Hi Marc,

We are aware of the issues with the Camera Calibrator at high zoom levels, and we are actively working on improvements for future updates.
Most Aximmetry users typically work in enclosed spaces spanning only a few meters. However, some users operate in large venues such as stadiums, and we are familiar with the calibration challenges that arise in those environments and lenses as well.

If you know you will not need high zoom levels, for example if you will be working in a small studio rather than a large venue, there is a helpful workaround. During the calibration process, when the Camera Calibrator prompts you to fully zoom in and out, only zoom in as far as you expect to use during your production.
After completing this calibration, you can add a calibration point at the maximum possible zoom using the Basic Calibrator. This way, even if you accidentally exceed your intended maximum zoom during production, you will avoid major discrepancies. 
If you plan to use the lens in a larger venue but do not have enough space to calibrate it beforehand, you can add multiple calibration points at different high zoom levels in the Basic Calibrator (levels you would not ordinarily be able to reach in a small studio with the Camera Calibrator). This effectively completes your calibration file. Note: Adding calibration points in the Basic Calibrator, especially with Lens Distortion values, requires some practice to master.
You can learn more about the Basic Calibrator here: https://aximmetry.com/learn/virtual-production-workflow/tracking/calibration/basic-calibrator/ 

Warmest regards,