Playing with my long (500mm) cheap mirror lens again. I set the shutter speed to 1/4000 (as fast as the Nikon will go) and let the auto-ISO handle the rest. It has a relatively fast f5.6 that cannot be changed. The other caution to watch for is the T-mount. It can unscrew a little and loosen while the lens is on the camera if it isn’t in tight. That will cause difficulty with focusing.
We have resident pelicans. They are supposed to be rare. Ours aren’t.
The herons perch on stumps out in our little branch of Lake Weiss.
The image quality isn’t perfect, but it could be a lot worse. Not sure how much is the lens and how much is the ISO/low light.