I am new to the forum and wish to share an experience which may hopefully assist other Pentaxians.
I know there has been lots of threads associated with DA* lense AF problems but I could not find a suitable one to add to so I have started this new one - I believe the AF problem I have encountered with my DA* 300/4 occurs with new cameras and as there are many new K3's being unboxed currently then this is as good a place as any (apologies if I am in the wrong place).
I have shot about 300 photos with my new K3 over the past couple of weeks. I bought a Pentax K1000 in 1981, upgraded to a Super A and then DSLR's K20 and K5 and have the DA* 16-50, 50-135mm, 300mm. I shoot mainly birds, kids basketball and family occasions.
It is early days and I am struggling with the setup and control differences of the K3 to my trusty old K5 especially the AF options. My 16-50mm and 50-135mm lenses have behaved as expected but I had an issue with the DA* 300mm.
My first outing with the 300/4 was a walk in a local bush park seeking some stationary and flying birds to play with the various AF options. I took about a dozen shots at ground level subjects, both near and far and then spotted a bird in a tree overhead. The K3 would not focus accurately, I tried to focus at a closer object and it failed and I could hear a crunching/grinding sound as the lense tried to travel. I turned the camera on/off/on, took the battery out and back in but no fix.
The symptons are:
- in AFC/AFS/AFA the lenses will only focus at or near infinity. It will not focus on closer objects.
- when trying to close focus the focus ring seems to hit a stop, and you can hear a scrunching noise that sounds like gears not meshing properly or a moving piece jamming on something. It is not loud but sounds like damage being done and would not be good longer term.
- the problem remains if the AF/MF switch on the lense is switched to MF while the camera is in AFS/AFA/AFC mode
- The problem disappears if the camera is put in MF mode and then lense immediately regains full travel. If the lens is close focussed and then put in AF* then it focusses properly at close subjects and then out to infinity but when asked to focus close again it can malfunction (some occasions it works for a few times before failing again).
So is the problem the K3 or the lens??
My other lenses work fine on the K3 - it is just the 300/4 that has the problem. So I put the 300/4 back on my K5 and it works. I put it on the K3 again and it fails. I repeat this and that same occurs. It is the combination of the 300/4 lens on the K3 that does not work.
I purchased the 300/4 and the K5 together in 2011 and had the same problem!!! On that occasion the problem started intermittently after (I guess) 100 shots and then became worse. The symptoms were exactly the same and it came good after I cleaned all the camera and lens contacts. The K5 & 300/4 have not missed a beat since.
I cleaned the K3 and 300/4 contacts this time but the problem remained. I am now worried because the lens is out of warranty and the camera will probably test OK!
I then closely inspected the two camera bodies looking for something different between the bodies that could cause one to work and one to fail. I noticed that the screw drive key on the K3 was in a slightly different orientation to the K3 drive. I rotated the K3 drive to the same position as the K5 drive. I also discovered the pin on the opposite side of the mount that pushes in and retracts the screw drive when a lens is placed on it. I pushed this in and out a number of times (it maybe my imagination but it was initially a bit sticky??) . I then put the 300/4 on the K3 and voila the lens worked and I have taken a further 200 photos without issue and have tested it a dozen times over the past week and no sign of the problem!
I am praying that my lens will now continue to perform as it did after the same incident with the K5 when they were both new.
I don't know the lens works and can only guess as to what actually happens. It may have been coincidence that the lens started working. However, I wonder if my lens and other DA* lenses are prone to malfunction when either they are new or when the cameras are new??? I wonder if that pin on the mount that retracts the screwdrive is too stiff when new and does not retract the screw drive enough or if it also switched the camera from SDM to screw. I wonder if the camera thinks it has a screwdrive lense when the SDM reaches a close focus and the SDM drive disengages, or partially disengages and that it causes the scrunching sound that I have heard. I also wonder if the malfunction continues and further damages the mechanism and this leads to failure of the SDM and grief/stress for Pentax SDM lens owners all over the world???
Enough wondering and back to praying that the problem has gone forever.
I hope this provides a quick fix for anyone with the same problem.
PS. I think the K3 is going to grow on me - it is not 100% love at first sight. The AF options will take time and as reported elsewhere there is a deal of learning to acquire to optimise your own operation of the camera. The IQ at ISO <400 is stunning however at higher ISO it is definitely noisier than my K5 ( I haven't tried any basketball yet but that is a concern for that activity). I snapped a couple of BIF's this morning not thinking they would be anywhere near acceptable and they nearly were (with spot expanded) so that is promising. I also photographed some Rainbow Lorikeets in a patch of magic morning light that I was very pleased with (was it the K3 or the luck & light?). See below:
Originally posted by RedBoomer 
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