Nowadays, infinity optics are standard in all commercial research microscopes. A tube lens is placed between the objective and eyepiece to generate a real intermediate image through the combination of objective and tube lens.
Between the objective and the tube lens the light beams are parallel. This so-called infinity space can accommodate additional optical elements such as a fluorescence filter in the light path without generating a shift of the focal plane or spherical aberrations.
Another advantage of the infinite light path is its flexibility with regard to length of the optical path and therefore the mechanical design of the microscope.
∞ = Objectives for infinite optics