A really really easy way to do it is to create a camera then in the hypershade middle clock the camera that you've created and plug the center of interest into the Fstop.
Then if you select the camera in the viewport and press T the center of interest is shown, move it to your object and render, adjusting the focus distance to suit how "tight" you want the focus. Thats how I do it anyway.
"No pressure, no diamonds" Thomas Carlyle