Originally Posted by
Jon Rista
I can't say much about the Mark I versions of Canon's TCs. The difference between the Mark II and Mark III versions is edge and corner performance. Center performance is similar, maybe a little better on the Mark IIIs. Edge and corner performance is improved considerably on the Mark IIIs over the Mark IIs. Distortion and CA are supposed to be better controlled as well. I've used the Mark II and III 2x TC, and the difference seemed very noticeable in the corners. I have only ever used the Mark III 1.4x TC, so I can't say explicitly if what the reviews say about it are true or not, but I'd offer the 1.4x III would perform better than the 2x III.