Canon 400mm f/5.6L. My impressions

I bought my 400mm a little more than a year ago as an upgrade to my old 300mm f4 L (non-IS) + 1.4 TC. IQ wise the 400mm didn't disappoint at all (but unfortunately my 300mm isn't top notch). Yes you do need to support the 400mm properly but good technique does help a lot. I have sharp captures with the lens a 1/80 hand held. Naturally keeper rate isn't great at 1/80 - actually it's quite low. At 1/250 I get an acceptable keeper rate with this lens. 4-stop IS would have been great though. One of the most important things regarding lens support is that you should support the front of the lens with your left hand. I think that this is a point that quite a few miss. I've even seen a book on nature photography showing a "stable" stance with the photographer supporting the lens quite close to the camera body.

About half a year ago I bought a used 500mm f4 L IS. Yes it's a better lens than the 400mm. But for most uses the 400mm will give you similar IQ if the 400mm has sufficient reach and you can arrange your shoot with sufficient distance between you subject and the background. The big advantage of the 500mm is that it's one stop faster, the more shallow DOF makes it easier to isolate your subject, and the reach - especially with TC added. The disadvantage of the 500mm is the price, weight of the lens, and that the significantly shallower DOF makes it harder to find you subject (with the 400mm finding your subject is a 2-D thing in most cases; with the 500mm it's almost always a 3-D thing) and for the camera to focus from significantly defocused. Basically you can point the 500mm lens in the right direction and entirely miss your subject because of significant defocus.

If I think I can get the image I want with my 400mm I will take it. If not I will bring my 500mm.