Micro-Nikkor 55mm f/2.8 aperture problem

I got a used lens and it has an aperture oddity.

The blades are free of oil, but is acts oddly on my FA.

When at the f/2.8 detent, the blades are visible. I can open the lens up a little more than the f/2.8 detent, and then they are completely recessed. When the ring is set past f/2.8 or completely closed for Program mode, the blades are visible.

They close appropriately when the mirror goes up, but then afterwards they completely retract. When I wind to the next frame, they come back out again.

It is consistent through every focal length, so the little arm that moves the aperture lever inside the lens seems straight. When I adjust exposure and let the camera pick an aperture, it seems to be able to close the aperture to various f-stops, so there the camera seems to be behaving.

When I cock the shutter, I can see the lever inside the camera lowers just a little, but this isn't enough on other lenses to show the aperture blades.

This doesn't happen with other lenses, it doesn't happen with the lens on my FM (different system), and when I looked at another 55mm Micro at a local shop, that lens doesn't do this on my camera.

I'm worried that this will affect exposure, but maybe only by a fraction of a stop. Any thoughts? Am I being overly neurotic?

Now that I'm dealing with the focus issue mentioned in another thread, I'm feeling hyper-attentive to the function of used gear.

Thanks!

-Jason.

Focal length/entrance pupil diameter as viewed through the lens front element=f stop.
Fl/Ep=f stop
Fl/Ep*Ep=f stop*Ep
Fl=f stop* Ep
Fl/f stop=Ep
55mm/2.8=19.642857142857142857142857142857 mm

Look through the front of the lens and measure the diameter of the lens aperture opening with the aperture blades out of view then with them against the f2.8 stop. If the opening with blades out of view is 20mm or greater and the opening is close to the 19.6mm mathematical calculation when against the f2.8 stop then all is fine, a 1/3 stop smaller aperture from f2.8 is f3.2, approximately 17.1875 mm in diameter. A 1/3 stop change in exposure is just detectable in wet printing, tonal shift in a scanned negative should be insignificant. If the opening decreases more than 1/3 stop the aperture or stop mechanism is worn or otherwise defective.

My 55/2.8 Micro-Nikkor, s/n 747xxx. mounted on my FM3a does not do this. At 2.8, no blades are showing; it's a perfect circle. I can move the aperture ring slightly beyond 2.8, perhaps by 1mm, but that makes no difference.

I suspect in practical use it won't matter much, but I'm curious about the cause. I've seen this effect with other lenses and cameras.

Wear.

Fixed it.

So I took it apart and figured out how to make it WAY worse.



This photo is linked to the extremely helpful site at http://conradhoffman.com/MicroNikkor55.html

These three screws are in oblong holes and the slotted screw at about 11:00 is eccentric for adjusting the relationship between the inner guts and the outer barrel.

In mine, you can see where the screws were originally placed by the wear around the hole, and they were originally all the way to one side. Somebody had it apart and they came out of adjustment a little and were closer to the middle. When they are all the way to the wrong end of the hole, I can't even open the aperture all the way with the ring.

When I adjust it back so that they are in their original position, the effect I described is still apparent, but there is only a hair of aperture visible.

Yay!

-J.

Is the stop down lever worn?
I've seen at least one nikkor where thee was a groove in it from use

That's what I was expecting, but it was fine. The dark coating (bluing? anodizing?) was not worn through on anything other than some superficial wear marks.

It looks like the lens was all the way at one end of the spec since the adjustment was maxed out, and it drifted with a cleaning.

-J.