d7k AF with AF-C & dynamic area doesn't work as the manual states

Lots of folks, including myself, have been having AF issues with the latest cameras, d7k, d800, etc.

There are a number of reasons for those problems, but one of them is the undocumented way that Nikon has implemented on many of their new bodies and firmware.  Apparently, this goes back to about July, 2008, when Marianne Oelund was having issues with her 2 d3 bodies when she changed the firmware to v2. She gives a lot of great info in several posts in that thread that I'd encourage you to read. http://forums.dpreview.com/forums/post/28716964

It's not important that the AF module is different on the d3 from the d7k, because the behavior is the same, in certain circumstances.  It is also important to note that the manual doesn't properly describe the new focus acquisition behavior for AF-C, dynamic area focus.

Basically, for purposes of initial focus acquisition, any of the multiple AF point dynamic area modes chosen will act like AF-A.  That means that if you have 21pt dynamic selected, any one of the 21 points can be selected by the camera for acquiring focus.  It doesn't matter which point you have selected, the camera will use the AF point that gives it the best AF target.

What this means to you is that it is MUCH easier for the camera to select an AF point that is not on the target that you want. For practical purposes, that would mean a lot more photos where the focus may be on something behind your subject.

I've tested this behavior on my d7k and d800 and both of them behave the same way that Marianne describes.  It's an issue that you need to take into account, any time you are using dynamic area AF, especially if you're accustomed to the behavior of earlier bodies, where this didn't happen.

If you're having issues with d7k AF in dynamic area, you need to use the minimum amount of AF points that you can. That means switching to single point, when possible.

I've been having issues with my d7k and this behavior is likely the reason why. I use AF-C, 9pt dynamic almost all of the time.

I've only just learned this behavior today, because of discussions on erratic AF on the d800 where a poster started a new thread, talking about the new behavior.  Here's a link to that thread, if you're interested. http://forums.dpreview.com/forums/post/50651413

good luck

Kerry

--
my gallery of so-so photos
http://www.pbase.com/kerrypierce/root