Sigma 17-70mm F2.8-4 Contemporary
Autofocus
The Sigma 17-70mm "Contemporary" features an in-lens HSM focusing motor. This motor delivers fast autofocusing that is almost completely silent. Unlike Pentax SDM motors*, Sigma's HSM system has enjoyed great reliability over the course of the past several years.
During our field tests (with the Pentax K-5 and K-50), the the 17-70mm's autofocus system never disappointed. The autofocus was quick for all applications, including close-ups, landscapes, portraits, and indoor photos. Because our subjective tests raised no concerns, we did not perform controlled tests of the 17-70mm's AF speed.
Note that the 17-70mm does not support Pentax's traditional screwdrive AF system, and it will therefore not be able to autofocus on early Pentax DSLRs (the *ist D series, the K100D, and the K110D). All DSLRs made in 2007 and later are supported.
*based on Pentax SDM survey results
