Originally posted by Rondec It is really tough to say anything global about specific lens types. The DA *55 has SDM but a really long focus throw and a really slow motor and is completely useless for tracking. The 50-135 is only slightly faster. The Pentax DFA 24-70 is also listed as having SDM, but it is a Tamron produced lens and quite a bit faster.
With screw drive, the motor is typically fast enough, the problem is there simply isn't enough precision in the system to keep the lens on track. Lenses like the FA 135 and DA 40 limited are very fast to focus though -- faster than SDM lenses -- I just wouldn't trust them to be accurate in a tracking situation. They still might be decent on a K-3 III, just a decreased hit rate percentage compared to other lenses.
This is something I would really like to be addressed in a Pentax Forum review. An assessment of how well the new AF system works in both single shot and continuous mode with ideally:
- F/FA-era screwdrive primes
- F/FA-era screwdrive zooms
- DA-era screwdrive primes
- DA-era screwdrive zooms
- Old SDM star lenses
- DC lenses (such at the 20-40 or 16-85)
- PLM lenses (well, it's "lens" at the moment, but anyway, the 55-300 PLM)
- New SDM (DFA50 or 85 at the moment)
- Sigma 18-35 (or another somewhat recent Sigma with a history of AF complaints)
It's a demanding list, so I know it's a lot easier said than done. But if any site has the knowledge to cover this thoroughly, it's Pentax Forums.
I think if it can be demonstrated that the K-3 III is going to give new value to the lenses we already have, it's going to encourage a lot more people to go for it.