[Leica] Burning a hole in the shutter

[pklein at 2alpha.net (Peter Klein)]

On another list, the subject of cloth vs. metal shutters has come up.  The 
danger of the sun burning a hole in the shutter curtain was cited as an 
advantage of the metal shutter.

In over 30 years of using Leicas, I have never burned a hole in my shutter 
(knock on wood, ptu-ptu-ptu!) .  I wonder how long it actually takes to 
happen.  Nowadays I often set my lens to close focus when I'm facing the 
sun on sunny days, just in case.  But I never did this before, and I never 
had a problem.

I suspect normal body movement usually moves the sun's image around enough 
so that the shutter doesn't "catch."  On bright days, lenses are usually 
stopped down, so less energy reaches the curtain.  I suspect that people 
have to leave the camera lying still in the sun for an appreciable length 
of time for the shutter to "catch." And probably the lens has to be fairly 
wide open and focused on or near infinity.

I'm curious how much of a danger this really is.  Has anyone heard of 
someone actually testing this out--hopefully on an already bad 
curtain!  :-)  Does anyone have any firsthand information about such a 
test?  Or, would anyone who actually burned a hole in their shutter be game 
enough to tell us how it happened?

--Peter

P.S.  I have seen one Leica with a burnt hole  in the shutter.  The 
original shutter on one of my cameras had a little patch over such a 
hole.  I don't know how it happened, because the previous owner said he 
bought it that way.  I happily used the camera until one of the curtain 
strings failed, at which point I had both curtains replaced.  It worked 
fine before, and it works fine now.