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10-16-2011, 09:07 PM   #1
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A DA* that just didn't want to focus...and then did

I was at a photo convention this weekend and decided to try out 2 lenses: A new Sigma 70-200 f2.8 and a "slightly" used Pentax DA* 50-135 f2.8. I'm trying to decide between the two of them and am completely split on the two after months of research. While, I would love to own the DA*, I've heard it has issues with its auto focus reliability. Today did not help its case: When I tried it, it would not focus. At all. The motors would not engage, yet it would not snap the pic until it detected focus lock (which I had to do myself manually). The auto focus switches on both the lens and the body were both set to auto yet it would not budge. I flicked it back and fourth a few times and worked. So I snapped a few tests.

How big a problem is the auto focus issue with the DA* 50-135? Is the DA* worth the loss in focal length compared to the Sigma?

10-18-2011, 10:09 PM   #2
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I have a DA* 50-135 with a failed autofocus module. It was repaired under warranty twice, but now would cost me big time to fix as it is out of warranty. The lens is optically stunning. Check my Flickr site for examples with the AFA 1.7x, which turned it into a screw focus 85-230 f/4.8 with little reduction in image quality. The cost of the AFA 1.7x is slightly more than the repair would probably be, but I already have it, and for the fun of it tested the combination before the AF failed for the third time. You can check out the tests on my Flickr site in my signature.

I've decided, after trying to manually focus a university hockey game, that I'm just not that fast any more. I'll use it as an 85-230 AF and a 50-135 MF lens.

My biggest problem with Sigma lenses is that the controls are in Canon orientation - opposite to Pentax and Nikon. I disliked having to learn to turn things the other way when I swapped a lens.
10-19-2011, 04:47 AM   #3
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QuoteOriginally posted by Twin-Reverb Quote
...yet it would not snap the pic until it detected focus lock...
Just an FYI that there is a menu setting (on all Pentax cameras) to allow the shutter to trip without the camera being 'in-focus,' that isn't something the lens would have caused.
10-19-2011, 07:14 AM   #4
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I have done a bit of reading on SDM issues as have many others, the most plausible theory in this type of thing you experienced is the SDM focus motor struggling to overcome the friction in the motor/lens. I wouldn't be buying the copy which had the symptom you described.

This isn't my theory, it's from someone who fixed they're own faulty 50-135 and posted on DPR (see the link "SDM fiction & fact" below). It does seem like a very plausible explanation though.

Stage 1: Lens will move but not focus lock. At full power for the lens to rack through to the approximate position its OK as it can overcome the static friction to start moving, Once it is moving its OK because the dynamic friction exerts less force. (remember High shcool Physics?)
But.. for small fine adjustments to lock on it hasnt got enough power to overcome the friction.

Stage 2: The lens tightens up so that it will not move at all.

Stage 3: Motor burns out SDM failure.


Stage 1& 2 might be intermittently experienced and manually working the lens has been reported to free it up again as i've read in several posts here and DPR. I appears you reported the same thing.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/156873-little-...-da-300-a.html
The last few posts on pg2 are mine, I had a DA*300 which did exactly the same thing straight out of the box. I took it back immediately.
I got another copy last weekend, this one is perfect, fast, silent and accurate. An awsome lens, definitely worth the spend for a DA*


The infomation I found most useful was from this thread on DPR. It also has a link to DIY fixing a stuck SDM (at your own risk)
SDM fiction and fact: Pentax SLR Talk Forum: Digital Photography Review

Big thanks to a very handy thread by "Solar1" containing the DPR link above and many other interesting ones.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/96946-one-stop...tml#post989879

10-19-2011, 07:42 AM   #5
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I own three DA* lenses with SDM, including the 50-135. I also own 3 Sigma lenses, though not the 70-200 you're considering. I've had a friction problem with SDM on one of the DA stars, which was quickly fixed by inserting a screwdriver in the screwdrive and manipulating it back and forth -- no problems since. The 50-135 produces stunning images. I use it as my portrait lens. My Sigmas are good lenses but they don't have the build quality of Pentax DA* or the weather sealing. They are also more likely to have QC issues (I had to return one for factory service right out of the box). And they don't hold their value as well. But no problems with HSM. There's no clear winner, but I would lean towards the Pentax glass, but not a used lens that already has SDM problems.
10-19-2011, 08:11 AM   #6
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QuoteOriginally posted by oeriies Quote
I own three DA* lenses with SDM, including the 50-135. I also own 3 Sigma lenses, though not the 70-200 you're considering. I've had a friction problem with SDM on one of the DA stars, which was quickly fixed by inserting a screwdriver in the screwdrive and manipulating it back and forth -- no problems since.
This may or may not work. I have two AF Pentax bodies, an SF-1 and an MZ-S. I used the SF-1 to focus the 50-135 for a good 10 minutes with no results in reviving the SDM focusing. It might be that the slow start ups have, in fact, fried the motor as mentioned above.
QuoteQuote:
The 50-135 produces stunning images.
This is true - I love the rendering and image quality of this lens. I am not going to get rid of it any time soon, simply because the image quality is so extremely good. For the odd occasion when I actually need AF, such as the above mentioned hockey game, I lose the wide aperture.
QuoteQuote:
I use it as my portrait lens. My Sigmas are good lenses but they don't have the build quality of Pentax DA* or the weather sealing. They are also more likely to have QC issues (I had to return one for factory service right out of the box). And they don't hold their value as well. But no problems with HSM. There's no clear winner, but I would lean towards the Pentax glass, but not a used lens that already has SDM problems.
10-19-2011, 03:33 PM   #7
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QuoteOriginally posted by Canada_Rockies Quote
This may or may not work. I have two AF Pentax bodies, an SF-1 and an MZ-S. I used the SF-1 to focus the 50-135 for a good 10 minutes with no results in reviving the SDM focusing. It might be that the slow start ups have, in fact, fried the motor as mentioned above.
This is true - I love the rendering and image quality of this lens. I am not going to get rid of it any time soon, simply because the image quality is so extremely good. For the odd occasion when I actually need AF, such as the above mentioned hockey game, I lose the wide aperture.
I don't think autofocusing with another body achieves the same effect as a screwdriver manipulation of the screw drive. I turned the screw drive until it hit a resistance point, and then turned it a quarter turn beyond that point and cycled back and forth until the resistance abated. I expect a camera body would stop at the resistance point.
10-19-2011, 04:26 PM   #8
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oeriies - I tried your suggestion with the screwdriver. Unfortunately, it had no effect on the focus mechanism. Thanks for the idea, though.

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