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Medical Forum / General / Vision / February 2005

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Lenscrafters ... so far

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abnerdoorman@yahoo.com - 11 Feb 2005 21:20 GMT
As I mentioned in an earlier thread, I was thinking about trying
single vision lenses after a couple years in bifocals (the small
eye place I've been going to got a new young doctor who said I
could do either). Well since the location is now some distance
from my home, and the doctor was different anyway, I decided it
was time to check out the chain place around the corner,
Lenscrafters.

They have a 30 return for any reason deal, so I said "Do you mind
that I want to get single vision lenses know that their is a
50/50 chance I'll be back in 2 weeks to change to bifocals?" They
said no so that's what I did. I got plastic lenses (CR39) and
metal frames -$205 with the random discount.

Anyway, I've had them for 10 days and so far I really wished I
had bifocals like twice (once with a menu in a dark restaurant)
but the rest of the time I've REALLY enjoyed not having bifocals
anymore.

There is one "issue". At a distance, things are a bit blurry when
I look straight ahead. We are talk street sign distances here. But
if I tilt my head back about say 15 degrees, it becomes clear as
a bell. I went back the the Lenscrafters today to see what the
deal was, figuring the had miss the optical center. After much
dotting of lenses and consultations with their optometrist, they
declared that optical center almost never mattered with plastic
single vision  and it was most likely as a result of my
astigmatism and that I just needed to adject to it. The OC of
the lenses was well below my eye, butI said I would give it
another week. They also looked at my old lenses and said they
had the same low OC which proved it wasn't an issue (of course
they are bifocals ...)

Can someone explain to me what is happening optically. I don't
see how this can work out, physics wise. Thanx for any light
you can shed on this.

I'm viewing this chain eye place thing as a learning experience.
If it doesn't work out, I can always go back to the small place
where the optometrist is only there like 7 hours a week and
its a road trip.

-A
Mark A - 12 Feb 2005 03:15 GMT
> As I mentioned in an earlier thread, I was thinking about trying
> single vision lenses after a couple years in bifocals (the small
[quoted text clipped - 39 lines]
>
> -A

You did not say which material you got in your SV lenses. Aspheric lens
designs do require that the optical center be fit properly on your face. You
can google "aspheric lens" to get the physics or more info.

Also, I have found that polycarb lens material exacerbates the situation you
described.
Robert Martellaro - 14 Feb 2005 18:54 GMT
>As I mentioned in an earlier thread, I was thinking about trying
>single vision lenses after a couple years in bifocals (the small
[quoted text clipped - 39 lines]
>
>-A

By following Martin's Rule we keep the principle axis of the lens passing
through the center of the eyes rotation.  If the Vertical OC is not dropped 1mm
for every 2 degrees of tilt (panto), and lens the power is high enough, the lens
will power will change and cylinder will be induced on the 180 meridian.

S'=S{1+(sin a)^2/(2n)}
C'= S'(tana)^2

S'= new spherical power
S= original sphere power
a= tilt
n= index of refraction
C'= the induced cylinder

If the lens power is -3.00DS (a reasonable assumption being a presbyope who
reads w/o glasses) with 20 degrees of tilt and the vertical OC level with the
pupil then the lens power will be -3.12 - 0.41 x 180. As you can see the lens
position DOES matter with single vision cr39. There is nothing to get used to
here-the lens is wrong and will blur the vision with possible fatigue/headaches.
In this example the OC needs to be dropped 10mm to eliminate the increased power
and induced cylinder. Your situation is the opposite with the OC well below the
pupil with, most likely, insufficient tilt. If this is true then the solution is
to adjust the glasses so that the panto or tilt is increased, increasing the
lens power and eliminating the induced cylinder. If this places the bottom of
the frame against the cheek then new lenses need to be made with more attention
to the vertical OC placement. It's either that or the doctors Rx is wrong. Or
both.

Hope this helps

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
robopt@execpc.com
~~~~~~~~~~~~~~~~~~
"If a million people believe a foolish thing, it is still a foolish thing."
-  Anatole France
abnerdoorman@yahoo.com - 19 Feb 2005 21:23 GMT
> By following Martin's Rule we keep the principle axis of the lens passing
> through the center of the eyes rotation.  If the Vertical OC is not dropped 1mm
[quoted text clipped - 23 lines]
> to the vertical OC placement. It's either that or the doctors Rx is wrong. Or
> both.

Excellent information. This whole vision thing was much more
complicated than I
realized.

Went back to LC yesterday. A different set of people there this week.
Didn't
try to tell me it was just astigmatism this time. First had them refit
the
frames (they messed with the nice fit last time trying to make them
work). Had
them double check the script was entered in their computer right and
had
them recheck my PD. So, after a bunch of measuring, they redid the
lenses.
First impression is that they are MUCH better, though I'll wait a few
days
to be sure. Oh, they raised the optical center about 6mm. It's now
clear when
I look straight ahead. Nice.

Thanks again for the people who wrote!

-A
 
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