Disassembling FA28-70 f4

Kim - they aren't holes. They are witness marks from the moulding. There are four little "fingers" sticking up, and it's only a couple of mm thick, and really not much to get a grip on. It's quite soft plastic, and I can't seem to find a way to budge it. The last thing I want to do is apply too much torque and rip the plastic

[edit] Just managed to do it. Two finger nails applying constant pressure and it unscrews
Now I can see the dud element more clearly. In fact, I can even feel it with my fingernail. Either there's a deposit on it, or it's been etched. Can't work out which, but I think the lens cloth has removed some of the stuff. It'd be easier to clean if I can get that out... so that's the next stage It appears to be held in by a piece of plastic that has three "pips" that fit into the tube where I just removed the rear element from. Whether it unscrews or unclips I can't quite see at the minute.

Might be able to get a clearer image today.
Matt

Ok, there are some more images: http://www.pbase.com/mattmatic/smcpfa2870f4

I'm puzzled here.
I might try some Eclipse solution on the element to see what happens with it. It would be great if it could be cleaned without more disassembly, but if you have any ideas what comes out next, that'd be great

Matt

Well, I've applied some Eclipse. I haven't managed to work out how to remove the element, but that doesn't seem to matter too much.
I found that using my thumbnail I could scratch off the deposit that was clouding the glass - still can't work out if it is a deposit, or one of the coating layers. It needed quite a lot of force, but the lens has cleaned up reasonably well.
There are still some trace marks, but overall it seems perfectly usable now. At least it doesn't warrant binning it, which is something!

If anyone has any more information on what causes this, or if there's anything else I can try to clean it up - do let me know!

And a huge thank you to Kim for being so patient with my lens disassembly skills

Matt

Hi Matt,
No problems, besides which I have added to my knowledge as well. Thanks for sharing it with us. We may even see you on the Camera Repair Forum
I am not sure what it might be. From what you describe, it doesn't appear to be fungal although csome forms can be hard to remove. I don't think it can be a balsam fault as it it is on the outside of the group. I wouldn't think it was a coating fault, I doubt this would come off with a fingernail. I wonder whether this is the famous 28-70 fault or a one off.

BTW, in such cases, I sometimes find that a cleaner called Flitz applied with a cotton bud can be useful in such cases.

Kim

Hi Folks,

Ok, now we're cleaning the element, and trying to figure out the cause. I have my usual dopey question. (Now, I DID try to find the answer myself in the directory etc. - no luck.) What causes balsam fault?

My guess has to do with lens elements being tacked in place with an adhesive, and this adhesive may break down chemically and contaminate some glass? (Now the plunge) Is this adhesive balsam pitch?

As a Kanuk, I'm well aware of how sticky this stuff can be, and how it seems to migrate everywhere like some plague. Smells great, though.

Now, somebody help me before I go sniffing my lenses, or decorating them for Christmas!

Cheers,

Hi Mac,
Traditionally, lens makers have used balsam to "glue" lens doublets together. It has very good optical properties and generally works very well. However, as with any glue, it can break down and the resultantant gaps etc disturb the light pattern especially as the refractive index of air/glass is different from balsam/glass.

Kim

For REAL???

Gee, I'm going out to buy lottery tickets! I can't believe how "in the ballpark" I was!

What really blows my mind is that this gooey stuff is optically clear when sandwiched between glass!

So, I guess smelling it is a bad sign - means your lens is falling apart.

I'll look at my balsam trees with a new respect from now on!

Thank's again, Kim!