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03-17-2014, 01:24 AM   #1
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Why deactivate SDM?

I see threads about how to deactivate SDM, and people prefer screw drive over SDM, can someone explain to me why this is?

I have recently purchased DA* 50-135mm f/2.8 because of its big eye, WR and SDM. And now I see people disabling their SDM left and right, leave me worrying if I have the right purchased....

03-17-2014, 01:47 AM   #2
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Earlier versions of the sdm-lenses were prone to break and when they did, the lens would not auto focus at all even if it had screw drive. The camera still detected the sdm and tried to use it instead of engaging the screw. Also, I guess some users didn't trust sdm because of this and decided so make a preemptive strike by disabling it, or they felt the screw drive was faster than sdm.
03-17-2014, 01:50 AM   #3
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QuoteOriginally posted by bengibollen Quote
Earlier versions of the sdm-lenses were prone to break.
If that's the case, how to tell if the lense it's the earlier version? I have just purchased a second hand (may be third hand, i forgot), but the seller reported there has been no SDM failure.

---------- Post added 03-17-14 at 04:51 PM ----------

Or why not take it to repair, because i read somewhere someone handed their DA* for cleaning. The repairman also upgraded their SDM.
03-17-2014, 01:59 AM   #4
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QuoteOriginally posted by leungclj Quote
If that's the case, how to tell if the lense it's the earlier version? I have just purchased a second hand (may be third hand, i forgot), but the seller reported there has been no SDM failure.

---------- Post added 03-17-14 at 04:51 PM ----------

Or why not take it to repair, because i read somewhere someone handed their DA* for cleaning. The repairman also upgraded their SDM.
It's hard to tell because externally they're the same, and the serial numbers aren't always sequential. But if it was originally purchased before 2013, it's probably the old version.


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03-17-2014, 02:45 AM   #5
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QuoteOriginally posted by leungclj Quote
If that's the case, how to tell if the lense it's the earlier version? I have just purchased a second hand (may be third hand, i forgot), but the seller reported there has been no SDM failure.

---------- Post added 03-17-14 at 04:51 PM ----------

Or why not take it to repair, because i read somewhere someone handed their DA* for cleaning. The repairman also upgraded their SDM.
You can spend you life worrying about 'SDM' failure or just take pictures and enjoy life.

My 16-50 is a late 2007 build never missed a beat never stuttered and is not de-centered ....I feel somehown cheated
But all is well my 2008 50-135 suffered 'SDM' failure 2012 and was replaced by Pentax

I could change my lens to screw but in my opinion SDM is queter and faster , a quick trawl of youtube will show that the same lens convertered to screw is generally slower unless the issue is stiffening AF (seals shiffted etc) then the greater torque of screw will overcome it.

ALso with the new converter SDM runs full speed but screw is reduced by gearing.

As to 'send to clean' to have 'SDM' fixed that would tie into my opinion the motor has never been 'upgraded' but the issue is AF stiffening causing the motor/electronics to burn out. I believe later builds have either different lubrication or mechanical changes which could be abpplied at service.

Last edited by awaldram; 03-17-2014 at 02:50 AM.
03-17-2014, 03:06 AM   #6
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FWIW, I also have an early 50-135 which has exhibited no SDM issues at all, which may (or may not) be partly due to my regime of running all my lenses on camera for a few minutes every two weeks or so (whether they've been out on a shoot or not during that period) making sure I operate the focus from min to max.

I get the impression that like us, lenses get lazy if they don't get some exercise!
03-19-2014, 05:12 PM   #7
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QuoteOriginally posted by JohnX Quote
FWIW, I also have an early 50-135 which has exhibited no SDM issues at all, which may (or may not) be partly due to my regime of running all my lenses on camera for a few minutes every two weeks or so (whether they've been out on a shoot or not during that period) making sure I operate the focus from min to max.

I get the impression that like us, lenses get lazy if they don't get some exercise!
I firmly believe that with SDM lenses. I rarely use my 50-135 and if I don't get it on the camera and exercise it by waving it about and letting the AF work through its range then it will stop working. When it does I find that if I point the lens down at the floor and keep pressing the AF button it comes back to life. But this is only after long periods of not using it - months rather than days. I've got into the habit of just pointing it around the room most days now, and it works perfectly.
03-19-2014, 05:40 PM   #8
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Thanks both, i will be give my SDM lense some workout every month if not in use.

03-20-2014, 09:12 PM   #9
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I am actually quite baffled that Pentax didn't spend the literally one hour it would take to add the programming for a focus type override option to the menus on all the camera bodies to turn off the SDM auto detect, and just include it in the next firmware patch. Considering it only takes changing one line in the programming in debug mode for the end user to disable SDM on a lens it would be near zero effort on their part to add the option to the body and dramatically increase piece of mind for the users.

Someone please bother to make them write that down next time we interview them.

EDIT: And to give my answer the original question, motors fail more often than just gears do, it would add to the theoretical life span of the lens to be able to use the motor in the camera body since DSLR's get replaced far more often than lenses for upgrade reasons.
SDM isn't junk by any means but I don't want to count on Pentax being able to repair it at a reasonable price (assuming they still have the parts) if it fails in 10 years, and for $1000+ dollars for many of those lenses it sure as hell better last at least that long.
I think I just have unreasonable expectations from all the Pentax A series lenses I use that are in the neighborhood of 30 years old now and still like new, to say nothing of the age and mileage of some of the Pentax M42 stuff running around. But then that's all manual stuff.

Last edited by PPPPPP42; 03-20-2014 at 09:21 PM.
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