HD Pentax-D FA 24-70mm F2.8 ED SDM WR

Focusing

Surprisingly. the HD Pentax-D FA 24-70mm does not take advantage of the newer, silent DC motor which we've seen used on nearly every recent Pentax lens. Considering that the lens is a rebadged Tamron design, it is likely (but unconfirmed) that the focus motor is, in fact, made by Tamron, using a technology closer to Pentax's older SDM than the newer DC. This hypothesis is supported by the fact that the other Tamron rebadged lens, the 15-30mm, also bears the SDM label, while every other D FA lens recently released uses DC focusing.

Since Tamron's lenses do not have a history of failing AF motors, the SDM label on the new D FA should not be the source of worry for new users.  The technology does not appear to have anything in common with the troublesome SDM technology found in DA and DA* lenses from the mid 2000's.

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, and not full frame. On the other hand, the newer AF systems are better adapted for contrast focusing using Live View.

Autofocus

The AF is almost completely silent, and will be inaudible in all but the quietest environments. The lens keeps the same length whatever the focus distance: all focusing is internal. The following video illustrates the low level of AF noise where the lens is set at infinity and tries to lock focus on a close black target in low light. The video also illustrates how decisive the AF system feels during use.

The AF is generally accurate and responsive. AF speed is impressive, especially in good light. A wider field of view delivers a deeper depth of field, which helps the AF system. Even so, the lens feels fast, although not as much as the recently-tested D FA* 70-200mm.

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 SDM focus motor is available.

Live View

Focus speed when using live view has improved greatly with the most recent lenses from Pentax. It does not feel slower, nor any less reliable, than using the viewfinder. The same level of accuracy was also obtained with live view as with the viewfinder. The 24-70mm is among the fastest lenses we have ever used when using Contrast Detect AF (CDAF).

Manual Focus

Like most modern lenses, manual focusing is not the main use case for the 24-70mm. Nonetheless, Pentax lenses made for digital always include some perks for users interested in taking control of the focus ring.

Like every other lens made for digital, the 24-70mm includes quick-shift for automatic override of the autofocus system. The focus ring also comes to soft stops at both ends of the range, meaning that it can keep on turning with increased friction, without affecting the focus distance. Reversing the movement immediately engages the focus system, without any backlash.

The movement of the focus ring is pleasantly smooth and dampened. Without reaching the level of the best contenders (such as macro lenses, or FA Limited), it is on par with other high-end lenses from Pentax, and certainly better than entry-level lenses such as the 18-55mm kit lens or FA lenses in general. It should not disappoint. The throw of the focus ring, at 100°, is longer than on many other AF zooms. As such, adjustments should be relatively easy, especially at the wide end where the short focal length yields a longer DOF.

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.

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

Focus speed

The chart shows that, in fact, the 24-70mm can be even faster than the D FA* 70-200mm when light levels are good. It can be compared to the D FA* lens at lower levels, which is probably more dependent on the camera than the lens. There is a steady increase until about 1.5 EV (a low level in itself), at which points the speed increases more gradually.

There is a measured difference between Contrast and Phase Detection AF, something that is not really felt in actual use. The two systems become similar as light levels increase.

Hunting

We experienced almost no occurrences of hunting with the 24-70mm during our tests. As can be seen on the video above, AF is very fast and decisive, going straight to the focus point without hesitation.

Verdict

The Pentax 24-70mm is fast, no doubt about it, and among the fastest-focusing lenses currently available for Pentax. It is somewhat sensitive to ambient light levels, but even in low light conditions, its speed is still better than most other lenses in the current lineup.


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