For these shots, I used the Sony 18-105 f/4 OSS. The day was very cloudy and the sun was not visible at all. The bounce house was also covered, which further reduced the available light.
I used the following settings:
- Lock-on off
- Focus area was sometimes Wide and sometimes Zone
- Face detection: off
- Continuous shooting: High
- JPEG only (I was running out of space)
- Exposure mode: manual with Auto ISO. I set the aperture at f/4, shutter speed at 1/2000, and I set the Auto ISO floor and ceiling at 100 and 25,600 respectively.
- I didn't turn off the image stabilization.
If you look at the shots, I had ample DOF to cover the subject but my DOF was still shallow enough that the background and foreground are out of focus. Therefore it wasn't just a matter of having enough DOF -- the shots were truly in focus.
At one point, the staff person at the bounce house asked me to move away from the entrance so I was forced to shoot from behind the net. The net decreased the contrast of the shots. Nonetheless, all shots continued to be in focus, even those with kids in mid-air.
So far, based on all the cameras I've had, including the Nikon D3 and Nikon D300, my impression is that the a6000 is as good as pro-grade Nikon DSLRs for capturing fast action, at least when using the right lens (some lenses are slower to focus, e.g. the 50 f/1.8 OSS) and as long as it's not too dark. Moreover, the AF points covered virtually the whole frame, therefore the camera never lost the subject. I also found the focusing to be very predictable. Going back to the original question: can the Sony a6000 focus quickly enough to capture fast-moving kids? The answer is yes.
As I was sorting through my images from this trip, I saw a Twitter post from Mohammad about The Camera Store autofocus shootout you've probably seen by now between the Nikon D4S, Fuji X-T1, Olympus E-M1, Panasonic GH4, and Sony a6000. At that point, I had already seen the shots from the bounce house, so when I was watching the video I felt good about the Sony a6000's chances of winning. Actually, I didn't know about the GH4's DFD technology (I never considered or researched the GH4 because it seems too large for my purposes), so I found it interesting to learn that there's another option to phase detection and simple contrast detection that can yield impressive results.
One observation I have is that if the tests were all done at f/4, the GH4 and E-M1 had a deeper DOF (similar to f/8 full frame) while the Fuji and Sony had effective apertures of f/6 full frame. I am curious to see if the GH4 would focus just as quickly at very wide apertures, or if DOF were equalized. Another observation is that for sports and fast action shots, you really need a very high shutter speed. Many people use shutter speeds like 1/250 or slower, which for fast sports can result in blur, and they sometimes think the camera didn't focus quickly enough. If you need a high shutter speed, then you can be pushed to use higher ISOs, which I think makes having a larger sensor more advantageous in mediocre light. Combined with the low cost of the a6000 compared to the GH4 or any other camera in the shootout, and how close it got to the GH4 (and by extension, the D4S), I was very pleased with the Sony a6000's placement in the shootout, and based on my own experience with it, I think the a6000's autofocus performs brilliantly.