Camera zooming in and out problem

I want to make a replica of this Projectile motion "game" for my physics project. The problem is that I can not make these zoom in and zoom out buttons working properly. I want background and all UI fields , so entire canvas, to be always seen like in this game and zooming affects only cannon and a trajectory of shot ( it becomes larger or smaller without changing position).

I have tried changing the field of view and changing the distance from camera to cannon but neither one of it is not working right since my cannon is not only smaller or larger but it also changes its position.

Maybe I am not precise with title of question or question itself so suggest changing any part of it if it makes it more understandable.
Thanks a lot in advance.