HD Pentax-DA 20-40mm F2.8-4 Limited

Focusing

The Pentax 20-40mm incorporates an in-lens "DC" ring motor for autofocus; this type of focusing technology has enjoyed very good reliability since its debut in the 18-135mm WR lens back in 2010.  Like other DA lenses, the 20-40mm lens is also equipped with "Quick Shift" full time manual focus override, which means that you can make adjustments to the focus at any time simply by turning the focusing ring.  You do not need to put your camera in manual focus mode first.

The focusing ring, which is located at the front of the lens, has a throw of about 60 degrees and it provides only a moderate amount of friction.  There is no lens creep, however.  Focusing is not internal and the front element extends by about a centimeter at the minimum focusing distance of 28cm.  Also, the focusing ring turns during autofocus.

The video below shows the focusing speed and sound of this 20-40mm compared to the traditional screwdrive AF system in the 18-55mm kit lens:

Although this lens is not as quiet to focus as internal focusing (IF) lenses with DC or SDM motors, it is still significantly quieter than any screwdrive lens, which is a huge plus.   The lack of IF does take a toll on AF speed, though, as the lens simply isn't very fast to focus.  Going from one end of the focusing range to the other takes about 0.6 seconds, which is roughly twice as long as the 18-55mm's 0.25 seconds. Micro-adjustments aren't the smoothest either because of the amount of weight that the AF motor has to move around. 

Verdict

Even though the 20-40mm isn't the fastest-focusing lens in its class, fast focusing isn't critical to its success, as this focal length range isn't commonly used for high-speed photography.  The speed is still more than acceptable for everyday use, and we're pleased to see that Pentax has fitted this lens with a quiet and reliable autofocus system, something that all future lenses should include.  So, all-in-all, we are satisfied with the AF implementation in this lens as it will get the job done without attracting too much attention to itself.