I do hear people report this, so I have to believe them. The 500mm f/4 AI-P's problems aren't helped by the position of the focus ring - in order to hold my hand where I could focus it, I found I had to extend my elbow to the extent of it being unsupported, and that lens was sufficiently front-heavy that I couldn't aim it stably. Newer versions may be substantially better. Well, give me one and I'll confirm it. I've tried them (pre-FL) a couple of times in shops; although they're heavier, the centre of gravity is close enough to me that I can brace an elbow on my conveniently-shaped belly. I'll have mixed feelings if I cease to be anatomically adapted before I get such a lens. The advantage of the 400mm is aperture - it'll give more subject separation than the 500mm (whereas for the 600mm it depends on the scene geometry). It depends what I'm pointing at, though; for wildlife near my house 400mm will often do, but the lighting is often limited. I don't really do the sports for which it's optimised. For birding, it's not long enough - the teleconverter option would be because I couldn't separately afford an 800mm. (200mm + 400mm will probably do me, and I'd be sane enough to hire when I needed more.) The argument for the 2xTC, for me, is mostly with the 600mm. The 800mm f/5.6 + TC14 only gets you to roughly the same place, while being a little less flexible; there's not really any such thing as an "old" 800mm, so there's no cheap route. Having seen someone with the 600 + TC20 in Yellowstone (with, admittedly, a D5 not pushing the resolution boat out much), it came to my attention that it's the main way of getting 1200mm with autofocus on a modern Nikon body - and it's quite a bit more reach than my puny 200-500 + TC14 combination. I've always been a little put off the 600mm partly because it's tested optically a bit worse than the shorter glass, but the FL versions seem to be improving that. On the other hand, less good, used versions for under £2000 (beaten up but hopefully okay optically) are a little tempting with autofocus. So far I'm restraining myself, since I have my eye on the D850 and 70-200 FL, and also not leaving myself indefinitely in debt. Besides, it would be most useful on a longer trip (near my house it would mostly give you a close-up of a magpie...) which means hiring the new versions are more appealing than trying to get a monster like this on a plane. I already stretch carry-on limits quite a lot. Technically it can also make an element thinner, which could be significant for both total weight and weight distribution. Barrel length doesn't do that much for total weight at the same absolute aperture (the 200mm f/2 is roughly the same weight as the pre-FL 300mm f/2.8, IIRC, and in the same ballpark as the 200-400 f/4 - all roughly 3kg), which - at the risk of being on topic - is a reason I'm dubious about the "lighter DX lenses" argument. You could get an FX lens to about the same weight as a DX equivalent by matching the effective aperture, to within an approximation. To be fair, I hear horror stories about this both from the Canon DO and the Nikkor PF, but people mostly seem awfully happy with them. I'd want to know before it went on my wish list, though. On the other hand, I can do about 1400mm f/5.6 with a 10" Dobsonian, but don't bother except for the sky because of the bokeh. Well, also it's manual focus and a bit hard to hand-hold.