A 400mm is a 400mm is a 400mm .....well not always. These two pictures were taken from exactly the same spot camera on tripod and luckily the bird didn’t move neither. It is easy to see; the fixed focal length delivers quite a bit more although I had to use two extensions rings (36 & 12 mm) as I was too close for the minimum focusing distance of the 400mm. This more true 400mm is the reason I sometimes really prefer the 400mm over the 100-400mm ... just a little bit more reach. It is about the same difference when they are both focused to 4m.