As a general rule 2x converters will only work well with a fast lens, ideally f2.8 but not bad on some f4's. Primes are also much favoured over zooms. In the Canon tele line up there are only a few super teles that take a 2 x well IMO (400/2.8, 300/2.8, 500/4 and 600/4).
Just my 2p's worth
|