You may need to select Oil instead of Air. Or select User-Specified and enter your own correction factor in the field just under Z Correction Factor.
What is it?
The Z correction factor is used to calculate the true position of the focal plane as the microscope is moved up and down. The focal plane moves a distance that may differ from the movement of the microscope. The difference is due to the refraction of light between the cover slip and the immersion medium of the lens. Under normal circumstances the predefined values for air, oil, and water immersion lenses can be used. There are circumstances in which the predefined settings should not be used. Contact MBF Bioscience for information about unusual combinations of lenses and mounting mediums.