Abstract: This article deals with optical laws that must be considered in camera lens system. With a brief introduction of the pin-hole model, it describes the view that the projection center of camera is the equivalent of two nodal points. This paper also presents a different view from the traditional standpoint of two-media photogrammetry, illustrating the collinearity relation for two-media photogrammetry owe to the shift of datum marks. With the base of the collinearity theory, it concludes that the principal length in two-media photogrammetry is approximately equal to the one in one-media photogrammetry multiplying with the refractive index of the object space in two-media photogrammetry. The experiment on one antenna shows that high levels of accuracy can be achieved.