What does FLD mean ?
“F” Low Dispersion (FLD) glass has been adopted and used in Sigma’s new lenses. FLD glass is the highest level, low dispersion glass available for Sigma’s lenses with high light transmission. With a performance equal to fluorite glass, this optical glass has a low refractive index and low dispersion compared to current optical glass. It also benefits from high anomalous dispersion. These characteristics should offer excellent correction for residual chromatic aberration (secondary spectrum), which cannot be corrected by ordinary optical glass and ensures high-definition and high-contrast images. According to Sigma, FLD glass offers superior optical performance, equal to fluorite, at an affordable price. The density of FLD glass is lower than traditional optical glass, ensuring a lighter construction of large aperture lenses.
FLD glass has been incorporated into three recently announced lenses. Four elements have been included in the Sigma 8-16mm F4.5-5.6 DC HSM and two elements have been included in both the Sigma APO 70-200mm F2.8 EX DG OS HSM (photo © Sigma Corp.) and the Sigma 17-50mm F2.8 EX DC OS HSM.