Introduction The new $999 model has a high-grade optical construction consisting of 18 elements arranged in 13 groups, with one FLD type low-dispersion element, which the company claims to have a similar performance to fluorite. It also has no less than seven SLD (ED type) elements, two of which are aspheres to reduce lateral chromatic aberration, field curvature, and spherical aberration. The lens also features a third, large-diameter aspherical element to minimize distortion.Like previous Art series models, it has sonic-type AF motor and has what Sigma claims is an optimized algorithm for smoother focusing. The lens
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