Connect Camera Focal Plane to Image Plane

I'm trying to connect these parameters so as I move my camera through a scene, the camera stays focused on a particular object.

I've used the whip tool to connect the Camera Focal Plane and the Image Plane Z position with the following expression:

ImagePlane3d.Transform3DOp.Translate.Z

The result did not fix the focal plane to the z position of the image plane.

Could somebody please assist?

Many Thanks