Well, the ratio is an indication of how long the tube will be, yes. But for anything significant there you actually need the focal LENGTH, not the ratio. So, the ratio itself is an indicator of how "fast" the scope is - ie: the length of the light path from the primary to the eyepice. The smaller the number, the shorter the path = the "faster" the scope. So, ok, so what? Well, the longer the path, (usually) the more concentrated the light particles - ie: the better the focus can be. So it doesn't really affect magnification by itself, but it does very much affect how CLEAR your max magnification can be. For example,an f8 scope will handle a mag of 300x with greater clarity than an f4 will - because its working with a more refined column of light in the first place.. See? Hope that helps some anyway..