I see you've already done "the right thing (tm)" but if you don't mind, I'm going to take a moment to explain why your initial "wrong thing" was even wronger than you imagined.
Hello
I have a new Nikkor 5.6/80mm. I'm going to use this small reversed lens.
It is a 34.5mm filter size lens.
It has a 34.5x0.5mm filter. The 0.5mm is the thread pitch, 0.5mm from the peak of one thread to the peak of the next. This thought is important: hold onto it for a minute.
Can I use a step-up ring of 34mm to 52mm?
No. The standard step-up ring will have a 34x0.75mm thread. 0.75mm from the peak of one thread to the peak of the next.
I just need to know if 0.5mm will make a difference or if I really need a ring with the precise 34.5mm on one of the threads. Thank you.
You do. With a 0.5mm diameter difference, and the fact that most thread adapters are made in China and deliberately undersized on the male side (and oversized on the female side) because their tolerances are low and undersized males or oversized females will not jam or get stuck. (although it may spontaneously loosen up, wobble, cross thread, or even "pop off" if it's too badly undersized). When you use a Chinese adapter on a Nikon (or other typical Japanese) lens, the effect isn't as bad as Chinese to Chinese but it's still bad. Going deliberately 0.5mm under will make it dangerously bad and you're much more likely to experience a pop-out or tilt-out.
With the 0.25mm thread pitch difference, properly fitting threads will rotate at most one full turn, then jam solid, to the point where you may not be able to unjam them without tools called "filter wrenches" and using freeze-mist to chill the male side.
With the combination of 0.5mm of diameter mismatch and 0.25mm pitch mismatch the lens and filter are likely to feel like they're threading together while the male and female threads grind the tops off each other, basically wrecking everything.
