Hello,

I've been using the Sigma 150-600mm C version for the last two years and I'm not quite sure where to go from here. I’ve used it with a Canon 7Dii for most of that time, and have recently purchased a Canon 5Div. I like the photos better with the 5Div, especially in low-light, but missed the perceived "reach" of the crop camera. I decided to purchase the Canon 1.4x Extender iii, to use with the Sigma, but unfortunately it does not work, except in live-view mode, and really not well there at all. I didn't realize the newest firmware made it incompatible.

My biggest problem with the Sigma is the autofocus. It honestly seems to be getting worse, perhaps there is something wrong with the lens, or maybe I'm much more discerning and putting it through its paces these days. I do get lucky and get sharp photos, but not when they are a good ways out, or flying usually. I've been using Servo with extended single point and single point one-focus. It’s just not very fast.

I primarily walk around and photograph what I can find, but I’ve been doing more “blind” type shooting with a tripod lately as well. I photograph waterfowl, small birds, & BiF. I'm still learning to get closer, instead of just relying on lens reach.

I like the zoom range of the sigma, which allows me to take some telephoto landscape shots when I'm out, but that is not a necessity for me. I also own a Canon 70-200 2.8 IS II. I've been looking at the Canon 100-400ii, as it is a direct comparison to the Sigma. I think it would probably be better, but by how much? I already have the extender, so that would help it's range a bit. I'm comfortable with the long end, as I use 600mm 95%+ of the time, and do not have problems getting the bird in the frame.

I think ideally I would want a 600mm f/4, but that is far too expensive for me at the moment, same with 400mm f/4 (& 2.8). Are there any other lenses I am not considering? How much of an upgrade would you consider the Canon 100-400ii vs. the Sigma 150-600C? I guess they would be about the same range with the 1.4x extender, but I go from f6.3 @ 600mm to f8 at 560mm. Not that the Sigma is super sharp wide open anyway (f/8 seems to be better).

Thanks for your time and consideration.