C2 the second condenser lens affects:

• The convergence of the beam at the specimen.
• Diameter of the illuminated area of the specimen.

Try switching between the three different focusing modes to see how diameter of illuminated area of the specimen changes.

Sitting at the microscope you can only see the image of the specimen on the fluorescent screen. How would you know when the condenser lens (C2) is focused on the specimen? If you now go away from this condition how could you tell whether the beam was overfocused or underfocused?"