Tannin another little point I found, I stand at the distance I want to take the photo then use the zoom to find the object and then fine tune with the focus. If it won't zoom and you can't get a clear focus, then you have to move yourself because you are not within the working length of the zoom position. I think I have explained it right. If not someone else might pop in to correct me.

Another example Tannin, what I like is the better close focusing you get. I used for next next test the 21mm Ext tube on my 80-400mm lens F8 and the on top flash on my 30D. The closest you can focus is 9ft. But with the 21mm tube I could focus down to 3ft at 400mm. The first photo was taken from a 3ft away from the grasses I did have to move my body slightly until I could hear the af wanting to focus. The second photo is a 100% crop. I have done nothing to the photos apart from the conversion and the crop, no sharpening.

You lose two stops with a 2x converter and that means the lens will not autofocus as the Canon body will not autofocus if it thinks the aperture of the lens is greater than f5.6 or with some models f8. Some people ''tape the pins'' to overcome the problem or use a converter without the relevent pins (Tamron) but I'm not sure which pins they tape. Puzzled by the weird manual focus bit, maybe just a very dark viewfinder due to the 2 stops loss making it hard to see properly.