Quote Originally Posted by HDNitehawk View Post
The Tamron is crap at 600mm, but compare the two at 400mm.

http://www.the-digital-picture.com/R...mp=3&APIComp=0
AF acquisition and accuracy is also something to consider between the two lenses. The Tamron produces good image quality at 400mm, but if it can't lock on to a target in time, then the EF 100-400mm L IS II may prove worthy of its price premium over the budget-friendly Tamron.