Weird Coating/Cleaning mark Phenomenon??

[Rus]

Hi guys, I've noticed an issue with the Canon LTM 50mm F1.2 lens that I have, and I'd be grateful if anyone could shed some light on it.

The Lens is fine when I look through it. But when I put a strong light through it at certain angles..I can see this bluish patch shown in the first photo below. Sometimes you can also faintly make out the edge of it when there is a strong light source, as shown in the 3rd photo below.

The lens works fine in daylight situations...nothing weird whatsoever...the "bokeh" of backgrounds without bright spot light sources is quite smooth...
However...
at night with bright spots.... the bokeh ball shows a weird shape inside them....

Here is the 100% crop:

And more similar shots:

100% Crop:

I have attempted a partial disassembly of the lens..(separating the front groups of elements and the rear groups, it's pretty easy with this lens)..and I know the defect is in the rear groups. What's more it does not seem to be on the rearmost element, nor the surface of the lens element immediately behind the aperture blades. I'm suspecting its on a surface somewhere in between.

Can anyone identify this bluish patch perhaps? Is it some dried up fluid? or is it a coating problem? (Since it leaves this distinct edges in the bokeh ball I'm wondering if it is in fact a coating problem...)
I hope it is not some problem occurring to the glue that holds the two lens elements together in the rear groups..coz that will be a huge pain to clean....

Anyway..all opinions and suggestions welcome!!

Thanks!!

-Ed

 

[Doug]

Hi guys, I've noticed an issue with the Canon LTM 50mm F1.2 lens that I have....

...I hope it is not some problem occurring to the glue that holds the two lens elements together in the rear groups..coz that will be a huge pain to clean....

Ed, sorry but that would be my guess: a partial separation of a cemented pair of elements. You might accept this behavior, as it's not terribly intrusive, or invest some money and send it to Focal Point (for example) for evaluation and repair.

 

[zuiko85]

This is why I no longer shine bright light through my lenses. It shows me things I don't want to see.

 

[maddoc]

It looks like element separation to me (if the lens has a cemented doublet in the rear group, e.g. double Gauss-design)

 

[Rus]

Thanks for the opinions guys! Really appreciate it!

Seems lens separation is the most likely cause then~~ 🙁 just what I have feared most..

I wonder what my options are now.. are there any particular lens repair workshops or individuals in the UK that are recommended？

Thanks again!

 

[farlymac]

Might be less expensive to just get another one. But then lenses of that age are going to be more prone to separation, so it might be hard to find one in better shape. I've read some home remedies that could do more harm than good (like heating the elements in boiling water to soften the glue, then mashing the elements back together). Water intrusion would be a problem there. If you're not getting any flare, then I'd just keep shooting with it, unless the outline in your bokeh bubbles really bothers you.

PF