Magnifying power of a simple microscope is inversely proportional to the focal length of the lens. What then stops us from using a convex lens of smaller and smaller focal length and achieving greater and greater magnifying power ?

Explanation

First, grinding lens of very small focal length is not easy. More imprortant, if you decrease focal length, aberrations (both spherical and chormatic ) become more pronounced. So, in practice, you cannot get a magnifyng power of more than 3 or so with a simple convex lens. However, using an aberration corrected lens system, one can increase this limit by a factor of 10 or so.