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07-29-2012, 09:54 PM   #1
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Does this CRIS diagnosis sound right?

CRIS has determined my Pentax DA* 50-135mm needs a new SDM motor, but I sent the lens in because of a consistent back-focus problem, not because of an apparent problem with the motor function. Does this sound right?

I've had my DA* 50-135mm for about two years now and in retrospect I believe it has always back-focused a bit, although I only determined this after the warranty was up. The good news is that the auto focus fine adjust feature in my K-5 has done a good job of correcting the problem. I decided after a few expenses were taken care of this summer I would get the lens looked at by CRIS. While I'm not planning on selling the lens, I wanted to be sure it was in tip-top operating condition in the off chance someone might buy it down the line. The last thing I wanted to do was hand off a lens that was not in good working order.

At any rate, I sent the lens along with an explanation that the lens consistently focuses a bit behind the subject, but it always did so to the exact same amount each time and the lens always focused quickly and quietly. To my surprise, CRIS replies that the SDM motor will need to be replaced in addition to having the focus adjusted in conjunction with my camera body. I know little about the impact of SDM on focus accuracy, and while I can understand how a failing motor would lead to poor focus performance, I can't imagine the failure would be so precise each time.

My other frustration with this diagnosis is that once upon a time, I sent a DA* 16-50mm to CRIS because of a legitimately bad SDM motor which got replaced, only to fail again five months later. I barely got the lens back to CRIS before their own 6 month warranty expired.

I haven't yet called CRIS back to give them the go ahead on the repair, and if I were to go through with it they would need my camera anyway, so it seems it's going to be a little while before this is sorted out. I've considered that I can ask them about this issue over the phone, but it seems when I've called them in the past they will only let me speak with a message-taker, not a technician. I suppose I could be more adamant about talking to somebody of authority, but I wonder if it will do me any good, since they're the "experts". Nevertheless, I have the weird feeling this is just a knee-jerk response on their part to a "typical" Pentax SDM/lens problem.

Thoughts?

08-01-2012, 06:04 AM   #2
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I would be so reluctant to replace a working SDM motor. It's pretty hard to know if they are just replacing it as a standard practice or if somehow the motor isn't properly calibrated to the AF sensor information it's getting to focus accurately. Only an opinion but I wouldn't think the SDM motor is the issue.

I don't know what other options you have since these guys are the only game available in the USA for service as far as I know.
08-01-2012, 06:09 AM   #3
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I don't know how flexible CRIS is, but I'd ask them to send the original motor back to you with the repaired lens. That way you have a spare considering it still functions (at the moment according to them).
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