Dirty aperture works, Clean does not - Long post!

[Zang]

Hi all,

I’ve run into a rather unique issue that I haven’t encountered before. While I understand the underlying science, the fix is not obvious.

I recently worked on a pre-AI Nikkor 50mm f/2, removing oil from the aperture and cleaning fungus from the internal elements. The work was successful: all mechanical components, including the aperture assembly and blades, are now clean, dry, and shiny. When the lens is mounted on a camera, it works perfectly, the aperture is responsive and smooth. Changing the aperture is also smooth and pleasant (I adjusted the spring tension and lightly re-greased the aperture ring thread).

The problem appears when the lens is off the camera. Changing the aperture from larger values (f/16) to smaller ones (down to f/2) becomes difficult. There is noticeable friction that makes the aperture ring hard to turn.

I believe I understand why this happens. Please refer to my illustrations as I explain how the aperture mechanism works in this lens.
Picture 1 shows the aperture blades (1) and the assembly top plate (2) with the coupling lever (3). The camera operates this lever to stop the aperture down.

Picture 2 illustrates how aperture changes are set. Ring (5) is coupled to the aperture ring on the lens and rotates with it when the aperture is adjusted. A round pin (6) moves along the curved edge of lever (4), causing that lever to rotate around its axis (the axis is the solid red dot in the picture). Lever (4) then presses on the top plate lever (3), causing the aperture to open and close in stop-down mode. When the lens is mounted on a camera, lever (3) is decoupled from lever (4), so the movement of lever (4) only sets the aperture value without actually opening or closing the aperture.

Picture 3 shows the same mechanism as Picture 2, with the aperture hidden for clarity.

The reason the cleaned aperture operates perfectly when the lens is mounted on a camera is that it relies on movement A, shown in Picture 4. This movement works flawlessly.

However, without a camera, lever 3 presses on lever 4 and turning the aperture ring causes lever (4) to move the top plate via movement B in Picture 4. This movement does not perform well when all mechanical elements are completely dry and free of oil. Movement B introduces a different type of friction (due to a different force distribution), causing the top plate to rub more firmly against the casing.

I don’t see many good options to address this.

Option 1 is to lubricate the mechanical components, which carries the risk of oil migrating back onto the aperture blades.

Option 2 is to use the lens as-is. This would be acceptable on film cameras, since this is an AI lens and the aperture only stops down during exposure. However, using it on a mirrorless digital camera would be problematic.

Any suggestions or comments would be greatly appreciated.

Cheers,
Zang

[Zang]

I suspect the aperture mechanism in this lens actually requires some lubrication to function properly. Has anyone else encountered similar issue?

[pluton]

Any chance that 3 tiny ball bearings are missing?

[Zang]

Any chance that 3 tiny ball bearings are missing?

There are no ball bearings in the aperture assembly

I have been able to fix the aperture using an unique method I was not able to send update here as NG was throwing errors. Will write that up again soon.

Thanks!

[Zang]

I finally managed to fix the issue. Like I mentioned, the oil in the aperture assembly helped the mechanism operate properly. However, putting the oil back was not something I wanted to do It took a lot of effort to remove it in the first place.

Based on my observations, the friction of the top plate plays a major role in the behavior of the entire mechanism. So instead of lubricating it, I decided to polish the edge of the disk. The finest sandpaper I had at home was 600 grit, so I started with that. After sanding, I finished the surface using diamond paste (the one I use for knife stropping).

Surprisingly, this approach worked, and the aperture now operates smoothly, just like new.