The relationship between the FOV, inter-pupillary distance, and head tracking has to remain constant for the user to feel comfortable. There are a few ways to do zoom. Here is a good thread on the subject: viewtopic.php?t=9641
Yeap I saw that thread time ago but all the methods seem deprecated. Still you gave me a tip on the interpupilar distance. I'll check the thread again to see if that is mentioned but otherwise any documentation where the relation formula is documented? edit: ok sorry I can see the ratio. Actual IPD vs Actual FOV.
We take care of the relationship between IPD, FOV, and head tracking for you. The best way to change the IPD is to scale the whole OVRCameraRig. To zoom I would recommend either scaling or moving the OVRCameraRig. If you're doing a "binoculars" feature, you could render the scene to a texture and then use a pincushion distortion shader on that to simulate the lens.