The Sigma 50-100mm f/1.8 DC HSM Art is a premium mid-telephoto zoom lens designed for APS-C sized sensors. It features an f/1.8 aperture all the way through its focal length  and factoring in the APS-C increase  the lens covers a highly useful range of 75-150mm. An enhanced and slimmer Hyper Sonic Motor (HSM) provides fast and accurate autofocus  while 3 Premium FLD and 1 SLD Elements produce incredible image sharpness. Inner focusing ensures stability while focusing without changing the lens physical length.