Weird Coating / cleaning mark problem??

[Rus]

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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 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:

 

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.

 

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

[Rus]

Oops...forgot to include a key photo:

 

[Fgcm]

Rus, a do it yourself is not advisable.

In your shoes, I would send the lens to a reputable shop for cleaning.

Ciao

Franco

[gbealnz]

I'm guessing separation?

Gary

[luigi bertolotti]

I'm guessing separation?

Gary

 

Me too think that is a problem of this kind... can be a degradation of the glue/cement that couples two glass element or, more probable imho, some kind of stress (not a shock, a longterm stress) that forced the two coupled surfaces to separate : I haven't idea on how is the repair situation on Canon lenses.... on many Leica lenses, even old, the issue can be solved by expert labs (I have it done recently, for a similar problem, on a lens dated 1964).

[pgk]

It looks like separation to me too - I can't see a coating problem looking like this unless there was an obvious ingress of something to cause it. FWIW I have a 35mm Summicron v.4 which had separation of the rear doublet - this showed as 'Newton's rings', like colour zebra's stripes, when looking through the lens and a slight performance loss in one corner. I am fortunate in that I was able to use a local optical expert to disassemble and then re-cement this doublet - this IS a specialist job as the elements need to be realigned to very tight tolerances indeed. Whilst I disassembled much of the lens, final removal of the doublet was undertaken by a Leica specialist, as I couldn't get it out, and who is rebuilding the lens at the moment. As I paid very little for the lens it has proved economic to do this.

 

If you want the lens repaired and rebuilt you may well need to source two services - a competent repairer who can disassemble the lens and reassemble it after another specialist has separated the elements, cleaned them and re-cemented them to relevant tolerances. Be aware that today they will use a UV curing adhesive (the Summicron does not readily transmit UV apparently and I was informed of this as it probably needed a strong dose) which once cured, I believe, will be difficult to disassemble again so it must be correct first time.

 

The problem you have is viability of repair cost - its perfectly possible to find specialists and repair such problems but is unlikely to be cheap (I was fortunate in that I knew the relevant person to re-cement the lens as a 'one-off' so it was not too costly). So I suspect that you may need to look at replacement value versus repair cost I'm afraid.

[bocaburger]

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I have a 35mm Summicron v.4 which had separation of the rear doublet - this showed as 'Newton's rings', like colour zebra's stripes, when looking through the lens

 

I've seen that exact issue before on that particular model lens. I'm hoping it doesn't happen with mine, but I have a v.3 to back it up just in case.

[pgk]

I've seen that exact issue before on that particular model lens. I'm hoping it doesn't happen with mine, but I have a v.3 to back it up just in case.

I'm pretty sure that it was an impact which caused the problem although it was there when I got the lens. I am under the impression that the cement was a UV cured adhesive not Canada Balsam though I can't remember where I found this out.

[TomB_tx]

Welcome to Focal Point - Lens Repair and Lens Cleaning

[pico]

Yep, the defect is replicated in the OOF.

[k-hawinkler]

Thanks for pointing this out!

I learn something new everyday.

[pgk]

On my lens I was told 'optical centring is better than 30 seconds now' - I don't know what is a good or bad figure but have been told that this is pretty good.... anyone?