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10-22-2009, 06:08 PM   #1
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SDM vs. screwdrive

I own two SDM lens (DA*50-135 and DA*200) and some screwdriven lens (FA50 f1.4, DA35 ltd, DA 16-45 and Tamron 90 2.8 macro). All of these lens are used exclusively with my K10D. Although I love my DA*50-135 and think it's the best portrait zoom ever, I fing it fairly slow focusing. On the other hand, the DA*200 focuses just as fast as my screwdriven lens. Anyone used the SDM lens on a K10D with the screwdrive? Are they faster when used with the screw?

10-22-2009, 11:58 PM   #2
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I haven't used it on K10D but the fisrt copy of my wife's GX20 had faulty SDM contacts so it was driving 50-135 via screw. Honestly the AF speed wasn't really different to the SDM on my K10D, maybe miliseconds one way or another but it's like tossing up a coin I'd say. It seems 50-135 is not the fastest focusing thing you can have in backpack, whether screw or SDM driven.

10-23-2009, 12:21 AM   #3

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At one point one of my K10D bodies had firmware 1.3 (supporting SDM), the other had 1.2 (no SDM support). The focusing on either body were about the same with the Pentax DA* 50-135mm.

I agree that the Pentax DA* 50-135mm is slow-focusing. It for sure is slower than my Sigma 150-500mm, even more than 2 stops faster.
10-23-2009, 02:03 AM   #4
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One thing that determines AF speed is focus throw - the distance the focus ring needs to travel from one extreme to the other. That's what makes the 50-135, 77 ltd and FA 100/2.8 macro, among some other lenses, considerably slower to focus (regardless of SDM or screwdrive) than others with shorter focus throws like the 16-50, Tamron 28-75 and the like.


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