REPAIR Tokina RMC 3.5/35-105: fogged rear group

[stevemark]

Not a complete repair report this time, but just a few hints to clean the fogged rear group of the Tokina RMC 3.5/35-105mm!

Well, this Tokina certainly isn't one of the best 35-105mm lenses, and usually I don't clean/repair/relube such lenses, especially zooms.
That said I have three samples of the Tokina RMC 3.5/35-105mm, all with fog, and some with fungus. Recently I got a sample which looks and feels pretty nice. Its front part (all lenses in front of the aperture) are clean and clear, but the rear optical block had quite some fogging and even whet looked to be som small droplets on some internal glass surfaces.

The internet and AI says the rear group is sealed and thus CANNOT be repaired - I'll prove you otherwise!

First image: two sample of the lens, the left one already partially dismantled:



Bayonet removed, but the rear group still installed:



Rear group removed:



Inspecting the rear group reveals no screws or removable rings at all. The rear group obviously is sealed; it seems the upper part (larger diameter) is glued onto the lower part (smaller diameter). Adding a few droplets of acetone reveals that the glue dissolves in acetone, but the two parts don't unscrew. To separate them, I had to soak (immerse) the entire rear group in acetone for about one hour. After that, using two pipe wrenches, I was able to separate the two parts pretty easily. Don't apply too much force!!


From left to right:
1) Large "upper" (= rear) part including one lens
2) smaller "lower" (toward aperture) part,
3) cemented doublet
4) strongly negative single lens
5) spacer
6) strongly positive lens

Cleaning all those lenses was easy, and so was re-assembling.

So - whatever the net says about this lens: cleaning the sealed rear group is not only possible, but rather easy (provided you have some acetone and two pipe wrenches of suitable size).

One day, I may clean one of the two remaining samples; they are much more fogged and have some fungus in the front part of the lens. Doing so will require to disassemble the entire zooming and focusing mechanism, and I'm not sure yet whether that's worth the pain ...

S