Fungus in 480mm Apo-Nikkor

[john wilton]

Picked up a nice recent Apo-Nikkor for about the same as it is costing for SKG to drill my lensboard for the flange (I plan to use it in barrel). Just a little (maybe) problem: there is a small (approx 1mm) object, which I think is inside the rear cell between the two rear (airspaced) elements. (See photo).

Does it look like fungus?

If it is fungus, what to do, if anything? First move was to make sure it's not on the front of the rear cell. I tried to remove the rear cell by holding the front and rear rims of the lens with rubber (bike inner tube) but too tight... it wouldn't unscrew. (Find a way to apply more twist, or wrong approach?).

Have given it a good sunlight soaking. (2 days x 3 hours). But worry that the warming of the whole lens in the sun will have negative effect on diaphragm lube.

I am tempted to just unscrew the rear retaing ring. Have read conflicting advice on this forum on this...centering is determined by the ground edges of the element whose seating is defined by the mount and retaining ring, versus send to technician...the latter not really an option because of cost.

Or do nothing...it's near the edge; and might not get worse in my lifetime.

[rob]

do nothing...it's near the edge, that's what I'd do.

[Carsten Wolff]

I'm with Rob here, John: If it doesn't look as if it's going to be an image affecting issue (esp. once stopped down) I wouldn't bother touching it with one caveat: If it is indeed fungus, it can eat over time into the coating and even etch the glass. The Apo-Nikkor is airspaced as far as I know; so if it really needs a clean you could probably do that yourself. Its dead-easy in e.g. a Goerz 480mm Apo-Artar (everything unscrews nicely and collimation stays put; you don't have to remove the glass elements from their mounts either).

[Dan Fromm]

Carsten, my Apo Nikkors are at home and I'm in the office so I can't check, but IIRC the cells are held in place (prevented from unscrewing?) by radial setscrews. I'm leery of trying to take mine apart, suspect it isn't as straightforward as just unscrewing ...