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I've read a couple of times that ED glass generally has an abbe number greater than 80, for example FPL-51, while SD glass has an abbe number greater than 90, for example FPL-52 FPL-53 or OK-4. I wouldn't trust this to be true in all cases though, since they are loosely used as marketing terms (just like how loose the term APO is becoming nowdays).
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