DA* 50-135 focus issue

Hi, folks. I'm in a bit of a quandary over my DA* 50-135. It started failing to lock focus at 135mm or slightly below, although it was fine at other FLs. This seems to relate to an SDM issue and I was getting ready to post it to Pentax repair in France (where I live) when the problem disappeared. I've been testing it for a week now and focus locks properly and quickly at all FLs. Admittedly, I hadn't been using the lens much but the same problem cropped up a year ago when it saw regular action at least once a week.

I'm concerned (and fairly confident) the problem will return when I'm relying on the lens later this year at some music gigs. It strikes me there's no point sending it in for repair while it's working properly. I suppose I could describe the problem and hope for the best, but this wouldn't be ideal. Any advice on the best way forward? I looked into switching it to screw drive but all the information on the net was quite old and some of the required software doesn't seem to be available from a reliable source any more. I'd appreciate any wisdom you can offer.

Thanks, Peter. I'll try your system for a while and keep my fingers crossed.

Sounds like you have something with AI installed... always when threatened with mechanic the symptoms miraculously disappear...our old Golf was a pig until it got onto the road the VW garage was, then it ran perfectly.

Is the problem with AF at low light? It's been a problem with my lenses and the lens hunts (not limited to DA* glass either). If SDM works on other occasions it doesn't sound like the SDM motor but maybe the sensitivity of the AF sensor...

Battery maybe?

Let's hope you get to the bottom of it by the time the gigs come around.

Light level made no difference. What I can't understand is why it would hunt only at maximum zoom or thereabouts. Knock it back a few millimetres and it would work normally. I made sure to test it with full batteries. My old Peugeot 307 did the same as your VW. Luckily, the problems disappeared when it spontaneously combusted in a car park

The 17-70mm is well known for developing the same fault. That problem is the SDM motor.

The repair people will know very well what the problem is, having seen it probably thousands of times before. They will be able to replace the faulty parts even if the problem isn't manifesting itself at the time the lens gets to them.

Many thanks, chaps. Off to the repair shop it goes.

Quick update: the French repair people, PM2S, are fixing it for the equivalent of £180. I think that's money reasonably well spent if the repair holds up for a while. Thanks again for your advice.