Separation or Fungus? Nikkor 28mm f2.8 Ai-S CIS

[woobl]

Hi there,

First time post but hope you can help.

I've just noticed on my Nikkor 28mm f2.8 Ai-S CIS coated lens that there is this marking on the rear element. I can't quite figure out whether it's separation or fungus. The rainbow effect leans towards separation but wonder if anyone can help decide. I guess if it's separation it will be harder to clean, if at all.

Thanks in advance.

[caspert79]

Never seen a fungus that resembles your picture. But the ai-s has 8 elements in 8 groups, so separation would be strange imo.

[D1N0]

More like separation but no cementing so maybe some pollution. You can take out the rear element and see if it is cleanable

[BrianSVP]

Normally, I'd 100% say separation, but if that optical diagram is correct, then most likely it's helical grease migrated onto the inner elements.

[visualopsins]

Welcome woobl!

Definitely some contamination. Could be fungus that has eaten into the coating giving the rainbow colors -- the lower part looks like fungus too but could be a light reflection. Could be an oil sheen giving the rainbow colors. I any case I'd try cleaning. Oil should wipe off clean. Fungus should wipe off. Fungus damage will not clean. Other contamination YMMV -- may clean; may have damaged coating.

[woobl]

Thanks everyone for your responses. This seems to have occurred fairly recently, as I only noticed it on a recent inspection. I'll look to get the rear element off and have a look.

[jamaeolus]

Let us now your outcome.

[Dejan]

Since it's at the rear, it could be oil that migrated from the aperture mechanism or something like that. Because the lens has floating elements, it could me more complicated to reassemble properly (at least I suppose so), so I'd try to keep it as simple as possible.

[BrianSVP]