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Hier eine Erklärung vom wunderbaren Henry Link aus dem "birdforum" dazu:

Keep in mind that "FL" and "HD" are meaningless marketing terms used mostly to conceal what kind of glass is really used.

Only Minox, to its @#$%&, comes right out and tells us that SCHOTT N-PK52A is the ED glass used in its so-called APO binoculars. It's almost certain that Zeiss also uses SCHOTT glass since SCHOTT AG and Zeiss AG are two parts of the same company. If Leica and Swarovski are sourcing their glass in Europe they're probably buying from SCHOTT also.

The current SCHOTT catalogue lists three glass types that could be called "ED":

N-PK51 with a Vd number of about 77
N-PK52A with a Vd number of about 81.5
N-FK51A with a Vd of about 84.5

None of these are very close to the "gold standards" of low dispersion materials, which are Fluorite (CaF2) and Ohara FPL53, both with Vd of about 95 (the higher the Vd, the lower the dispersion). The most likely candidate for "FL" glass is N-FK51A, since it's a fluoride containing glass with the highest Vd number of anything in the current SCHOTT catalogue, except Fluorite.

BTW, the type of ED glass only tells us what CA correction is potentially possible. The mating glass types and the total design will determine whether that potential is reached. I suppose that's one good excuse for not revealing the ED glass type, so that consumers aren't tempted to rely solely on the Vd number as a shorthand predictor for quality.:

(leider in Englisch)

Grüße

R.K.