I have not been doing photography for too long but have got a secondhand canon 100-400 L mark one lens. I have a friend who also started around the same time he has the Sigma 150-600 contemporary on a Nikon body. He seems to get some good shots with his set up. I like mine but his just seems slightly sharper. I use a 1.4x canon extender on mine. It does AF but only in the center. My question is do you think I would be wise trading the canon 100-400 in and trying the Sigma or will it be my lack of skill and I just need to keep trying. I think I have it in my mind that because the Sigma is newer then it must be better than my older lens. I dont think I have the guts to go for a 100-400l mark 2 at the moment.