I played around a lot with Depth of Field in mental ray but it took ages to render out! So in the end I did the DOF-effect in post. The problem was that I wanted to have focus on the two glasses in the second row. The solution I figured out was to render two Z-Depth Passes; one short from the front glass to the focal point, and then a long pass from the focal point to the last row of glasses. I then subtracted the short pass from the long pass in post to end up with a mask that was dark at the correct focal point.