Why Does Tube Length Affect Microscope Focal Length?

[akinokoe]

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!

 

[Valerie Park]

The focal length of the objective lens does not actually change; it stays the same. The tutorial is referring to the effective focal length of the entire microscope system, which changes with the addition of the tube lens and camera. The tube length matters because it affects the distance between the objective lens and the tube lens; this is known as the intermediate image distance. The intermediate image distance sets the size of the intermediate image, which is then magnified by the tube lens and then further magnified by the camera. All of these components come together to form a complete microscope system with a specific magnification and effective focal length.