3 aspherical, 5 super low dispersion, 1 fluorite-like dispersion
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[Tested on 42/61 MP camera] It’s a nice blend of build, autofocus, and optical performance in a price that is reasonable ($1400 USD) compared to the Sony alternatives [...]. It’s chief competition in many ways will be the much cheaper Tamron 17-28mm F2.8 RXD ($900 USD). There are many reasons to choose the Sigma, despite the price difference, including a more useful focal range, more feature-rich build, better edge performance, and lower coma.