help with Apo Nikkor 760 not focusing

[Andrea Gazzoni]

Hello, I have an Apo Nikkor 760mm which seems impossible to focus, the image on the ground glass is blurry.

I suspect it was modified at some stage in the past.

If I unscrew the front and rear elements, here is what I find:



1) front cell: the optical unit is simply placed inside the barrel, once you lift it out, you find that silver spacer I'd say about 6-8mm high. I've thought of removing the spacer, but in that case the optical unit would sit too low and there would be a gap between it and the outer retaining rim (the one with the letterings that closes the front cell).







2) rear cell: since in its closed position it doesn't focus, I've tried loosening the rear element by small bits before, then by 2-3 complete turns, no big changes.

unlike the front, this element is screwed inside the barrel, so by turning it you change the distance between the elements. once completely unscrewed and pulled off, I find a black spacer above a gold color collar which requires a spanner wrench or similar tool to be moved







I was wondering if anyone with such lens could look inside their copy and tell me if there's any visible difference.

Also any suggestion for basic troubleshooting is appreciated. So far I've tried moving the rear element a bit backwards, but to no avail. Also tried to reverse the front cell, but it doesn't fit.

thank you in advance

Andrea

[Kevin Crisp]

This is a really basic question, and not intended to be insulting. Do you have enough bellows for it? I've seen stranger problems here.

[Andrea Gazzoni]

yes, I have enough bellows ;-)

[Tracy Storer]

Have you tried infinity focus? That is the first test. Aim your camera out a window and see if you can focus a distant subject. Maybe you already did this.....

[Drew Wiley]

Something looks fishy about that lens. I'll have to take a look at mine.

[Andrea Gazzoni]

that's what I am doing. the buildings outside my window never come into focus on the groundglass

Have you tried infinity focus? That is the first test. Aim your camera out a window and see if you can focus a distant subject. Maybe you already did this.....

[Andrea Gazzoni]

An update: I finally got the Apo Nikkor 760 focusing correctly.
The culprit was the inner concave element in the rear group, it was just mounted reversed.
Plus, the outer convex rear element was reversed too (but reversing just this one didn't solved the issue).
Both glasses in each group are not symmetrical..I mean, they are symmetrical to the front elements and maybe even interchangeable with them but each glass is not symmetrical per se. The major convexity of the outer elements goes toward the outside.
A real mess was made very likely in the last cla by previous owner.
Since it was all taken apart, I did a good deep clean up of each piece of glass, then put everything back to its place and closed the lens.
What I now have is a very happy almost new Apo Nikkor 760...no, wait... two of them, since Seth here sold me his copy when I was desperate..gotta place a For Sale ad I guess
thank you Juan, Drew, Dan and everyone else who helped

[StoneNYC]

An update: I finally got the Apo Nikkor 760 focusing correctly.
The culprit was the inner concave element in the rear group, it was just mounted reversed.
Plus, the outer convex rear element was reversed too (but reversing just this one didn't solved the issue).
Both glasses in each group are not symmetrical..I mean, they are symmetrical to the front elements and maybe even interchangeable with them but each glass is not symmetrical per se. The major convexity of the outer elements goes toward the outside.
A real mess was made very likely in the last cla by previous owner.
Since it was all taken apart, I did a good deep clean up of each piece of glass, then put everything back to its place and closed the lens.
What I now have is a very happy almost new Apo Nikkor 760...no, wait... two of them, since Seth here sold me his copy when I was desperate..gotta place a For Sale ad I guess
thank you Juan, Drew, Dan and everyone else who helped

How much for the second one? Lol.

[Andrea Gazzoni]

How much for the second one? Lol.

now try to convince me that I don't need two...