"CAM AF function...Amount of Acceptable Contrast and AF Point position"
Yesterday I was shooting a sailboat entering the harbor from a distance, I had AF set to AF-C, dynamic 21 points, shooting 70-200 VR1 (left the VR on had forgotten to turn it off...)
See below, I had the middle sensor locked onto the boat hull, but was surprised where the camera chose to lock the final focus point. Boat could not have been moving much more than 7-8 knots.
Even more strange is where the bird focus point landed...i had VR turned off in this instance. The bird was descending and I was panning down.
Basic questions are:
1) how much contrast is needed on the target in order for it to lock in 2) does the AF point need to be completely inside the target, ie if you fire and lets say 1/2 the af point selected is on the target, and the other half off the target - how does the CAM system know what to focus on?
#1. "RE: CAM AF function...Amount of Acceptable Contrast and AF Point position" In response to Reply # 0
You're asking the camera to track the initial point at which you focused. In the case of the sailboat, if you focus on the point where the sail intersects the horizon, do you intend the camera to focus on the sail or on the horizon? From what I can tell, the system prefers the nearest object, but it's not an absolute.
In general it's best to have the entire subject covering the focus point if possible and if the subject provides a contrasting target. In the case of the boat, I might have put the focus point on the hull at the sail. If the relative motion of the boat was small, I would have used single-point focus, or perhaps 9-point dynamic since there is no possibility that the actual focus point I want would be out in the wider field of 21 points.
You said you had the middle sensor locked onto the boat hull, but the hull isn't close to the middle of the frame. Did you assert the focus, then reframe, expecting the focus point to follow your reframing? If so, that will work only briefly, until the camera decides that you really meant to focus on whatever is now under the selected focus point, depending on the setting of CSM a3.
As for the bird, it's a small AF target. If your focus point slips off of the bird and you have CSM a3 set to off, or you are off the bird for long enough, the camera is going to switch back to your initially selected focus point, which is what appears to have happened in this case.