I recently bought a used 500mm f/8 Sigma Mirror Ultra telephoto for a song. I know ,you get what you pay for. But for $65 it was worth the gamble. The lens looks like new. The problem is the image I see in the view finder is very sharp. But the film doesn’t show that at all. I shot a series using tree branches about 500 yards away on an angle to reduce the problem of depth of field, in other words something should have been in focus. The lens was mounted on my heaviest tripod which I use for my 4x5, the Olympus OM attached to the lens. With mirror up the was shutter tripped by cable release and exposure were 1/1000 and 1/500 Oddly enough the 1/1000 was the worst with the typical out of focus donut highlights, Obviously something moved. What mystifies me is I am not getting what I see. If the lens were of coke bottle quality I would expect to see that in the viewfinder .Just the opposite the image in the viewfinder is as sharp as my prime lenses which are pretty sharp Anybody have suggestions or experience with these long lenses?