Focus Oscillation in K3 iii?

Started 3 months ago | Discussions thread
Paul_R_H Senior Member • Posts: 1,153
Re: Focus Oscillation in K3 iii?
1

This is fascinating. I have found the same, but only at focal lengths LESS THAN about 25mm.

Given the deep DoF at those focal lengths (even at F4), it represents quite a movement of the focus motor.

I had it with the DA15 Limited, and the DA16-85 at its wide end. I wondered if it was errant dust on the focus sensor but couldn't blow it off.

I shoot with back-button focus C-AF, but went through S-AF, shutter-button focus C-AF and S-AF, and changed all the parameters of focus hold, used all the focus points, and it (mis)-behaves consistently.

SRS Microsystems took the body back for repairs under warranty and they sent it to the UK repairers. Some weeks later they returned it to me saying they could find nothing wrong.

I made a video with my phone pointing at the DA15 as I held the C-AF button, the lens aimed at a fixed target about 2m away. The lens can be seen shifting focus every few seconds, oscillating between one position (correct) and another (too close). It does the same for distant targets.

So SRS negotiated from the UK repairers to give me a new body. Fantastic service, all credit to SRS.

And… The new one does it too, it but only slightly. When focussing on static subjects at very wide angles and wide open aperture I focus in live view and then switch back to the viewfinder to compose.

I can now live with it. If it was doing it at long focal lengths I'd be really upset. For fast-action work with the 55-300PLM it is amazingly good. It can track cars coming towards you at high speed as they half-fill the frame.

Paul

JoeOptics314 wrote:

I just received a Pentax 70-210 mm zoom and am comparing it against two other lenses that cover that zoom range (150-450mm and 55-300mm PLM). My initial shots comparing against the 55-300 mm showed it better in terms of blur, which would be quite surprising. But when I started taking pairs of shots a few seconds apart and compared the pairs taken with identical manual settings I see what appears sharpness differences.

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