SDM Failure

David thanks for your replies, yes I got just couple of days before I make final decision. I really didn't know I would start thinking all over again. Well, your DA* 300 is prime, we both know that its focusing mechanism is less complicated and of course should be more accurate but we didn't signed to that kind of inaccuracy in 50-135 lens. I should ask you how old is your 50-135 and do you find similar backlash in focus ring as in my copy?

In terms of IQ, I absolutely agree it is just fantastic.

Hi Lev

I don't have any backlash at all in the focusing ring, it is just fine. The lens was bought approx. 3 years ago, but it wasn't new. When I bought it the motor had just been replaced with a so-called 'updated' motor that didn't have the reliability issues, but that is not the case, even the new motors suffer the same problems, mine is starting to show signs of failure now, but still works at the moment. I have learned that it is best to store this lens in an upright position if not being used for a period of time, as this puts less stress on the motor. It does seem to help, but I don't think anything will solve the erratic focusing at wide apertures. When I say erratic I mean that focusing tolerances have to be very small when using F2.8 at telephoto focal lengths, due to the wafer-thin DOF and the truth is my lens does not have the required accuracy to be reliable enough at focusing in these circumstances. As stated though, I would not sell it, I like it too much.

Regards
David

EDIT: The advice to store the lens upright is from a former Pentax engineer who worked on the design of the SDM lenses and who has stated publicly that the motors are a flawed design. They are so sensitive to alignment that even gravity over a period of time can cause friction to occur, which the weak motors cannot cope with. Since I started storing the lens upright, the problems with it 'not waking up' have not got any worse, but I am sure it will fail altogether eventually.

Well, what to say, would you buy it again? I mean, now I have a choice to return if, replace it or exchange it to something else. I can't see other WR portrait lens out there with similar IQ rather than 70-200. Did your lens behaved like that when you got it or it started to do so later?

man now I'm really scared

Hi Lev

There were no issues when I first got the lens, they started to occur about 12-18 months ago. I don't use the lens heavily, so it is stored for periods of time, which appears to be one of the problems. I've been storing it upright for the last 6 months or so and I don't think it has got noticeably worse during this time. It usually wakes up after being on the camera for about 30 seconds, it helps to manually focus it a few times to free it up.

Regards
David

What is the difference between an SDM motor and a PLM - as in 55-300mm and are there any issues with those?

As far as I know, there were no issues with PLM. It is lot faster, almost instant focusing, but it cannot be used to drive large focusing elements like in SDM.