On Tue, 18 Sep 2007 18:43:25 -0700, 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?


Nominal magnification is inversely related to the normal
reading distance of 10" so a 1" focal length lens used as a
magnifier is rated as x10.

Spectacle glasses are specified in Diopters which
determine the way in which they modify an eye focussed on
infinity. 1 Diopter is a focal length of 1 meter (39.37") and
would cause a normal eye focussed at infinity to focus at 1
meter.

With normal sight your 1.5D safety glasses would focus at
39.37/1.5 = 27". Because you find your working distance is less
than 22" it means that you are short sighted and need a WEAKER
lens maybe 1.0 or 0.75D

Try store bought reading glasses of this power. It is
possible they may not give enough improvent. If this is the case
you also need correction for astigmatism (different focal power
on horizontal and vertical line objects) and this means a visit
to the optician for tailor made glasses.

Jim