Well, I decided to buy the new Nikon 28-400mm zoom, despite concerns that it might not be as sharp as I'd like.
The first shots I took were of the distant landscape across the river from the rear deck of my house, at 400mm, and I was shocked to find in Photoshop how blurred and distorted the images were when enlarged.
Then it suddenly occurred to me that it was the effect of 'heat shimmer'. However, the next day was cooler and cloudy so I experimented with a few shots from the same deck. After taking a shot at the widest angle of 28mm, I noticed a Kookaburra on a branch, in the shade, and was able to quickly zoom out to 400mm, and take a few shots at the same ISO but used a slower shutter speed, before the bird few off.
The attached wide angle view is F8, ISO100, and 1/320th sec exposure, reduced to HDTV resolution. The shot of the Kookaburra is at F8, ISO100, and only 1/160th sec exposure. I've reduced the cropped image of the Kookaburra to the higher UHDTV resolution, to show more detail.
This is an example of why I felt the need for this 28-400mm lens. There are so many occasions, as I take my walks, when I see something interesting, but am disappointed that I'm not carrying an appropriate lens.