RikWriter wrote in post #17407224
Mike, having used the 100-400 MKI with a TC, I would highly disagree that the IQ is better than the Tamron at 600. And even with a camera that will AF with the TC on, the AF is much slower and less accurate. I sold my 100-400 MKI to buy the Tamron and don't regret it at all.
I appreciate your first-hand experience with the two models. To be fair, I didn't say the 100-400mkI with the TC has higher IQ than the Tamron at 600mm. I did read that they offer similar sharpness though when comparing the entire image frame (the Tamron is usually a little sharper in the center, the Canon a little sharper at the edges). I haven't tried both next to each other though, and it seems that the 100-400 has a wider sample variation with respect to image quality.
I agree that the AF of the Tamron is probably faster than the 100-400+TC (and when it comes to requiring a certain camera body to get AF to work at all, that's certainly a benefit of the Tamron). Without the TC, my impression is the 100-400 edges out the Tamron with respect to AF.
I suppose it depends on what focal lengths you'll be using. I figured most of my images would be at 400mm or shorter, so I went with the 100-400. If you use the longer end more often and don't mind the extra weight of a bigger lens, then the Tamron probably makes more sense. Either lens is a good choice for someone looking for a quality super telephoto at a price that won't break the bank.