Just wanted to pick folks brains with this one - I regularly shoot wildlife and obviously the best shots are always when the subject is close but sometimes there is an interesting bit of behaviour across the other side of a lake - for example, yesterday I was trying get shots of an Osprey with a large Pike trying to get peace to land on a post to eat it. Looking through my bins this was a great sight, the bird splashed down to catch it, took off and wheeled around looking for a pullout area and was getting hassle from crows and herons as the fish (about 18" long and at a guess around 2-3lb)was being swung about below. Eventually it did settle down to eat. The shots looked usable in the viewfinder at 400mm, f5.6, ISO200, 1/640th, centre spot, single point focus but even modest cropping was a waste of time, poor quality. The action was perhaps 500 yards away. Should I just accept that I will not get a decent shot at that kind of distance or would stopping down, choosing more focus points or changing to say, centre-weighted average have improved things?

500 yards is too far away to get some nature stunning pictures even with a 400 mm lens.
To get sharp pictures of birds in action with this lens, you should have a "speed" of 1/2000 s and so increase the ISO settings

"500 yards is too far away to get some nature stunning pictures" Agreed.

I see a lot of long exposure pictures that are great. Usually they use wide lens and pick sunset or sun rise to shoot. In sea view or broad sight.