I've had an MC4 1.4x and an MC7 2x and was disappointed with the IQ and AF speed of both. I've now got a Pro300 DGX 1.4x which is great quality with IQ, fast AF and it reports the f no. correctly. It will even let a 100-400 L lens retain AF at f8 on a 7D and a 5D2 at least. Well worth paying the extra out for the DGX version.