11-08-2017, 05:46 AM
It is pretty difficult to measure/calculate the true focal length of a lens at different focus distances. It already is a near on impossible task to find an actually correct formula which takes into account the distance from the lens' focal point to the imaging plane, and then to find a correct way to establish the front and rear nodal points, needed to asses the distance to the subject correctly.
To measure the focal length at infinity to some degree of accuracy is easiest, as the errors from subject distance and not taking into account the imaging plane distance become more or less insignificant.
To measure the focal length at infinity to some degree of accuracy is easiest, as the errors from subject distance and not taking into account the imaging plane distance become more or less insignificant.