Hi,

For doing an "autofocus" (without killing my animation, because interest is animated) i want the null interest on my camera move only on it own X axe (red) (like a piston) following the other null position - wich can be for instance animated on a curve - like that the second null stay always in focus.

How to do that ? i can find. Ice maybe ?
Thank you very much

Probably missing something but why wouldn't using the animated null as the target work?

That don't work, its very bad that we can't have 2 nulls, one for interest, for animate the camera and one other for adjusting DOF focus.
In my case, the camera is animated by the interest, so, the focus point is not at the good place for me...

exemple : you make a scene with an object in center, and rotate the camera around the object, interest is at origin (0;0;0)
If you want defocus the object or have a focus on another object, how you do ?
I know i can do it by animating focus lenght in camera, but in my case camera avec to stay focus on a fly who move very fast...

Yes, you can do it. You can do it with Linked Parameters, or you can do it in ICE.