Write an expression for the magnifying power of a simple microscope. How can it be increased?

The expression for the magnifying power (M) of a simple microscope is as follows where f is the focal length of the lens, and D is the least distance of distinct vision. The magnifying power of the microscope can be increased by using the lens of short focal length (i.e., shorter the focal length, more is the magnifying power). But it cannot be increased indefinitely.