umphotography wrote in post #17909432
Jake
I have never heard of using an extension tube on a 500F/4 v/s a TC. She said she doesnt use TC's only extension tubes
Whats the advantage/disadvantage
Met her at Yellowstone and we were pointing at the same grizzly
With the 500mm f/4L IS the minimum focus distance (MFD) is 177" ( a little under 15 feet)
What this means is if a subject gets closer to you, say 12 feet, you can't get a shot, The subject is out of focus.
If your subject happens to be a tiny little bird like a wren or a hummer, then even at the very closest you can get focus, 15 feet, the subject is very small in your frame, very few pixels on target. Adding a T-Con helps when the bird remains outside your MFD, but again if you can fill the frame by getting closer you have no shot as it's all a blur.
Extension tubes allow you to reduce MFD, and this allows you to fill the frame with a smaller subject, and at NO impact to optical quality.
I keep a pair of tubes with me at all times when I bring the 500mm out, for this very reason. If I am lucky enough to get THAT close to a smaller attractive subject, I do not want to miss the shot 
This is the primary reason to use a tube. Using them to fit T-Cons together is more of a hack.