SDM to Screw Drive or Not

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Link Posted 17/11/2014 - 22:53
I had a go at converting my 50-135 to screw drive the other day to compare focus speed. I recorded the full focus travel back and forth a few times using the lens on a K3. Video was recorded at 30 fps and played back frame by frame to get results.

Full focus travel, infinity to near (results were the same for near to infinity)

SDM: 0.633s
Screw Drive: 0.5s

So the screw dirve is just over a tenth of a second quicker than sdm or about 20%.

It was also quicker at changing direction at the closest focus point (where it seems to lag forever, it sounds like it tries to go nearer and then decides it has to go the other way).

SDM: 0.833s
Screw Drive: 0.7s

Infinity change in direction was the same for both 0.23s

The above 2 times are taken from the top display light going off after ive half pressed the shutter button and the focus barrel beginning to move.

So... If you had this lens would you choose to stay with sdm or use this as a screw drive lens?


Link Posted 18/11/2014 - 13:20
Have you tried the same test on another body or only K3.
Save Pentax


Link Posted 18/11/2014 - 14:15
I'd be interested in knowing how you did the conversion. I have seen some instructions on the Internet, which suffered in the translation from (I think) French. It seemed a rather formidable undertaking. I'd be happy with screwdrive if the SDM motor failed!
Regards, Christopher



Link Posted 18/11/2014 - 18:15
I converted my 16-50mm to screw after SDM failure and quite happy now, especially for what I use it for.

K-3's, K-3II.
SMC PENTAX-DA FISH-EYE 1:3.5-4.5 10-17mm ED [IF], SMC PENTAX-DA* 1:2.8 16-50mm ED AL [IF] SDM,
SMC PENTAX-DA 1:4 15mm ED AL Limited, SMC PENTAX-DA 1:3.2 21mm AL Limited,
SMC PENTAX-F 1:2.8 28mm, SMC PENTAX-FA 1:2 35mm AL, SMC PENTAX-FA 1:1.4 50mm,
SMC PENTAX-D FA MACRO 1:2.8 100mm WR, SMC PENTAX-DA* 1:4 300mm ED [IF] SDM, SMC PENTAX-FA* 1:5.6 400mm IF & ED,
HD PENTAX-D FA 1:4.5-5.6 150-450mm ED DC AW, HD PENTAX-DA 1:5.6 560mm ED AW,
SMC PENTAX-FA* 1:5.6 250-600mm ED[IF],
PENTAX AF160FC Auto Macro Ring Flash.


Link Posted 18/11/2014 - 18:51
pafu_pafu wrote:
Have you tried the same test on another body or only K3.

No I only tried it on a k3.

Ive got a k7 which I used to do the conversion so could try it on that too.

The conversation is a doddle but I think it depends what camera you've got to do it with.
I couldn't do it with a k3 (the lens copy eprom wouldn't work), worked fine on a k7.
Last Edited by richardmills on 18/11/2014 - 18:51
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