[Kobayashi.K]

I inherited a 'Sigma High-Speed Zoom Multi-Coated 1:3.5-4 f=80-200mm' (Sigma 529301) with M42-mount and I would like to try it on my K5 with an adapter, but unfortunately it has a large stain on one of the inner elements.



The front element looks weird in this photo, but it is actually clean. I think it is a reflection from the environment.

I have found threads in 'Maintenance and Repair Articles' for removing fungus, but it doesn't look like a fungus to me. It has rainbow colors as if a very thin layer is damaged, running right across the lens, from one side to the other. Could it be that the glue between two elements has become loose? The lens has no signs of damage at the outside.

Before trying a repair, I would like to have some advise from the experts.

---

Last edited by Kobayashi.K; 08-18-2013 at 03:31 AM. Reason: Image link changed.

[Just1MoreDave]

That looks like element separation, which is glued elements becoming unglued. When I see this on a Pentax lens, I look up the optical diagram and see where the glued elements are, to see how easy they are to access.

Pentax puts the full coating on each element even if they glue them together. So the separation doesn't always have a visible effect on images. I expect it to hurt the resale value anyway.

[vonBaloney]

The lens has very little (if any) resale value -- only a select few zooms that old do. Not worth putting any money into for repair. (You could just buy another one for less than 10 bucks.) But the good news is that it is quite likely that the problem you see has no effect on the image quality anyway, so give it a whirl. If you don't have an adapter now, I wouldn't buy one just for this -- go get some nicer m42s, millions to choose from...

[Kobayashi.K]

I already feared that it was as a separation. This lens has almost no value so, I could use it as a guinea pig and try to dismantle it to gain repair-experience.
In such a scenario I would separate the lens-parts, clean their surfaces, and re-mount them with the correct glue (and post the procedure illustrated with photo's on PentaxForums of course). Does anybody know what kind of glue to use for lens-elements? I have heard of 'Canada balm'.

[TomB_tx]

Canadian Balsam used to be the common optical cement, but it is rarely used anymore. Now nearly all is done by UV cured glues, like:
Crystal Clear Glue Adhesive for Crystal, Glass, and Metal

[Just1MoreDave]

There are some posts here on dissolving the old cement. It looks like a lens spanner might be useful for disassembly. Look under the rubber grip for hidden screws. Good luck!