Bringing Special Low Dispersion Glass to Life.

The development and mass production of SLD (Special Low Dispersion) glass enabled the mass production of high-quality lenses and was a turning point for SIGMA’s principle to provide “great lenses for as many people as possible”.

Photo : The ‘APO zoom 70-210mm F3.5-4.5’ was realized through the mass-production process technology of SLD Glass created by the know-how of lens experts.

The development of the APO 70-210mm F3.5-4.5 telephoto lens, released in 1986, was a major turning point for SIGMA. With its strong optical performance, overall high quality and produced with an improved efficiency, it realized SIGMA’s policy to provide great lenses for as many customers as possible.

It was a groundbreaking lens in part for SIGMA’s ability to keep the recommended retail price at 63’000 Yen (about $600), despite the generous use of SLD (Special Low Dispersion) glass, a material that reduces optical aberrations.

SIGMA had been working on a telephoto lens with reduced optical aberrations since the early 1980s. At the time, the best material for this purpose was fluorite, but due to the high cost it was difficult to use fluorite in anything but high-end glass lenses. SIGMA looked for a replacement material that would allow mass production but despite best efforts the cost efficiency problems remained.

After countless experiments, SIGMA concluded that a joint development together with a capable materials manufacturer was necessary to enable mass production and realize a high-quality lens at a mass-market price point.

The APO Zoom iota 50-200mm F3.5-4.5, released in 1984, was the first SIGMA product made with SLD glass. Mass-production technology had not yet been established, and SLD glass was exclusively used in high-end lenses.
SLD glass is succeeded by ELD (Extraordinary Low Dispersion) glass and FLD (“F” Low Dispersion) glass, whose optical characteristics are very similar to fluorite. These materials made it possible to offer high-performance lenses at an affordable price point.

The mass production of SLD glass was a painstaking process. Due to the material properties of the glass, manufacture was difficult and the material defect rate high. In addition, serious problems kept occurring: shape and dimension of the lens were unstable when stocked; cords (1) appeared in the lens with high frequency; the glass scratched too easily during the polishing stage; high abrasion making it impossible to control the machining accuracy; differences in temperature caused the glass to expand too much and break, and so on…

However, based on the principle of their joint venture to profit from and improve each other’s technologies and knowledge, all problems eventually were overcome one by one thanks to the cooperative efforts of the materials maker and the factory in Aizu. In the end, we managed to establish a manufacturing process and a stable materials supply. The dream of a mass-produced high-performance lens was realized. Development of the APO series (2) with its rich use of SLD glass followed. The foundation of SIGMA’s policy of ‘high-performance lenses at reasonable prices’, exemplified by our 150-600mm F5-F6.3 DG OS HSM | Sports / Contemporary lens, was laid.

(1) A foggy streak in the glass. Cords are caused by a lack of homogeneity in the glass.
(2) Before the launch of SIGMA GLOBAL VISION, this meant any telephoto lens with more than 2 SLD glass elements.

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