indeed, and this also makes the DOF calculations difficult. As for focal length at macro distances, Here is what I get with my Canon EF 100 f/2.8 Macro USM:
The measurement at 305mm (between sensor and object) is out of spec but provides acceptable sharpness for negatives that have emotional rather than technical values. The horizontal axis represents a scale of 1:1 up to approx. 1:2.5.