Parameter adjustment of the transmitted light source yields a good-quality image for video measurement microscopy. However, a disadvantage is that the measured size of the objects changes simultaneously while the parameters of the transmitted light source are adjusted, which directly affects the measurement accuracy adversely. Furthermore, the variation of the objects' measured size leads to the magnification variation as well. In order to analyse how the measured size of the objects varies with the parameters, we apply a photometric method and an operator algebra method to derive a unified mathematical model that correlates the parameters of the transmitted light source with the measured size of the objects. Theoretical results show that factors including the illumination intensity, the diameter of the aperture stop and the field stop and the object thickness are important to the measured size of the objects. The simulation results have been verified thoroughly using a wide variety of internal and external diameter measurement experiments. The proposed method has proved to be suitable to determine how these parameters affect measured size of the objects.