I am trying to design a microscope and I am confused about some concepts. I plan to buy a Olympus 4x infinite-corrected objective lens and 6 inch long tube and a tube lens (f= 75mm) and then a CCD camera. By chance I found a tutorial online http://www.olympusmicro.com/primer/java/microscopy/focallength/index.html I have questions about the tutorial: why is the focal length of the objective lens changing? Since the light beam come out of the objective lens is parallel, why does the tube length still matter? Really confused ... Need help!