I would be using a 210mm or 300mm for the heavy compression you get with these telephotos. Using one (and just one) extension tube would add a little more Bokeh while letting the subject stay in focus. One tube isn't going to do a whole lot to a 210mm or 300mm (or even 180mm)lens as far as making it macro. The 300mm is extra tempting because the N version has some nice ULD glass in it that is lacking in most lenses in this range.
Originally Posted by paul ron