The Nikon D850 is equipped with numerous autofocus settings and options. Autofocus (AF): Single-servo AF (AF-S); continuous-servo AF (AF-C); predictive focus tracking automatically activated according ...
If your camera doesn’t have an AF-ON button, you can often customize the action of the AE-L/AF-L (auto exposure lock/autofocus lock) button in the Custom Settings menu, to act as AF-ON instead.