I while ago I bought my self an old canon x 1.4 extender (ver. 1). I got it for a reasonable price and I think it might get in handy when the 70-200mm or the 400mm is lacking reach. This is a test picture with my f2.8 70-200mm + x1.4 extender. I think it works quite well. Maybe just slightly softer than the f2.8 70-200mm. I haven’t tried it yet with the 400mm lens. Will test this under the Christmas holidays.