Originally Posted by
rpcrowe
Even if a TC can physically mount to a camera/lens, usually only the very best quality lenses will retain enough image quality to make their use worthwhile. Additionally, most often 1.4x converters will provide far better image quality than 2x TCs. Often the quality achieved with a 2x TC (Canon or third party) is not satisfactory.
There is at least one exception to the above blanket statement. The Canon 70-200mm f/2.8 IS ii lens will match with the Canon 2x iii TC and provide very acceptable image quality.