Calculating size from electron micrographs requires the use of conversion factors. 

3.5 cm/(50,000 magnifications) = .00007 cm actual size.  = .0007 mm actual size  =  0.7 μM actual size. 

                                                  (division)                     (10 mm in a cm)            (1000 μM in a mm)

*M. smegmatis is about 4 μM long.