Pentax 17-70mm Lens Comparison
Sigma 17-70 v1: Focusing
The focus ring is large enough and well-textured. It falls easily under the hand when using the camera. The ring bears distance marks in meters and feet. Its total throw is about 55°, including a few degrees past the infinity mark. Because of this, manual focus is not particularly easy. The ring is surprisingly stiff, however, more so than most current AF zoom lenses, making MF more reliable than on common AF zoom lenses. The lens is not parfocal : zooming in or out will change the focus distance. In fact, rotating the zoom ring with the focus set to manual will also rotate the focus ring! This behaviour does not occur in AF mode, however, and the lens appears to be parfocal when in AF mode.
This is a lens that will be used, most of the time, in AF mode. In this regard, it delivers satisfying results. It is not the fastest AF lens around, but it is more than fast enough. In use, it feels responsive and AF is very accurate at all focal lengths. The focus ring rotates when AF is engaged.
AF is driven by a screw: the lens does not offer silent, SDM-like focusing. The AF gears generate a low level of high-pitched noise, not worse than any other screw-driven lens. Refer to the comparison section for AF speed figures.