I've got a new K-3 and Sigma 70-200mm/f2.8 DG HSM OS lens and I'm trying to take photos of cross-country runners. I've been using the K-7 and DA* 50-135mm for the past 5 years and got comfortable using AF-C and the center focus point to capture the runners. Not every photo was in focus, but I could manage to get enough in focus shots to be satisfied.
I was hoping the K-3 would improve the AF performance, but I'm getting worse performance, not better.
I'm using the following settings:
Av priority, SR off, lens OS off, generally 1/500 or faster.
AF-C, center point, SEL-S (center 9-points)
16. 1st frame action in AF-C - Focus Priority
17. Action in AF-C Continuous - Focus Priority
18 - I've tried AF hold off, low, med and high
I've also tried using shutter-release AF and back-button AF, with little difference.
I can see the center AF point light up on the runner, yet I'm getting entire sequences out of focus. It's not camera shake or slow-shutter - the camera just seems slow to recognize that the runner has moved forward and is no longer in focus. You can see the 3rd shot below is very out of focus, but in the 4th shot the camera has tried to refocus. It is still way behind the runner though.
Here's an example of what I mean:
(all f4, 1/1250, 200mm)
50% close-up:
The focus was on the center of the runner's chest throughout the sequence.
I know the K-3 and the lens can do the job. At 100%, you can count his eyelashes. Here's a crop at 50%:
I've got way too many out-of-focus sequences where the runner is simply ahead of the plane of focus, even when the initial shot is pre-focused on the spot where I want to begin the sequence.
With the K-7, I could prefocus, get 2 or 3 shots in focus, then lose 1 or 2 shots while the camera refocused, then get another 1 or 2 shots in focus. With the K-3, I'm getting complete sequences where the runner is never in good focus at all.
I'd really appreciate any thoughts on settings or technique that can help me out.