Pentax-D FA 100mm F2.8 Macro WR

Focusing

The D-FA 100mm macro WR does not come equipped with SDM or DC motors, relying on the screw-drive of the camera for autofocus. This is somewhat surprising, since even when it was released in 2009, SDM was already available for several new lens models. On the other hand, a screw-drive means that the lens is compatible with every legacy camera model.

Autofocus

The AF generates a moderate amount of noise, which can be annoying when the lens rakes through all its focus range, but won't be disturbing for fine tuning between shots.

AF speed is slow when large adjustments are necessary, which is to be expected with a lens offering such a long throw. Luckily, small corrections are fast enough for everyday use. AF is very accurate and reliable even for macro work. However, we did observe some hunting when the AF system could not easily find the subject within the macro range (think about a leaf moving in the wind, fooling the AF system). In those cases, the lens can hunt all the way towards infinity before coming back and finally finding its subject. A limiter would have removed this problem. Quick-shift is a partially working solution, imperfect but globally effective. This behavior is still undesirable, but not unique to this lens.

Focus outside the macro range is in another category, being in fact surprisingly fast. The lens never hunts towards the macro range when not needed, so we never felt the need for a limiter in those circumstances. Small AF corrections near infinity were almost instantaneous. This lens will easily serve as a short tele for everyday use.

Live View

Live view AF (using contrast detection) can be pretty good or pretty bad. When it acquires the target in one pass, it can be reasonably fast. However, when it misses, things get worse, as it needs to scan the whole focus range before coming back to the focus point, which takes more than a second.

In one occurrence, using a K-3, the AF system behaved in a surprising way, one which we have not seen with any other lens. The AF point was about one meter in front of the lens. The AF system missed it, went all the way to infinity, as we have come to expect. Finding no lock, it proceeded to move back towards short distances, but at a very low speed, one that would be expected for movies, not stills. In that occurrence, it did lock focus eventually, but it took a very long time.

In all cases, focus has been accurate and precise.

Manual Focus

Macro photographers often disregard AF completely, and depend solely on manual focus. These users will be pleased to know that manual focus is extremely pleasant. The lens offers a very long throw and butter-smooth dampening. The focus ring intuitively falls under the hand and is easy to operate. Live view available on modern camera bodies allows very accurate manual focus, especially by using focus peaking. We never observed any creeping of the focus ring.

AF Speed

We tested the AF speed with a K20D and a K-3, the latter using both live view and the standard PDAF (phase detection AF, through the viewfinder). We concentrated on the intended use of the lens, which is close focusing. The subject was a black line on a white background, about 40 cm in front of the camera. We used the central focus point. The lens was set 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.

AF Speed Test Results

This chart shows that Pentax has made visible improvements to the AF speed of its cameras, especially in low light with PDAF. In both cases, the lens is far from being a speed demon, although focus speed improves greatly when lighting conditions get better.

Contrast focus using live view is a letdown, but is to be expected with a screw-drive lens. This is one area where SDM and DC motors provide a significant improvement on AF speed. This behavior is not unique to this lens, but is observed with any lens using screw-drive.

Verdict

While not particularly fast, the AF performances of the lens falls within expectations. Previous comparisons performed with the Tamron SP AF 90mm F2.8 Di Macro showed that the Pentax lens has a small edge regarding AF speed. The accuracy of the autofocus is excellent in most cases. When used outside the macro range, AF is excellent and reliable.