Hello harrymac1953,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with using your Nikon D850 with FaceTime. We're happy to help.
"If your Mac is connected to multiple cameras, microphones, or output devices, you can choose which one to use for your FaceTime audio and video calls." You can find this information and steps to help in the guide below.
Choose a camera or microphone for FaceTime calls on Mac
Should the issues continue, the following steps may assist:
Make sure the USB or USB-C cable is properly connected to your camera and the computer.
Run First Aid on Macintosh HD - Repair a storage device in Disk Utility on Mac
Check your camera to make sure it’s turned on and set to the correct mode
You can also check the Nikon D850 website for any firmware updates here: Nikon Download Center
Best!