Hi,
I understand that a prime will most of the time be sharper and quicker than a zoom and that the 400mm is one of the best - I thought I would put that out straight away.
In fact my lens history includes a Sigma 150-500 > 100-400 > 400mm, so I'm familiar with the old tech Bimga and how it compared to the 100-400 and that for 400mm only teh prime was better than that, however times have changed and a new model is in the mix, especially as from what I've read its much better than the old 150-500.
I have seen a few reviews of the 50-500 OS but have yet to see one that includes the 400mm for direct comparison of IQ and AF speed.
(magsnorton)
