When the highest accuracy is required it is a good idea to calibrate your camera at the distance and focus setting you will be using for your real projects. If your real subjects are smaller than one meter across then Camera Calibration software that comes with PhotoModeler can meet that requirement easily. On the other hand if the subjects are bigger than what can comfortably be done with a projected slide or printed calibration pattern then you may want a more accurate calibration than the Camera Calibrator can give you. Field calibration fills this need.
You may wish to first do a standard calibration project with the Camera Calibration choice in the New Project Wizard. You can use this calibration data for smaller subjects. You then find a suitable subject (side of a building for example) that you can take photographs from all angles (side to side and up and down). You photograph the building and set up a normal PhotoModeler project using the previously calibrated camera. You mark and reference as many points as you can (using the sub-pixel target marker if you have circular targets) on these photos and when you process you ask for field calibration as discussed above.
When the processing is complete and you have reviewed the quality of the solution (by reviewing residual values, tightness, model shape etc.), you can then save the camera to disk with these updated camera parameters and use in your future projects of similar size.
See also Upload Camera.