Accuracy of Correction Collar scale markings?

[Lou Jost]

Thanks everyone for the good advice. I managed to fix my second high-end objective in spite of my fears (it was very expensive). Here is a brief description of the process, which seems to apply to many modern Nikon objectives with correction collars.

1. Before removing the shroud, make a mark that crosses the top of the shroud and the brass part with the threads. That way you won't lose the collar's factory calibration.

2. Work above a large white cloth sheet. The screws involved are "microscopic" and bouncy, and you will drop them many times. I would have lost them if they had bounced on a smooth hard surface. I worked in the center of my bed, because there is a risk of dropping the objective too. I did drop mine once during the process. No harm done as it bounced on the bed.

3. The screws holding the shroud do not have heads and are so tiny they are hard to see. I had to file down a tiny screwdriver so that it would be narrow enough to engage the screws. The screw threads only engage the shroud, not the inner brass objective assembly. Best not to remove all three; they are devilishly hard to get back in their holes during reassembly, so leaving one of them in will save you a lot of time. Just unscrew it enough to loosen the shroud.

4. After removing the shroud, remove the collar with the thickness markings. Only one little screw holds this to the inner collar that moves the cams.

5. Remove the screw that serves as the axis for the main rotating peg that engages the inner collar and actually moves the internal lens elements. Note that the (hollow) peg is asymmetric; the distal end is narrower. That's important. It should rotate freely as the collar is turned; if it doesn't, it may need cleaning and/or lubrication. Mine rotated freely in both my objectives so I didn't lubricate it.

6. Remove the limit screw which stops the inner collar from rotating too much. I do not know why this screw is there, unless to avoid stress on the rotating peg mentioned in #5. I lost this screw for one of my objectives, but it doesn't seem to matter.

7. Now the inner collar comes right off. I cleaned it with alcohol to remove old grease; that old dry oil was the reason the collars were stuck. It is especially important to clean not only its inner surface but also its flat top and bottom; old grease there caused a lot of unwanted friction. I also cleaned the brass objective assembly with a cloth moistened with alcohol. Then I lightly touched the relevant objective assermbly and coillar surfaces with a cloth carrying a little oil. This left an almost undetectable trace of oil on the surfaces.

8. Reassemble. Reassembling the peg that engages the collar can be hard. One of my objectives seems to have a more complex optical correction assembly, with two different moving assemblies, and these needed to be aligned in order for the peg to seat properly on its inner screw. Remember the narrower end of the peg should be facing outward.

9. Reassembling the shroud is the hardest part of the whole process and it took me as much time as all the other steps combined. The screws are diabolically small.

Both my objectives had collars that were so tight as to be unusable. Now both of them work very smoothly and easily.

[PeteM]

Congratulations on the successful surgery, Lou!

A pair of just-right tweezers can help hold and align the screws. A jeweler's screw-holding screwdriver can sometimes stay more or less centered on the screw itself.