[Tfug] ... and, yet another, optics question! ;-)

[Jeremy D Rogers]

Hi Don,

Diopters is the optical power of the lens in inverse meters. Being
near sighted, my glasses are -3 diopters, so they have a focal length
of -1/3 meters or -30 cm. That would not help you. Glasses for far
sighted people (reading glasses) are positive power which is what you
want.

A magnifying glass works by increasing the optical power of your eye
or equivalently shortening the total focal length of the eye/lens
system. Since I am near sighted, my glasses have a negative power.. so
all I have to do is take off my glasses and put the object closer --
no pocket magnifier needed. :-)  So what you want to look for is
higher power or more diopters and you want them to be positive (which
all reading glasses are). The magnifying power that you get from a
given lens depends on your eye's near point though. The only drawback
to using a lens from a pair of glasses is that the lens is usually
curved, so give a focal length, the thickness might be more than a
lens made to be a magnifying glass. On the other hand, sometimes
glasses use a higher index glass or plastic that requires less
curvature to achieve the same focal length which would make it
thinner. Probably your best bet is to play around with the pair of
glasses in the store and see if it works for you better than what you
have.

One other thing you might look for is a loupe. This is what jewelers
and such use, and it isn't flat, but it gives more like 10x and still
is easy to carry around.

--
Jeremy D. Rogers, Ph.D.
Postdoctoral Fellow
Biomedical Engineering
Northwestern University



On Thu, Aug 21, 2008 at 1:31 PM, Bexley Hall <bexley401 at yahoo.com> wrote:
> OK, eyeglasses are measured in diopters.
> Magnifying glasses are expressed in *power*.
>
> I want a small magnifying glass that I can
> carry with me (ideally, in my *wallet*!).
> Most of the magnifying glasses that I find
> are "3X" and very large.  (of course, there
> are some cheap fresnel style lenses that are
> *flat* but I suspect they wouldn't work well
> over the long run)
>
> Can I just buy some COTS "reading glasses"
> and pop the lenses out of them to act *as*
> a magnifying glass?  If so, how do I decide
> which (diopter) to buy?
>
> [<grin>  you can see I find optics an
> interesting field given my inexperience with it!]
>
> Thx,
> --don
>
>
>
>
> _______________________________________________
> Tucson Free Unix Group - tfug at tfug.org
> Subscription Options:
> http://www.tfug.org/mailman/listinfo/tfug_tfug.org
>