For those of who use the AF-ON button on Nikon DSLR's may have noticed a change in the way it functions with AF-C, on current generation of cameras. I did not mention this in my D800 review last year, because it was not until recently that I realized what was going on. I was often puzzled why I would have a number of out of focus shots at the start of a sequence, but didn't pay much attention, figuring that it was simply user error. This issue did not come to my attention until one morning, while preparing for an outing, something strange happened. While setting up the camera for the day I bumped the shutter, and it fired. I wondered why, because I had the camera setup for trap focus. Under said condition that would not happen with my older Nikon DSLRs. I did some testing and found that the shutter would fire, even when auto focus was not active (fully depressing the shutter, without pressing the AF-ON button). I brought this up with some other photographers I know, and some didn't understand what I was talking about. Thus I made a video, seen above, to show the problem.
Settings used to achieve trap focus are:Focus mode:
AF-C in single point, dynamic or auto area *
a1: AF-C priority selection: Focus *
a4: AF activation: OFF (AF-ON Only)
On all the Nikon camera bodies I've shot with, D80 (with AF-E/AF-L button set to AF-ON), D300, and D700, when using these settings the camera would not fire the shutter unless the camera was in focus. With the D800, and apparently the D4 as well, the camera will fire even if focus is not achieved (as seen in the video).
The only way around this problem is to set AF activation to ON (AF-ON and shutter release). Not a big deal right? Sort of. The entire point of using AF-ON is to decouple auto focus from the shutter button, allowing the user to keep the shutter button depressed (ready to fire) whenever the subject falls into focus. Forcing the user to use shutter button AF kind of defeats the purpose of having the AF-ON button altogether. If Nikon continues to make the cameras function in this manner, the AF-ON button might as well be removed from the camera.
* Further testing shows this affects AF-S focusing mode as well.