In article . com,
Gerry wrote:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

Assuming you are starting from perfect (or perfectly corrected) vision
at "a long distance" (infinity, or at least the mountains across the
way) the focal length is related to diopters as the inverse in meters -
0.5 diopter will change "infinity" to 2 meters (and everything beyond 2
meters will be out of focus), 1 diopter will change "infinity" to 1
meter (and everything beyond 1 meter will be out of focus), 1.5 diopters
will change "infinity" to 0.75 meters (and everything beyond 0.75 meters
will be out of focus).

Since 0.75 meter is roughly 29.5 inches, it sounds like you should start
with a visit to an optometrist if you can't see the lathe at 22 inches
with 1.5 diopter glasses.

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