@HD Black is tough on AF. For the pup, to help put some context around your focus rate, I'm curious which lens, and rough distance to subject when it had trouble. If the missed shots were start or mid-burst and what your tracking settings are (I'm assuming R5 has similar options as other recent cameras... I've never seen one).
Lenses vary in focus speed and accuracy. Everyone knows this. What fewer realize is how subject distance can affect tracking. Using the 70-300mmL @ 300mm. I can change focus from ~2.5 to ~10ft with about a 30 degree turn of the focus ring. I can focus from 10ft to around 60ft with a bit less, 22-25ish. That means means it less effort to switch the focus over 50 distant feet, and it takes to switch focus by 7.5 near feet. I find tracking running dogs at short focus lengths to be an impossible task... you want them somewhat distant with a tele lens.
I'm not sure if there's a mirrorless equivalent to the old f/2.8 or faster lenses can use more accurate cross-type sensor. Probably not directly, but the extra light from a fast lens probably helps.
I know the 7D factory settings for bursts were first shot is shutter priority, then Nth shot is focus priority. This is great if you need a perfectly timed shot, and works best if you can prefocus, but if you're after focus while tracking, the start of your burst (and any single shots) will be tracked, but not predicted, so the focus will lag. You need to change the priority to be focus. It may mean that you don't get a shot when you press the shutter... it won't shoot until the tracking locks on and predicts motion. Other tracking settings change how quickly it can respond to changes, which may help with the sudden stops.
The camera matters here too... with my old T1i, it would always lag. Lots of shots with blurred heads and sharp tails, which is how I ended up with a 7D. I'm guessing the R5 won't have this problem... just a warning to anyone reading this in the future looking at an entry level camera... it may not track fast motion very well by design.
On Flickr - Namethatnobodyelsetook on Flickr
R8 | R7 | 7DII | 10-18mm STM | 24-70mm f/4L | Sigma 35mm f/1.4 | 50mm f/1.8 | 85mm f/1.8 | 70-300mm f/4-5.6L | RF 100-500mm f/4-5-7.1L
@DavidEccleston for the problems I was having before I sent it in across the board there were inconsistences. For what I was describing when I got the R5 back from service I was using the RF 70x200 usually at 200mm shooting at or near wide open. I had it on the 4 setting for tracking and face eye detect for an animal. The pups are field trial stock and these two are especially fast, neither is the slow moving lopping lab you usually see. So they are tough subjects. The R5 picked them up instantly running away and held on their butts. When they hit the brakes I would loose several frames the camera couldn't compensate quick enough. I had several shots that the dog would blink, and it was odd because the camera would go slightly OOF when it did this but immediately catch up. They were anywhere from 30 to 50 yards away when they stopped and picked up. Overall I think it did great, most people do not go out and try and push their camera to its extreme limit.