HD Pentax-D FA* 70-200mm F2.8 ED DC AW

Focusing

The HD Pentax-D FA* 70-200mm features the newer silent DC motor which has proven to be far more reliable than SDM, as well as near-silent.  In-lens focusing means that the lens won't autofocus with cameras older than the K10D (2006), which are quite dated by modem standards anyway. On the other hand, the newer AF systems are better adapted for contrast focusing using Live View.

Autofocus

The AF is almost completely silent. Even knowing that the lens uses DC focusing, this is impressive considering the amount of glass that potentially has to move. The lens keeps the same length whatever the focus distance: all focusing is internal. The following video illustrates the AF noise (or lack thereof), where the lens is set at infinity and tries to lock focus on a close black surface in low light.

The AF is also accurate and responsive. With the latest Pentax bodies, focus errors during our tests were almost non-existent with static subjects, and rare with moving targets. The few errors, when they did occur, were most often caused by the use of multiple AF points (auto 9, for instance). Using spot or 1-point select greatly reduced the risks of the camera selecting the wrong AF point. With lenses offering a very shallow DOF, using one point for AF is recommended.

AF speed is impressive. Subjectively, this feels like the fastest AF lens this reviewer has used. Subject tracking depends more on the camera than the lens, but at no point did we feel that the 70-200mm was slowing down the AF system, quite the contrary.

There is no provision for older camera bodies (before the K10D), no screw-drive option like what is found on several DA* lenses. Only the internal DC focus motor is available.

To put things in perspective, we compared the focusing speed D FA* 70-200mm to Nikon's 70-200mm F2.8 VR II when moving from minimum to infinity.  The Nikon lens operates more quickly.

Live View

Focus speed when using live view is impressive. There is no visible difference, in use, between the two AF modes for static subjects. The same level of accuracy was obtained with live view as with the viewfinder. This feels like the fastest lens we have ever used when using Contrast Detect AF (CDAF).

Manual Focus

Manual focus is not likely to be of paramount importance for most use cases of this lens. Still, at this price point, sub-par performance would be unacceptable.

The good news is that manual focus operates quite well. There is a good level of dampening on the focus ring, and better feedback than with most consumer lenses. The movement of the ring feels smooth (without reaching the level of a macro lens). The ring is correctly positioned, even though it would, maybe, be more intuitive to have the ring near the front element. The tripod foot does not hinder the ring's operation, and the distance scale is clearly visible behind its plastic window. The throw is long, at 180°, but adjustments are still very easy to perform, especially if the lens has reached the vicinity of the focus point beforehand. It is possible to easily move the ring with just two fingers thanks to gearing.

Quick-shift Modes

Quick-shift allows full-time manual focus override. The lens offers two quick-shift modes in addition to the standard Manual mode. In QFS/M (the "regular" quick-shift mode, a manual-focus priority mode), AF is disabled as soon as the focus is manually adjusted; autofocus will resume if the AF button or shutter is released and pressed again. MF is thus accessible at any time. QFS/A is an autofocus-priority mode, which allows the user to switch to manual-focusing operation only when autofocus operation is completed. This allows the user to make fine adjustments after the AF operation, but prevents unintentional disabling of the AF system.

AF Speed

We tested the AF speed with a Pentax K-1, using both live view and the standard PDAF (Phase Detection AF, through the viewfinder). The subject was a black cross on a white background, about two meters in front of the camera. We used the central focus point. We set the lens at infinity before each test, and three measurements were averaged for each data point. Measurements were performed by recording the AF noise, at various levels of ambient light. The AF range was set to full. We also tested the Tamron 70-200mm lens for comparison purposes, using the same parameters.

For reference, the Pentax K-1's autofocus sensor is rated for ambient light levels as low as -3EV.

This chart shows a number of interesting results. The first is that, with the Pentax lens and under our specific test conditions, contrast focus is slightly faster than phase focus. This is a first for any of our tested lenses. The differences are minimal, and will likely depend on the shooting situations, but they are consistent across the various light levels tested. At all times, the lens performs (and feels) fast and responsive. Results stabilize at a little below 1 second, but CDAF appears to gain a little at higher light levels (remember that the sun is about at EV 15, while 5 EV is reached by using one 65W light bulb in the ceiling, no more).

The Tamron lens is much slower to focus when light levels are low. In PDAF, its performance reaches that of the Pentax only at the brightest level tested. CDAF is never really in the race, and has a much higher rate of missed shots, enough for us to be wary of using it.

Both lenses show a visible improvement starting just below 2 EV. This is probably more a property of the camera than of the lenses given the consistency.

The Tamron's CDAF performance also showed some erratic behavior at very low levels. Screw-drive lenses are not designed for live view focusing, so this is not surprising. At other times, both lenses performed very well, as they should.

Verdict

The Pentax 70-200mm performed above expectations for AF, both using the viewfinder and in live view. Speed and reliability are probably the best we've tested. Using the focus limiter appropriately improves performances even more, especially on the near range (1.2 to 4m).

The Tamron can be very good in the right conditions (and using the viewfinder), but slows down when light gets low. This is a visible difference between the two lenses, although many other lenses are no faster than the Tamron.

In short, users should feel comfortable with the DC autofocus motor implementation in the D FA* lens, as it comfortably exceeds the capabilities of screwdrive.