"Richard J Kinch" wrote in message
. ..
BQ340 writes:

For example how would I specify a pair
of bifocals that give my say 3X at 2'?

Eyeglasses do not do that.

Reader eyeglasses do not magnify per se; what they do is move the near
point (how close you can hold something in focus) closer to your eye.
The standard distance of 10 inches is considered normal closest reading
distance and could be considered 1X. Now someone with emmetropic but
presbyopic vision (focused at infinity but no accommodation to focus
closer like a younger person) will need a +10D (+10 diopter) lens (100mm
focal length) to see at 10 inches. This is also about the strongest
eyeglass Rx. A +40 diopter lens (25mm focal length) is about the
strongest simple magnifier possible, and this lets you focus on items 1
inch away, which is in effect a 10X magnification from the standard 10
inches. But at +40D you are into loupe magnifiers and not eyeglasses.

To have true magnification the lens system has to be away from your eye,
not a contact lens with zero vertex distance or eyeglasses with a small
vertex distance. This is why people with high-power prescription needs
want to have small lenses that can tuck closer to the eye; then the
world is not distorted bigger, just focused properly. A true simple
magnifier produces a larger virtual image of the subject at infinity
focus. But it has to be away from the eye to do so, otherwise you're
just changing the refraction of your eyeball (and its focus point), not
magnifying or minifying anything. Another way to think of this is to
observe that putting a lens directly on top of an object doesn't magnify
the object; a simple magnifier must be some distance in between both the
object and the observer.

If you really want things 2 feet away magnified by 3X, the device is
called a "telescope" and is made of multiple elements. One example is
the binocular scopes worn by surgeons and dentists, the simplest types
being little Galilean opera glasses that are compact but dismal. Another
is a stand-mounted operating microscope like a Zeiss OPMI (called a
microscope when it actually is a close-focusing telescope).

As a machinist you can improvise your own binocular "operating
microscope" from a $10 pair of import roof-prism binoculars. All that
is needed is to extend the travel of the eyepieces further back to bring
the close focus point within a few feet:

http://www.harborfreight.com/cpi/cta...emnumber=92442

Richard, thanks for the great explanation. After reading it 4 times, I
think I understand it. Isn't the internet wonderful?

Thanks again for your wisdom.

Ivan Vegvary