I have a vintage circa 1970 Nikon Nikkor 500mm F/5 Reflex lens that will not focus at infinity with any of the Nik-NEX adapters I have to connect this lens to my Sony A7RIII. The lens works great on a Nikon 7000 camera. The obvious problem as I see it is the adapter is just a tiny bit too long. I am an amateur astronomer and my primary use for it at this time is astrophotography and my stars are not perfect in focus with the adapter and the Sony camera. Any suggestions?
This from the mir.com website, I'm not sure if it's relevant but worth mentioning because of the higher resolution sensor in the a7r III:
A filter must always be in position at the rear of the lens,when the lens is mounted on the camera and ready to be used. Five filters, neutral density, UV, Yellow, Orange and Red, are supplied with the lens. These filters are specially designed to eliminate focus shift, when used interchangeably. When no correction is desired, the UV filter should be in position. L39 and ND filters can be used also for color photography; the others supplied, however are recommended and used exclusively for black-and-white photography.
Adapters of this kind being too long is an exception - instead, they often are intentionally a bit short to allow dealing with lenses with slightly miscalibrated infinity stops.
If you think a shorter adapter will help, get a couple of cheap adapters, one of them is likely to be short ... or a "universal" adapter (like https://www.amazon.de/Universal-Adapter-Objektiv-Konica-Olympus-Polyester/dp/B019BBR7I6) which can be tuned to a random flange distance.