I think the TC16A was designed to allow the N2020 to autofocus with certain manual focus lenses. The improved AF sensors on later cameras allow it to work with somewhat slower lenses than originally claimed. Also, I think the published limits are due more to the amount of illumination of the scene. It may well work with the 500 f/8.0 at noon on the beach, though I haven't tried it. The TC16A works best on shorter focal lengths that do not require the range of movement that the longer lenses require, but if you manually focus the longer lens, the TC16A will critical focus quickly within its limited range. I have used it with a first model 80-200 f/2.8 on an F4 at football games and auto races and it has proved to be a great combination. It's also a bargain at $145US.
