Using a tube length of 160mm works well, unless optical accessories such as a vertical illuminator or polarizing accessories are added into the light path of a fixed tube length microscope. If items are added, it changes that tube length to more than 160mm. In order to solve this problem, in the 1980s the Infinity Corrected Optical System became common place.
Microscopes that utilize the Infinity Corrected Optical system have an image distance that is set to infinity. A tube lens is placed within the body tube between the objective and the eyepieces to produce the intermediate image (see image below). The Infinity Optical System allows auxiliary components such as illuminators, polarizers, etc. to be added into the parallel optical path between the objective and the tube lens with only minimal effect on focus.
|Fixed tube length versus Infinity Corrected Optics|
The Infinity Corrected Optical System typically makes it easier to maintain parfocality between different microscope objectives, even when auxiliary components are added to the microscope.
Learn more about Infinity Corrected Optics here.