Huygens Remote Manager - Help : Tube factor
The magnification is dependent on the tube length of the microscope, with most objectives and eyepieces having optical properties designed for a specific, fixed length. Variation to the length may lead to changes in the magnification factor, and although additional lenses can be used to compensate, this in turn will introduce a magnification factor that must be taken into account (known as a tube factor usually in the region of 1.25-1.5x).
On some microscopes the tube factor can be adjusted using a wheel with the different possible tube factors written on it.