Pentax-DA* 200mm F2.8 Review
Autofocus Performance
The primary autofocus mechanism of the DA* 200mm is the Supersonic Drive Motor or SDM. This technology departs from Pentax' usual screw-drive mechanism, in which the focusing mechanism is driven by a motor in the camera body. Instead, SDM uses a motor that is built into the lens. This technology provides a much smoother and quieter focusing experience, at the expense of reduced speed in many cases (unlike newer "DC" lenses featuring ring motors).
To ensure compatibility with older bodies, the screw-drive mechanism is also included in the DA*200mm f/2.8, but it won't be used on modern DSLRs.
In the following test, we shall evaluate the autofocus speed of the DA*200mm f/2.8 when using the SDM mechanism. The body used for this evaluation is a Pentax K-5, firmware version 1.14.
Our test aims to measure the time that the lens takes to displace its focusing distance from the closest focusing distance (1.2 meter) to the farthest focusing distance (infinity), and vice-versa. These tests were done 5 times each, then the resulting time values were averaged to provide a more representative figure. Our test scene has a luminosity equivalent to 8.3 EV and it was illuminated using tungsten balanced CFL (compact flurescent lamps).
The results were the following:
- Infinity to 1.2m: 1.44s
- 1.2m to infinity: 1.84s
We have noticed that the DA*200mm f/2.8 displays some latency between when the user activating the autofocus (half-press shutter release or back-button focusing) and when the SDM mechanism actually starts moving. We attempted to measure this period to determine how severe this latency can be (when compared to the actual autofocus times). We measured the latency on both directions for 5 trials each, and the average is 0.95 seconds. The direction of the focus displacement seems to not impact the amount of latency.
Verdict
The autofocus speed of the DA*200mm f/f2.8 is slower than expected, especially when compared to older screw-drive lens like the FA* 200mm. When evaluating the numbers carefully, we find that most of the time is spent on the latency, which accounts for 50-65% of the autofocus times. If this latency were to be eliminated, the actual time (where the SDM motor is moving) is quite acceptable and in-line with the speed of screw-drive lenses.
We conclude that the SDM autofocus speed is one of the most glaring weaknesses of the DA* 200mm f/2.8. Other than that, we find that other aspects of the SDM to be quite satisfying, such as its near-silent operation and its smoothness. The DA* 200mm would certainly benefit from a DC AF motor upgrade.
