Canon EF 200-400mm f/4 IS USM Extender 1.4X review: does the performance justify the price?By Kevin Carter - Tuesday June 25 2013 Lens Review
Achieving an excellent DxOMark score of 24 points overall and 19P-Mpix for sharpness mounted on the EOS 5D Mk III, the lens performs very well, edge-to-edge, but the image quality drops noticeably when the 1.4X extender is engaged. Then it’s just slightly the ahead of the $999 Sigma 120-400mm F4.5-5.6 DG APO OS HSM, in terms of sharpness at least. In all other respects; Distortion, Vignetting and Chromatic Aberration are all very low indeed. Slightly more than one stop (1.2EV) is lost in Transmission when the extender is used.