square on in the face, it will look as if they have smaller eyes and the cheekbones will appear closer together through the lenses than the actual cheekbones of their face outside the lens' refraction. In actuality, we know their face isn't really shrunken, and the entrance pupils are actually normal sized, larger than they appear through the lens.

The smaller entrance pupils couple equally to the demagnified exit pupil, so in all reality the proportional scale remains constant. The only factor probably at work here would be light transmission lost through the eyeglass lens.

#8 Guest_**DONOTDELETE**_*

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Posted 10 May 2005 - 05:43 PM

Bill, I am not an optometrist but...

Pupils with spectacles:
Left 6,85 right 6,8

Pupils without spectacles:
Left 8,2 right 7,9


seems like dilation of the pupil to me. His pupil diameter grew. I did not follow your alternate description.

#9 KennyJ

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Posted 10 May 2005 - 06:13 PM

< What about the fact that you can see the stars naked eye without your glasses and you get really tired of taking them on and off as you switch from naked eye to binocular viewing? >

Bill -- shouldn't that have read "CAN'T" rather than "CAN" ?

If not , then I'm afraid even my malleable mind is struggling to understand what you are trying to say here :-)

Regards , Kenny

#10 John Hoare

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Posted 10 May 2005 - 06:44 PM

I'm myopic (-7, -7.5) and my pupils become proportionally larger compared to the irises when I remove my glasses. I checked three times.

#11 btschumy

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Posted 10 May 2005 - 07:05 PM

< What about the fact that you can see the stars naked eye without your glasses and you get really tired of taking them on and off as you switch from naked eye to binocular viewing? >

Bill -- shouldn't that have read "CAN'T" rather than "CAN" ?


Kenny, right you are.

#12 btschumy

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Posted 10 May 2005 - 07:24 PM

NW,

Maybe I have this all wrong. I read Norrman's original message to mean that with glasses, the exit pupil appears smaller when viewed from the front of the glasses. I didn't think he meant his pupil size actually changed size. He will have to clarify which it was.

My thinking (which I now suspect was erroneous) was that all the light that would enter the larger true pupil would have to fit through the smaller apparent pupil that the glasses present. Thus wearing glasses for myopia would effectively decrease you entrance pupil. But now I'm not so sure that's right.

#13 EdZ

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Posted 10 May 2005 - 07:41 PM

What happens to your eye when you focus on something? Glasses or not, the muscles constrict and the pupil gets a little smaller. Go ahead look in the mirror, in a darkened room, and measure your pupils. Then relax and let someone else measure your pupils. The self measure should show you a smaller pupil because you were focusing on yourself.

I'm assuming the doctor measured your pupils. With your glasses on you were watching something, you were focusing. Without your glasses, you are so myopic that you couldn't focus on anything in the room. Your eyes were relaxed. The measurement would be larger.

When you are using your eyes to focus, the smaller measurement is what you are going to get. That's what you would be getting when using any optics. Try the mirror/other person test and see if it bears out what I'm saying.

As far as coatings, my polycarbonate lenses have FMC Nikon coatings. Light transmission is about 99.5%. With contacts the inside surface is acting like part of the eye because of the fluid layer (just like an oil seperated achromat), and it would not need to be coated. The outside surface is not losing a whole heck of a lot of light. Tranmission losses are not changing the pupil, or if so, then not much, not enough to see measurments like are noted here.

edz

#14 MaritimeSky

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Posted 10 May 2005 - 07:59 PM

An eyeglass lens which corrects for myopia is essentially just a demagnifying glass. If the measurement was taken with this lens between the measuring instrument and the pupil, the demagnification would make the pupil measure smaller. If the measuring instrument was on the same side of the lens as the pupil being measured, the pupil would measure the same, but the increment on the measuring instrument (ruler or what not) would appear scaled down as a result of the demagnification of the myopia correcting lens.

#15 stendec

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Posted 11 May 2005 - 06:09 AM

Ever since I got contactlenses I have noticed a change in pupil size when I take out ONE contactlens and look at myself in the mirror. But I have never think about the change in pupilsize when focusing the eye like EdZ mention.
Maybe it is the lack of focusing that makes the pupils bigger OR the demagnifying effect makes they smaller. Interesting!

I will try measure my pupils tonight when it get darker.

#16 Guest_**DONOTDELETE**_*

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Posted 11 May 2005 - 07:12 AM

That's true EdZ. The eye focusing, or de-focusing, will certainly effect the pupil size. Perhaps that's going on there when his glasses or contacts are removed. Or it could be something else. Hard to say.

#17 btschumy

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Posted 11 May 2005 - 09:53 AM

Either way, it shouldn't make a difference when viewing should it? You are still focusing on infinity.


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