Thanks! I guess I should thank the kudu for standing still that long! 🙂
@Dan-Cope, yes I’m a crop sensor shooter!
Just curious, was/is your 200mm a “fast” lens? f/4 or lower? If so, you’re better off putting your 2x converter on it to get 400mm. The problem with TC’s is that they increase your effective aperture. For example, if you have a 100mm f/4 lens and add a 1.4x teleconverter, it will increase the aperture to f/5.6 If you add a 2x converter to an f/4 lens, you’re going to get a maximum aperture of f/8! That’s not very fast! 🙂
So, I’m just guessing your 55-300 lens has a maximum aperture of f/5.6 on the long end. Just say you add a 2x converter to that, you’ll be turning it into a 600mm f/11 lens! That’s really slow! Your camera won’t even be able to auto-focus through that. It will have a hard time even with a 200mm f/8 lens (100mm f/4 + TC). That is, unless you have the new Canon 1DX II pre-ordered. It will AF with all the focus points at f/8! Most cameras will only AF at f/8 with the center focus point.
Another thing to remember is that adding another piece of glass will inevitably decrease your IQ. A 1.4x TC is a compromise between not using one at all, and going too far with a 2x converter. Unless you have really good glass, you’re just as well off cropping probably…
So the short answer is: No, a 2x TC on a consumer zoom lens won’t work very well.