What is this inside my Tamron lens?

[Lloydy]

I bought another Tamron 07A 28-50 / 3.5-4.5 at the Camera Fair the other day, I've had a 07A for ages, it's a nice little lens, but my 'new' one was very cheap, filthy dirty but undamaged, I buy lenses like this to clean up and sell on at the next Camera Fair. And it did clean up very well, but then I noticed something that looked like an iris of an eye inside the lens. So I tested it alongside my old 07A.

And this is the result, the first picture is the Churchyard and the horizon is the treeline over a mile away, the other two are the best I could get wide open and closed right down of closer objects.






This is inside the lens, and it's weird. the yellow with the black border bit appears to be fixed and 'solid', with clarity, wheras the outer part seems to be soft, and not clear. There is also a thick liquid that can be seen in the botton of the outer ring, and this slowly sinks to the bottom if the lens is rotated.
If this is just some weird effect of lubrication leaking onto the optics - I don't know? Or is it some kind of filter that's been inserted into the lens? The lens was dirt cheap, so I'm quite prepared to take it apart to find out, but maybe it is some kind of soft focus effect?

[marcusBMG]

Very interesting Lloydy. Looks like a big bubble. Is it in between cemented elements?

[Lloydy]

[stevemark]

[visualopsins]

Oil is my guess...take apart & let us know!

[jamaeolus]

Very strange and very interesting. How could they center the oil so perfectly? Keep us appraised.

[RokkorDoctor]

I would say oil. Probably between a pair of convex & concave lens surfaces, that almost touch at the center but have a bigger gap at the edge. That yellow clear center is then probably a circle of oil held between the lenses by capillary action where the lenses almost touch. This would also explain why it is slightly off-center, as gravity will drag it down a bit against the capillary forces. At the edge the distance between the elements is likely big enough that gravitational forces can overcome the capillary forces, and there the oil just flows to the bottom. In the last two images you can just about make out that the suspected oil is also yellow.

Possibly a failed lubrication attempt, or an attempt to fill the gap between a separated pair of lenses with oil.

Alternatively, this isn't oil but a (fairly fresh) solution of Canada balsam in xylene. If that is used for re-cementing a crown & flint, but an almost symmetrical flint is accidentally reversed, the same effect can occur as with the oil mentioned above, as then the crown and flint won't match precisely and the gap won't be uniform.

But it could be something else.

Do let us know when you find out

[eggplant]

I am not really sure why people have jumped ahead claiming cement/glue without looking at the optical scheme- where there are no cemented elements:



I am very, very curious as to what's going on here, and why it's so perfectly round in the middle.

A disassembly guide for this lens, in Japanese, is here https://yossyheim.com/tamron-07a-28-50mm-macro-repair

It's a two group zoom, and the front (negative) group comes out as a unit, as seen below, after you take the rubber of the focuser off and loosen a set screw for it. Won't come out otherwise. Good place to start and get a clear view of the rear section/parts around it.

[/img]

[stevemark]

[stevemark]

[eggplant]

[RokkorDoctor]

[Lloydy]

[D1N0]

O darn an adaptall-2 I don't have yet. No I don't need it ...

[mr_tibbs2004]

Great topic and great responses! Sorry, I have nothing to add but to say this is why I love this forum. Lots of knowledge and ideas running around here!

[guardian]

This to me is a very worrying malady which I fear could have origins elsewhere in our galaxy.

Be extremely careful with this one. You never know.

[stevemark]