Magnification
The objective magnification is calculated by dividing the reference focal length of the tube lens by the objective focal length.
What determines the microscope PSF, and therefore the resolution and the ideal sampling rate, is not the magnification but mostly the Numerical Aperture. Still, the magnification is relevant to properly map the resolved points onto the detector, specially in the case of a CCD camera. You need a total magnification that correlates the pixel distance in the detector with the ideal sampling distance on the object you are imaging. For more details and examples, see Sampling Density.