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05-01-2018, 05:30 AM   #1
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50-135mm sdm lens auto focus failure

My Pentax 50-135 sdm lens won't auto focus. I noticed it slowing down while it tried to focus a while ago. I've read on here that there is somebody that changes them to screw drive focus? I saw it a few months ago, but now I can't find that thread! (This forum is huge!) I would appreciate any help in finding this person. Thanks!

05-01-2018, 05:38 AM - 1 Like   #2
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Hi Rose, the gentleman you are looking for is @UncleVanya. Here is the thread you will need. Good luck.

SDM to Screwdrive Conversions -
05-01-2018, 05:59 AM - 1 Like   #3
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Or you can do it yourself. It's all in THIS THREAD.

I managed it, so it can't be too hard

But Brad's a good guy, and if you're not confident he's definitely the go-to option if you're in the US.
05-01-2018, 06:20 AM   #4
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Thanks to both of you! I've sent him a message.

05-01-2018, 08:12 AM - 1 Like   #5
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Sdm 50-135

I have had this lens since it first came out.
It has a tendency to not no auto focus after a period if disuse.
I have always brought back to life by leaving it connected and try to focus till it works.
The last time I put the camera in live view after several attempts to focus "normally" and it focused in live view immediately. Normal focus then worked again.
Make sure your camera contacts are clean and a good charge is in the battery.
05-01-2018, 09:08 AM   #6

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My experience is the same as BFD.

I try to run my SDM lenses every two weeks, regardless of whether they've been used in the interim. I subscribe to two theories about SDM issues.

The first is simply they get 'sticky' if not used, so run them for a few minutes between near and far focus.

The second is that there are capacitors in the SDM units that lose their charge, and the units won't run until they're re-charged, so attach the lens, turning the camera, then leave alone for a few minutes.

I've found that sometimes when first mounted the lens won't run at all, but will after some minutes, but slowly. I keep working it, and after 5 minutes, good as new.
05-01-2018, 01:54 PM   #7
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Thank you both! I did what you suggested, and it seems to be working! Still a little slow when it's not in live view, but hopefully it will get better.
05-01-2018, 03:03 PM   #8
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I think I'm just lucky (knock on wood many times) ... I've had mine, an early version since about a year after release and it's been just fine without problems. I very much hope that doesn't change. But having the conversion operation available, with many thanks to @UncleVanya and other forumers here, I've held onto it and will continue to do so.

05-01-2018, 04:16 PM - 1 Like   #9
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Ok! Thanks to all. I have a 50-135 that stopped working two years ago. I tried in vain to get it to work on my K3 and considered converting it to screw drive. Well, after reading this I thought why not dig it out. Nothing to lose! So I mounted it on my K1 and set the auto shut down to 10 minutes. Walked away. A short time later I came back and tried this cure. After several attempts and no response I moved the Af/Mf switch on the lens several times and got a sound! Went to live view and continued to work with it. Hesitant at first, but now fully functional! I'm amazed! Miracles do happen.

Color me happy, Ray

Last edited by Ray Cox; 05-01-2018 at 04:23 PM.
05-01-2018, 04:26 PM - 1 Like   #10
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My 50-135 takes such beautiful pictures I briefly considered holding onto it as a manual focus lens, but reading in these forums about the screwdrive conversion, and having a camera old enough to get to its config file (a K-x), I'm glad I went through with it.
11-15-2018, 12:44 PM - 1 Like   #11
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I'd like to thank those who suggested the trying the AF on the SDM failed lens several times and using live view as well. my lens started working and avoided me going the convert to screw drive route. a bit of perseverance was all it took and I never would have thought of that. so my 200mm lens now works perfectly. thanks.
11-15-2018, 03:47 PM   #12

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I bought a 50-135 used and the SDM was DOA. I spent a few hours trying to get it to work, like I've read on the forum, in vain. Someone suggested I open up the lens and "massage" the motor. I tried it and, after a few times, it worked. I lost an o-ring at the front of the zoom ring in the process but it works. Now I just need to find some o-rings that would fit so I can have a fully weather-sealed lens again.
11-25-2018, 09:50 PM   #13
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Thanks so much for posting this...!!! I bought another SDM lens used & it wouldn't power up either. After switching AF/MF several times and letting it "power up" properly, it's now working perfectly. Happy Thanksgiving!!
06-12-2019, 12:32 AM   #14
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My camera with the 50-135mm lens fell off from the car chair when I brake. Now the lens have focus failure more specifically back focus. Can I find solutions for my problem into the previous entries? Please help me.
06-12-2019, 12:01 PM   #15

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QuoteOriginally posted by gabosalbin Quote
My camera with the 50-135mm lens fell off from the car chair when I brake. Now the lens have focus failure more specifically back focus. Can I find solutions for my problem into the previous entries? Please help me.
Your problem is evidently a different issue than this old thread was addressing.

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