Hope some wise person/optic Physicist will weigh in. The Camera lenses we use are composite "lenses"; however, I am having a hard time reconciling the numbers: A 600mm lens (the Nikkor for eg) is reported to be 19 inches in length (outside dimensions). Without measuring the thickness of the flanges and mount etc, 19 inches translates to 482.6 mm. I would have thought a 600 mm lens would at the very least be 60 cms in length (from Objective to image plane)= 23 inches. Anybody have any opinions on this?