anonymous one , Mar 10, 2007; 10:26 p.m.
The way I understand it is that anytime you add more glass infront of your camera the quality is going to deteriorate. That being said, the higher quality the added glass, the less the deterioration. So if you were using a high quality 70-300 and a high quality telecoverter, it would be better than using an average 600mm. Thats just my take on things, I'm an amateur with little experence in teleconverters.