1. Reading your eye glass prescription http://www.allaboutvision.com/
What is PD-Pupil distance Where to get it or measure it.
Miller Optical had this measure for me.
2. Converting the prescription to reading glasses (Zenni had instructions for this)
My prescription for example:
Using sphere reading:
OD -1.00 add the ADD reading 1.75= OD .75
OS -0.25 add the ADD reading 1.75= OS 1.50
PD # less 3 if PD for reading glasses.
3. Reading the eyeglass frame numbers from a current pair of glasses. http://www.selectspecs.com/
4. Comparison of Online Eyeglasses companies.
Paul & I spent a lot of time finding this information . And I spent hours going through frame sizes until I found one that closely matched my current frames. My frame # is 51019140 That translates into
Lens width 51
Bridge width 19
Temple Length 140