I've been testing out some of my new gear and I must say that it seems to work out extremely well. It 's the Canon 400mm f/2.8 IS mk II lens and the 1,4x teleconverter mk III. This combination gives me a focal length of 560mm with a maximum aperture of f/4. I was a little bit worried about the AF speed and the image quality with this combination, but it's seems to be even sharper AND faster than my old 300mm lens. While getting those 560mm for birds, I still have a long lens with big aperture, perfect for mammals in dusk and dawn! So I'm super stoked to do some further tests with this lens and see what it can deliver in the future!