Field calibration is much like a Camera Calibration project and so has similar requirements:
• there should be at least five photographs,
• the photographs should all be taken with the same camera at one focal length setting,
• the photographs should be at strong angles to the subject,
• the photographs should all be at similar distance to the subject (and hence all at the same focus setting),
• the distance of the camera from subject should be similar to the distance used in your real projects,
• there should be one or two photos that have rolled angles (portrait type shots), and
• there should be at least 25 well marked and referenced points on all photos, with the points having each at least four references. The minimum number of points can be reduced on Preferences - Processing and Cameras – though not ideal, a field calibration with a smaller number of points can sometimes improve an approximate (or poorly defined) camera definition.