Unknown ring mark in image of Canon 50 F1.2 LTM

[Infantasy]

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I recently bought a Canon 50 f1.2 LTM. There is no haze or had been cleaned by previous owner but with some minor hairline in the internal lenses elements. The bokeh is good and it is sharp in the centre in wide open. It has special character and worth to buy as it is low in price. However, I find an unknown ring mark in the bokeh of spot light. It is obviously not from the light source as it appeared in different scenes as shown in the attached samples. I don't see any same pattern or mark in the lenses elements. Does anyone know what cause the mark appear or anyone who has this lenses with the same phenomenon?

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[Ecar]

Is it the same across the frame? If so, I would look for separation with the help of a flashlight. If not, perhaps a coating issue in a small area. But I may be wrong.

[PCPix]

Maybe some balsam separation? Or two elements which have not been re-balsamed after being separated during a service??

[Infantasy]

Thank you all. I have used a torch but can't figure out any separation. Perhaps, it is and I hope so, a coating problem only.

[AndreasG]

vor 1 Stunde schrieb Infantasy:

Perhaps, it is and I hope so, a coating problem only.

I am afraid, most probably not, for me it also looks like a lens separation.

[hmzimelka]

Usually clearly defined patterns or marks in the bokeh balls are down to dirt or issues with the rear lens groups. I'm also going to guess this is due to lens seperation. 

Perhaps if you shine the torch from the front of the lens at different angles, and inspect the rear elements also from different angles, it may be visible. You may also have some luck polarising the light or viewing through a polariser which may help making it more obvious during inspection.

 

[Infantasy]

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I had dissembled the lenses and inspected element by element. I found there was no rainbow ring in the cemented lenses groups and no any sign of separation. However, some transparent cement residual were found in the rear group which located behind the diaphragm, so I guess the lenses was re-cemented by previos owner and the said pattern may be due to uneven distribution of the new cement. Do you think is it possible?

[Ecar]

7 hours ago, Infantasy said:

I had dissembled the lenses and inspected element by element. I found there was no rainbow ring in the cemented lenses groups and no any sign of separation. However, some transparent cement residual were found in the rear group which located behind the diaphragm, so I guess the lenses was re-cemented by previos owner and the said pattern may be due to uneven distribution of the new cement. Do you think is it possible?

Absolutely. As a matter of fact, I had separation appear out of the blue on one lens recently. The diagnostic was that it had been poorly re-cemented at some point. I hadn’t noticed any issues before.

In your case, perhaps someone tried to cure the recurring haze that is known to affect this lens and ended up separating the elements. Re-cementing is not a trivial job, as the alignment must be perfect. If it’s not, IQ is likely to be affected to some degree and the elements may end up separating again.