I did once. A 2X teleconverter. It doubled the f-stop (went from 5.6 to 11), was noticably darker in the viewfinder and fuzzy around the edges. Hix
As ever, you get what you pay for. I regularly use a Nikon TC1.4 with my 500mm Nikkor: this doesn't give as much reach as a 2x converter, but it only loses 1 stop and there are times when a 700mm f/5.6 is much better than a 500mm f/4 (although that may seem excessive, anyone shooting wildlife will know it's true). A good converter matched to a good lens can undoubtedly give good results. One other point is of interest, the minimum focus distance of the lens doesn't change when the converter is added, which can give opportunities for 'close-up' shots that you can't get any other way. Alan
I used the Kenko on a Canon 70-300L and it autofocused on centre point although it struggled when the lens was almost fully zoomed out. Up to about 270-280 was ok.
Any modern teleconverter that is properly matched to its lens mount should autofocus properly too - if it doesn't, it's not fit for purpose so ask for your money back. But remember that a teleconverter works by magnifying the image formed by the lens in front of it, so any focus errors will be magnified too and autofocus is likely to be slowed by the effective aperture reduction. When I was an impecunious student I tried a cheap OEM converter. I wouldn't do it again - stick to one made the manufacturer of the lens that you will be using it with. As I said before, you get what you pay for. Alan
Most camera bodies won't autofocus with an f8 lens aperture and adding a 1.4x convertor to an f5.6 lens makes it f8. The 70-300L is f5.6 at its longest focal length and f8 with the convertor added. Hence the focusing problem.
I work in the photo industry, am a part time professional shooter, so I will tell you what I know about Canon equipment. Other brands may or may not be similar. There are two main types of extenders (also called teleconverters): a 1.4X and a 2X. The former magnifies things 1.4 times but also uses up one stop of light. So a 300 f/4 lens becomes a 420 f/5.6 lens. The latter magnifies things 2 times but uses up two stops of light. So a 300 f/4 lens becomes a 600 f/8 lens. (I should note that Nikon makes one between these two that is a 1.7X and uses 1 1/2 stops of light). Extenders are meant to be used with telephoto lenses only, in most cases fixed (non zoom) lenses, with the exception of a couple pro grade zooms. In the case of Canon, the only two zooms I know of that use the extenders are the 70-200 f/2_8 and the new 100-400 f/4_5-5_6. It is widely reported that 1.4 extenders give good results with a good lens and 2x extenders give mixed results. With most Canon bodies, you need an effective maximum aperture of 5.6 to maintain autofocus, though a couple pro bodies will center focus at f/8. Some aftermarket extenders (like the previously mentioned Kenko) trick the camera into reading a different aperture so that it will still autofocus. If Jackwow had tried to use a Canon brand extender on his 70-300 the focus would not have worked as Canon says that lens is not meant to take extenders. I may have helped you or I may have caused more confusion. If so, sorry!
I am now using a Canon 1.4x III extender with a Canon 100-400 L Mk2 and a Canon 7D Mk2 and I have to say that it works great. Having said that I have so far only used it in good light. Of course at the long end the 100-400 is f5.6 so with the 1.4x attached it becomes f8, however the Canon 7D Mk2 is one of only a few cameras that can autofocus with an f8 lens.
