I’ve recently seen a reference to FLD glass being used in Sigma lenses, is this different from ELD and SLD glass?

FLD ("F" Low Dispersion) glass is the highest level low dispersion glass available with extremely high light transmission. This glass has a performance equal to flourite glass which has a low refractive index and low dispersion compared to current optical glass. FLD glass offers superior optical performance, equal to flourite, at an affordable price.

ELD (Extraordinary Low Dispersion) glass has lower dispersion characteristics than SLD (Special Low Dispersion) glass, which Sigma has been using in its APO lenses (and some non-APO lenses as well) for many years now. It has other advantageous properties as well.

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