When there are sufficient 'good' points and photos in the project, the Include Camera Optimization check box is checked in the Processing Dialog, and the field calibration check box is set in Process section of Preferences - Processing and Cameras then the field calibration process will run along with the global optimization. A warning appears if there are fewer than the threshold number of good points, but you can still attempt the processing. It is a good idea to save your project before field calibrating in case you need to revert after an unsuccessful or sub-optimal result.
Field calibration is very much like the Camera Calibration path of the New Project Wizard in that is it trying to solve the internal parameters of one camera. Field calibration can be a replacement for full Camera Calibration in some circumstances.
For high accuracy projects, Field Calibration (along with appropriate photographs) can be very important to get that extra bit of accuracy.
The camera parameters to be solved during Field Calibration can be set on the Processing dialog – expand the Include Camera Optimization control to reveal the camera parameter options to solve.