Obviously I am a AE newbie, I am well versed in vegas and key frame animation but I am having a hard time learning the AE UI.
I am using a Null object with motion tracking to composite a face image over a moving body, but I can't figure out how to limit the length of the Null object in the time line to end when my tracking point is out of, (it's a back and forth dialogue shot)
Can I do this without cutting the clip up and doing each segment individually?
Using the basic motion tracker, you should be able to stop tracking your shot when the picture cuts to the other shot. Then delete the tracking points you don't want (if you tracked a few frames past). Apply that to a null object.
Then you can delete the tracking data from the tracked shot. Move down your timeline to the next section you want to track and do the same thing. If you apply that tracking data to the same null object, it will simply add the new tracking data and leave the first data alone. It will interpolate movement between the last of your first track and first frame of your next track. Not sure if that's a problem for you.
If you want, you could create multiple null objects and adjust their length to the area of the clip your tracking. On a mac you can hit Opt [ or ] to trim your null layer to desired length. If you are tracking 3 sections of video, create 3 null objects. Parent the second to the first and the third to the second. Then parent your face to the first null.
Hope that helps and is kinda what you were looking for.