There was a very insightful post on here a while ago describing the varying grades and designations of the various types of ED glass. Does anyone have a link to that? Thanks.
Nope, although I do recall that discussion. But at the risk of seeming presumptious, there are three grades of fluoride bearing abnormal "ED" glass, referred to by some makers ( cross referenced to other maker's nomenclature, all of whom share the same final numerical suffixes) as FPL51, FPL52, and FPL53, in ascending order of abnormality. Only very slightly better than FPL53 is pure calcium fluorite crystal, commonly called "fluorite".
When paired with the best mating element in a doublet lens, FPL51, 52, and 53 provide reductions of color error, compared to a normal crown/flint doublet, of factors of about 2, 4, and 8 respectively.
I am not posing as an optics expert by any means, but have read some things on this subject at the amateur level (I would recommend Rutten and van Venrooij's "Telescope Optics"), due to my interest in astronomical telescopes. My scope being, after all that, a crown/flint doublet!
Otherwise, to find that original thread, I would search on "Abbe number".
Ron
Just to expand a little on Ron's information, most every glass maker seems to make an Ohara FPL51 equivalent with an Abbe# around 82, but AFAIK the only Fluorite equivalent glass in current production is FPL53 made only by Ohara with an Abbe number of 95. Ohara FPL52 (Abbe# around 90 I think) is no longer in production and I don't think there is an equivalent glass available from anyone else.
We aren't often told what glass is used in spotting scopes or binoculars, but since they (especially binoculars) represent a low magnification non-critical use it's probably safe to assume that most "ED" glass used in them is an FPL51 equivalent. I've heard of one spotting scope that is said to use FPL53 (Kowa 773/4) and no binoculars. I noticed one binocular maker specifying Schott FK5 with an Abbe# of 70, which barely qualifies as ED at all.
As Ron mentioned, the ED glass type determines the potential color correction, but the mating glass types and the overall design will determine if that potential is reached.