Mysterious M3 Light Leak

darkosaric said:

Did you taped with black tape the place where the vulcanite is missing? Do you have some other lens to test it?

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Great questions. I used another similar lens (M mount summicron now using a cleaner screw mount summicron) and the negatives have the same leak in certain frames.

As for the black tape, I suppose that is next. My lab is willing to run a test roll this afternoon, so if I can shoot a roll with some variations like that, I may narrow it down. Anything else I should try on this roll?

Take one shot into the bright light. Take exactly same shot, but all of the windows on top plate covered.

Ko.Fe. said:

Take one shot into the bright light. Take exactly same shot, but all of the windows on top plate covered.

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Great idea. I will do that, as well as covering the vulcanite chip with tape for a few frames.

Ko.Fe. said:

Take one shot into the bright light. Take exactly same shot, but all of the windows on top plate covered.

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THIS WAS IT!

I happened to mention to my coworker my light leak issue and he said "You know, I used to work in a lab and we had to investigate light leaks for all sorts of equipment"

Why did I not ask this guy earlier? I shared with him the APUG feedback, and we were going to do a test roll together. He had a eureka moment that I should have had a while ago...

This image is with a flashlight held directly up to the rangefinder window! and the second image is the reverse, holding the light diagonally up to the open back, with the curtain down.

My sanity has been restored... Youxin will receive this puppy on Tuesday, and soon, my Leica will be flawless.

I greatly appreciate all of your help, thank you!

Hi
This is just awesome. A friend of mine has a very similar problem with his M6. I tried to figure it out to no avail and gave up.
He sent it for CLA/repair and it will be very interesting to see what the shop will find (extremely reputable place in Paris).
Tanks for the excellent read.

interesting -- so the light must be piping down around the hole in the top where the rangefinder post that holds the coupling ring comes through. Never would have guessed it.

Someone removed a bit of flashing in there at some point and forgot to put it back.

Leak from the top windows? Is the leak only confined to the frame?

Well, this is an exciting thread for me. I've had a similar light leak in the lower left corner of my M5 that I've been trying to diagnose for over two years!

Just like for OP, it shows up on bright days (and interestingly, usually only with a wider angle lens). I had tried everything mentioned in this thread - searching for pinholes, internal reflections, even light leaks through the rangefinder, around the back door, or through viewfinder or strap lungs into the film take-up chamber. No success so far.

However, looking at LMNOP's solution, I realize I don't think I've tried looking for light leaks through the rangefinder window with the shutter held open. I'm looking forward to checking this when I get home - It would be amazing if I could finally solve an issue that's been driving me crazy for two years! Thanks for posting such detailed pictures and coming back to share your solution.