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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?

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    Are you sure it is an issue with infinity focus rather than an issue with lens resolving power? The D7000 is 16.2 MP. The α7rIII is 42.4 MP. Blur that looks decently sharp on a 16MP sensor may look blurrier on a 42 MP sensor. – Michael C Jan 19 at 3:11
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    Sample photos from both cameras demonstrating the problem, as well as photos of the lens front and mount, would be helpful. – xiota Jan 19 at 6:20
  • Are you absolutely sure that the mass produced, used by many, would have junk reviews if it was too long because that would screw up a lot of lenses, adapter is the problem here? I mean, it could very well be...but do you have some actual measurements that can aid that investigation? – Hueco Jan 30 at 4:58
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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.

Nikkor 500mm f5 Reflex

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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.

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The infinity stop on most lenses is adjustable... often by removing a couple screws and repositioning the fixed distance scale for MF lenses. IDK the specifics for that lens, it would require some research or take it to a skilled repair person.

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