After comments from my friend Griff, I decided to mount the Vivitar 400mm f/5.6 telephoto on a tripod and shoot a few frames with my Nikon D300. The D300 is a crop-sensor dSLR with an APS-C sensor size and a crop factor of about 1.5. So, the 400/5.6 is equivalent to a 600/5.6 on a full-frame camera. I shot the lens wide open, f/5.6 , stopped down one stops, f/8, and stopped down three stops, f/16. The first frame (follows) is at f/5.6 (wide open). It is not very sharp.
The next frame is the test shot at f/8. It’s better.
The final frame is the test shot at f/16. It’s reasonably sharp.
So, here’s my take-home lesson. The Vivitar 400mm f/5.6 telephoto lens is unacceptably soft wide open. It’s better at f/8 and good at f/16. That means if I need a large aperture because the light is failing, this is not the tool to use. If it’s reasonably bright (where Sunny-16 is good), then the lens is useable at f/16 (maybe f/11) and can produce decent images. But, don’t let your friends shoot one of these wide open. It’s irresponsible photography and will frustrate everyone.