Pentax 17-70mm Lens Comparison
Pentax 17-70: 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, with different colors for each. Its total throw is about 45°, and does not move past the infinity mark. The ring is completely lifeless, with next to no feedback and very little friction but very smooth movement. In use, MF is easy but the short throw of the focus ring means accuracy is average without the use of an LCD. The lens appears to be parfocal, which means that the focus distance does not change with focal length. Thanks to the Pentax quick-shift system, MF operation is possible at all times by simply moving the focus ring in any mode.
Like the Sigma, this lens will be used, most of the time, in AF mode. In this regard, our sample delivers average results. SDM means that AF is virtually silent, which can be a very important feature in some situations. However, in all but the brightest conditions, we have found the AF system to be hit and miss. With our K20D, the lens would often reach a visually satisfying focal point, and then hunt near it for a second or so before giving up and giving the signal that focus has failed. Curiously, this occurred more often with the center focus point, which should be the most sensitive and reliable. Focal length and focus distance did not impact this behaviour.
Tests performed with a Pentax K-5 exhibited a comparable behaviour. In that case, the center point was very reliable, but focus problems occurred with the other focus points. The levels of contrast and the brightness of the focus target appear to have no link to the rate of success.
When it does lock focus, the lens does so quite fast. We have compared the noise it makes with that of the Pentax 18-135, another silent-focusing lens, and the 17-70 produces much less noise. While the 18-135 emits a low, very high-pitched noise, the 17-70 truly is almost silent.
Refer to the comparison section for AF speed figures.
