[rfg]

Thanks for the tests and all this info crewl1. One quick question - did you have SR on or off?

[crewl1]

I did use SR in the body for all the lenses tested, I did not use lens OS on the Sigmas to keep things consistent.

If I decide to test again I will use OS on the Sigma and see if it improves the performance.

It steadies the visible image quite well and may help keep the target centered, where SR doesn't change what you see.

[Ben Searle]

Thanks all, especially Crewl1 for all the ideas and shared experience.

So far as the K5 goes I've found centre focus point and continuous auto focus (focus priority) works best. But I don't always want the subject in the centre so hope the K3 can manage on other focus points should get one.

A general point - I assume longer lenses focus more accurately on quickly moving subjects as subjects stay within a given part of the frame for longer (due to field compression), despite the reduced depth of field? And neighbouring areas are more likely to be significantly out of focus and so less of a distraction to the autofocus system? My 15mm seems the hardest to use on action subjects and the 70mm the easiest, but my 21mm has done well.

An example with the 21mm attached. I was pleased with how the camera held focus through about 6 shots through the river. The focus centre spot was on the handlebars and in this case worked well for being on such a small area. ideally I would have chosen a spot to follow my son's head, or body.