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

[Bexley Hall]

Hi, Jeremy,

Come to my rescue, yet again??  :>

--- On Thu, 8/21/08, Jeremy D Rogers <jdrogers at optics.arizona.edu> wrote:

> 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.

Are you playing fast-and-loose with terms or strictly pedantic?
I.e., "optical power"... so, is a "2X magnifying glass" (2 power?)
a "-2.00"?

Or, am *I* playing fast-and-loose?  :-/

> Glasses for far sighted people (reading glasses) are
> positive power which is what you want.

Yes, which is why the thought struck me that I could just
buy some El Cheapo (reading) glasses and steal the lenses...
But, how do I convert/compare diopters to "2X, 4X, 7X, etc."
encountered on magnifying glasses?

> 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. :-)

OK, I'm far sighted so don't normally need glasses for close in
work.  *But*, reading the little tiny writing on surface mount
components, etc. is now beyond me.  :<  Even bringing the item
"up close" ... I just end up feeling like I am looking
cross-eyed!  :<

> 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.

OK, what's a "near point"?

> 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.

I had such a thing (clipped onto my eyeglasses) when younger.
But, realized I rarely needed it back then so...  :<
(I also used to have a nice *metal* shoe horn that I never
needed but sorely mis now!)

The problem I have found with most things (like *worn* reading
glasses is that I end up with the lens close to my eye and.or
the "target" close to my face.

Ideally, I want to be able to hold something (lens) in my hand
at a comfortable distance with my head far back from both.
For exemple, I may be trying to solder a component on a circuit
board -- I don't want my face so close that I can feel the heat
(and fumes!) from the soldering iron.

And, I think (?) to get any decent magnification, I would have
to have a large (diameter) lens so both eyes could see through it
if it was close to my face -- hence a smaller diameter lens would work
if it was close to the "target" (i.e., both eyes could converge on
the lens more easily).

Sorry my explanation isn't very precise.  :-/  I don't know enough
about the science to put it in better terms.