In my non-professional experience, unless you're actually dealing with a pixel resolution issue (which you can in some cases, though I don't know about a 5D2... teleconverters are usually worse than just cropping, but it will depend a little on the lens too. Some lenses seem to be more friendly to a teleconverter. If the pixel resolution doesn't make full use of the lens, it may very well be better to use a teleconverter.


Is that ambiguous enough?