Focuses Image of the Specimen at Focal Plane of Objective Lens into Focal Plane of Ocular Lens.
Data about Lens usually indicated on Lens Barrel:
"X"=Magnification;
Abberation Correction: "Ach"=Achromatic=Corrected for Spherical in G, Chromatic in R and B wavelengths. "Apo"=Apochromatic=Corrected for Spherical in 2 wavelengths, Chromatic in 3 wavelengths.
"0.000"=Numerical Aperture