Field test: Sony FE 400mm f/2.8 GM OSS

Sony's Alpha A9 is a brilliant camera for shooting action, thanks to its incredibly quick AF system and 20fps burst shooting. However, it's been hampered a little by the limited selection of Sony telephoto lenses, with the FE 100-400mm f/4.5-5.6 GM OSS the longest dedicated optic for Sony's range of full-frame mirrorless cameras.Now Sony has launched the much-anticipated FE 400mm f/2.8 GM OSS telephoto prime, which offers a focal length that's all but essential for sports photographers, and for