taob wrote: Hrow wrote:
One then depresses the shutter button all the way but the camera doesn\'t fire because it has been instructed not to fire unless in focus. When a subject enters the focus zone, the camera automatically fires. Is this close to right?
Correct.
If so, how could this possibly work with AF-C as the AF system isn\'t running (or it would have focused on something behind the \"empty area\" already)? If it only works in AF-S that seems pretty limiting for shooting something like a hurdler.
I think you may be confounding single vs continuous burst mode with single vs continuous AF. Remember, the AF drive setting is irrelevant when it comes to trap focus, because AF is not involved at all! You disable it either by switching the lens to manual focus (thus it stays at the desired focus distance) or you turn off the AF function on the shutter button (so the camera does not attempt to refocus). AF-S, AF-C, face-detect, etc., etc. are all out of the picture at that point. Trap focus is essentially a manual focus technique.
If you are shooting 400 f2.8 at 2.8 you\'d be lucky to get one frame in focus if the runner lifts is head and the AF point is triggered by a hit on the body.
That would be addressed by choosing the shutter release mode: S, CL or CH. It is independent of the AF drive mode.
OK, so I am not missing something (which I usually am!) I see this as very useful in some situations but for narrow DOF shots it would seem too hit or miss to be useful. I realize that the AF isn\'t at play but acts as the trigger. In the case of the hurdler with a long lens wide open if the trigger point is the body rather than the face then the face has already pasted the point of critical focus and it doesn\'t matter how many frames get fired off as the base shot was already missed.
Even if it isn\'t missed, shooting at 150 ft with a 400mm at f2.8 you have approx. 7.2 ft of DOF with about 3.5 ft of that being in front. Assuming you hit the face perfectly, the runner is traveling at 22 ft/sec which means that they are out of the DOF area in less than 1/7 of a sec. If you are really lucky, you might get two frames with a usable focus but I would hope that the Nikon AF-C would be way better than that so I would question why one would use it in this case.
That\'s not to say I don\'t see the value of having this as an option. More options are almost always a good thing and I can see the merit of remote triggering based on focus, especially as a second camera but it doesn\'t seem very useful for something like track or racing.
Another quick question based on the bee example. Would it fire only when the bee hits a specific focus point or could one use focus point expansion so that you aren\'t reliant on a bee flying over a very specific and small focus point?
