Think about how you might solve this in the real world. Grab a two by four of the appropriate length. Use duct tape to attach the ball to one end, and to attach the camera (aimed at the ball) to the other end.

Now you can carry that rig around with you. The ball will always be in the center of the camera view, and will be a constant distance away from the camera.

You can build the same rig, virtually, in SceneKit. Create a new SCNNode to be the rig (taking the place of the two by four). Add the ball as a child node, at (0, 0, 0). Add the camera as a child node too, at (0, 0, 5) (camera looks down the -Z axis, so this position should put the ball in the center of the view). Now you can move the rig node anywhere in the scene you want, and you'll have a consistent ball position.

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