Camera FOV modification with ZOOM fixe on object scene

I’m working on project about 3d object integration inside a picture.

I use the FOV to adapt the perspective of the picture, but on each modification I need to adjust the ZOOM on my object, and it’s complicate because it’s not UX.

To resume my question, we can use the example of (
On it, I want change the FOV but i want blocked automatically the ZOOM on the cube.

Do you know a solution for that ?

Thanks all.

The link of scene was wrong. I changed it.

I’m finding a solution here :

What do you think ?