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| p.1 #18 · 7D: AF strategy for subjects coming straight on |
One thing that I would point out is that most of the respondents to the OP are using long lenses, whereas the OP is talking about using a 17-55. This is a significant difference, because with the shorter lens, the target is much closer to the camera, and the target speeding toward the camera is moving a much larger fraction of the distance from the camera to target than with a longer lens. For example (and just making up numbers without checking to see if they are quite right for the focal lengths involved), with the 17-55, maybe the dog runs from 20 feet to 10 feet away during an interval of time. Using a 100-400, the dog might start at 150 feet and run to 140 feet. The former case, I believe, is much more demanding on the focus tracking and focus motor driving than the latter case.
This is not to say that it can't be done, but the experiences being shared by respondents are not necessarily relevant for what the OP is trying to do.
And now for my gratuitous shot of my running dog when she was a puppy (taken with a long lens and a 7D):
You are absolutely correct about shorter camera to subject distance making a difference, but it is not just the camera's tracking ability that is stressed more when the subject is closer. The necessary shutter speed also becomes much higher the closer the subject is, which is why I tried to stress the importance of shutter speed in my initial post. If the subject is closer, it will be traveling through a greater portion of the available depth of field during the time that the shutter is open than it would be if the subject had been further away, and it is more likely that, just during the time that the shutter is open, the subject could have moved from being perfectly in focus to being completely out of the depth of field. Thus, higher shutter speeds are required.
While it is not exactly the same thing, a similar principle applies to subjects moving laterally across the frame. If you want to stop all motion blur, you will need faster shutter speeds with a subject that is moving laterally across the frame at a close distance compared to the shutter speeds needed to stop motion blur with a subject moving laterally across the frame at a greater distance, and the reason is that, when the subject is closer, it is traveling through a greater portion of the frame while the shutter is open than the same subject would be traveling if it were further away and shot at the same shutter speed. And, like with the subject moving toward the camera, higher shutter speeds are needed when the subject is closer in this case, too.