I have a Vivitar 400mm f/5.6 in my stable. The 400mm is not a focal length I use a lot, but it has its place for long shots, particularly of wildlife of both four- and two-footed varieties. The shot was handheld (braced against the door) from my back door. The birds were in a tree about 80–100 feet away. They are sharp enough, but not the kind of razor-sharp image I’d like.
I’ll have to put this lens on a tripod and reshoot. I don’t know if it’s inherent in the design, the focal length, or is a reflection of poor camera technique.