I’m thinking about buying a Tamron SP 500mm lens from a friend, so he’s let me borrow it for a few days to try it out. It’s a manual focus lens and appears to be a fixed aperture (or I need to RTFM), so shutter speed and ISO speed are the things you play with to get the right exposure.
I set up my tripod on the balcony and aimed it at the moon. Other than cropping and some light processing using Lightroom Mobile on my iPhone, these are pretty impressive. 

One of the appealing things about the lens is that it’s roughly the same length as my 18-200mm at its shortest length. The lens is more like a little telescope than a traditional telephoto lens because it has a configuration of glass and mirrors that’s used in compound telescopes, aka a catadioptric lens. 

The bokeh (the blur of things outside the in-focus range) is pretty cool. Instead of the traditional “dots” you end up with “rings”.