It's really a shame that manufacturers did not develop a standard used by all, and use standard nomenclature for there diopters. Bronica, for example, lists their standard eyepiece as a negative diopter value but the lens is certainly a positive lens. Go figure.
That's more what I'm hoping to find... so after your paragraph saying "+1.5 offset my eye needs a true diopter -2.5" and so on... and then standard eyepiece and I need a +1.5 or +2.5... not sure if I'm reading this properly, but in the simplest terms - if I need a +1.5 reader for reading and 1-meter-ish work, then I need something around a +1.5 diopter, depending on how the manufacturer dealt with it??
Right...the key question is "How did the manufacturer deal with the labelling of the Accessory's strength?"...Do you
buy an accessory called '+1.5' which is stronger relative to the standard eyepice by that amount, or
do you buy an accessory called '+2.5' which has an absolute strength optically which can be measured by optician?
Going back to Bronica which was brought up, Bronica state they offer range of -4.5 to +1.5...but standard eyepiece is -1.5
so, from Bronica, you buy -2.5 or -3.5 or -4.5 if you need Negative correction from standard or
you buy -0.5 or +0.5 or +1.5 if yoiu need Positive correction from standard Absolute strength of the diopter lens.
But for another brand you buy -3 or -2 or -1 for negative offset from standard, or
buy +1 or +3 or +3 for positive offset from standard
for the exact same six corrections!
Canon, for example, says their dioptric corrections are 'not true'. for the Canon F-1 accessories..you buy Relative offset strengths for the Canon F-1