To get a good solution, we recommend measuring a minimum of twelve well-distributed points (and preferably fifteen to twenty) in each photograph. Also, the entire measurement should have at least twenty (preferably thirty) well-distributed points. When in doubt add more points. It’s quick and easy to do, so go ahead and do it.
Of course, measuring more points will lead to a better solution, however you quickly reach a point of diminishing returns. In most cases, measuring more than forty well-distributed points in each photograph, and more than sixty well distributed points overall will not significantly improve the solution.
Notice we always qualify the number of points with the term well distributed. The distribution of the points can often be much more important than the number of points. It is better, for example, to have twenty points which are spread out over the entire area being measured than to have fifty clustered in one small area and fifty more clustered in another small area. Points which are added only to improve the distribution of points are usually called “fill-in” points. See Resection and Self-Calibration for more details.