Good article about diopters

http://medfmt.8k.com/mf/diopter.html

Diopter Lenses Page

by Robert Monaghan

And make sure to check out the chart for achromat (the good stuff) diopters - which can tell you what "5T" or "500D" means:

http://www.angelfire.com/ca/erker/closeups.html

Cheers

James