First off, the Minolta 70-210mm f4 is a very inexpensive full frame legacy lens.  It is sharp sharp at all f/stops, well almost.  However, if you get passed f14 and narrow aperture, image diffraction sets in.  So what is 'diffraction' you ask?  I'm glad you asked!

Lens Diffraction is an optical effect which limits the total resolution of your photography It happens because light begins to disperse or "diffract" when passing through a small opening and in our case, our camera's aperture.

Now, I'm no pixel peeper or junky or something like that, but this lens is awesome.  It is even comparable to the Sony sal 70-200mm f/2.8 G lens!  You will not see a difference with your naked eye!  I use this lens all the time with my Sony a99.