I've spent a half hour today with the 70-400mm on A900 (not a combination I regularly used, because the A77 seemed obviously a better choice - even the A580 is a better choice in terms of telephoto resolution). First of all, the bokeh of the 70-400mm at 400mm and f/5.6 does produce a distinct halo from marginally defocused bright objects and in daylight, where the light is of course from above, it's nearly always visible on the top edge. It disappears completely if the focus is perfect - and by perfect, I'm talking a matter of maybe 1 or 2 metres in 100-300mm.
I've been able to tell quickly that my lens on the 'new' ex-demo body before any lens corrections has consistency back, not front, focus tendencies and also by studying 3D detail that it is back focus which creates the most visible focus halo. I will adjust my AF micro setting and hope to eliminate this.
I have also been able to see from some of the complex subjects I've shot that at 400mm and f/5.6 the lens is critically sharp corner to corner - it may not have been focused on my 'layer' of trees and woodland, but the layer immediately behind in depth is rendered to pixel perfect level. Also, there's no trace of halo on absolutely perfect focus. I'll follow this up on the A99 when I have time and also on the A77, but I suspect 'just within depth of field' is fooling the DPReview poster along with subtle aspects of subject and light.
Here is a quick snap taken today of a chiffchaff (I think - smaller than a robin, tiny sharp beak, very yellow, very fast moving) - not a bird I can remember seeing in April in Scotland before - which was darting from branch to branch in a tree over the riverside this evening. Light was low but the excellent ISO 320 peformance of the A900 enabled a 1/1000th shutter speed- still not enough to freeze the extremely fast movement of this tiny bird. I used single shot, AF-S, centre focus and to be honest with the complex web of branches between me and the bird, I was amazed to get all three shots I managed in ten minutes to be sharp, ansd I do not believe there's any way I could have seen and targeted this subject with the EVF. Others will disagree but recognising the bird in the finder and moving the focus target to it before it had flown off to the next branch required concentration and a very clear view of fine detail.
This image is reduced to 50% size, this area was just over 2500 pixels wide in the original shot.
Let's just say that right now, I'm very happy to have received my ex-demo A900 at exactly one half of the last 'new' price WEX had it listed for, missing only its flash shoe cover, otherwise in excellent order with a tiny mark on the rear screen.