Extension tubes

I always carry a set of extension tubes with me when I travel.  Most people think of these as macro photography aids in that they allow a macro lens to focus closer to tiny subjects.  That's true.  They do, in fact, do this.  But they also work with telephoto lenses, reducing the minimum focus distance.  For example, this mother and baby baboon I photographed in Kenya during a photo tour were sitting just 8 or 9 feet from my vehicle.  With my 70-200mm, I couldn't fill the frame with them as much as I wanted, and my 500mm lens couldn't focus at this distance.  The minimum focusing distance for this huge lens is about 14 feet.  Therefore, I put one extension tube between the lens and body and now I could have my cake and eat it, too.  In other words, I could fill the frame with the animals and still focus on them with the 500mm lens.