This lens doesn´t have an automatic aperture function (therefore the “auto” doesn´t appear in the name).  It is a so-called “preset” lens, and the aperture ring is on the front end of the lens.  There are actually two aperture rings.  One has defined click stops and this ring determines the smalles aperture you want to use.  The second ring turns continuously and lets you vary the aperture between “wide open” and the respective aperture set on the other ring.  I assume this is to allow easier handling for manual focusing.