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The term tube length refers to the optimal distance of an eyepiece from its objective where the distance results in the clearest image. There are actually two different types of tube length. These are: 1)The Mechanical Tube Length(MTL) and 2)The Optical Tube Length(OTL). The mechanical tube length is the distance from the top of the tube to the nosepiece without the objective in place.

The OTL is the distance between the back focal plane of the objective, and the primary image plane in the eyepiece and, all things being equal will vary with the focal length of the objective. Shorter focal length (higher power) objectives will need to have a back focal plane deeper within the objective to maintain parfocality.

Some pertinent equations:
Magobj =OTL/FLobj
FLobj= OTL/Magobj
Total Magnification of Microscope=MagObjective X Mag eyepiece

OTL=optical tube length   FL=Focal Length   Mag=Magnification   obj=Objective