Re: [Phys-l] optical center : real-life physics

[Roger Haar <haar@physics.arizona.edu>]

Hi,

	A few years ago I had to go to bifocals.  I had broken the frame on my 
previous glasses and wanted to do the replacement process quickly, so I 
went to one of the places in "the mall."  Something made me 
uncomfortable about their fitting process and as they were tallying up 
the bill I decided that I did not trust them and said "Sorry" and left.

	I went to an independent place and they had a very good person ( 
opti-something) doing the fitting process.  She carefully adjusted the 
frame and then had me look straight ahead and marked the lenses with 
where my eyes were.  She explained that was necessary to get the 
position of the near lenses correct.  The mall place had not done that, 
so who knows where the bifocal part of the lens would have been.

	So I am not very surprised that someone really screwed up a pair of 
bifocals.  Just another buyer beware tail.

Thanks
 Roger Haar
   Physics U of AZ

***********************************************************
John Denker wrote:
> Hi --
>
> One of my relatives recently had some bifocal eyeglass made at
> Wal*Mart.  The distant-vision lenses seemed OK, but she complained
> of big problems, including double vision, when she tried to use
> the reading lenses.  The Wal*Mart guys said she'd get used to it
> in a few days.
>
> The glasses "looked OK" to me at first glance, but on a hunch I
> used one of my dollar-store lasers to locate the optical center
> of the reading lenses.  The results were as shown below, where
> the optical center is indicated by a "+" mark:
>
> 	 ________________          ________________
> 	/                \        /                \
>         |                  |      |                  |
>         |       _________  |      |  _________       |
>         |      /      +  \ |      | /      +  \      |
> 	\    |           /        \           |    /
> 	 \    \         /          \         /    /
> 	  \____\_______/            \_______/____/
>
>
> It was kinda fun to be able to do some physics that took only
> a minute or two, and was of immediate value to a real person
> (as opposed to long-term research that requires a big lab full
> of equipment, and is supposed to benefit "somebody" in the
> distant future).