Archived posting to the Leica Users Group, 1999/07/08

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Subject: [Leica] Light leak in M6?
From: (Pieter Bras)
Date: Thu, 08 Jul 1999 11:21:13 -0400

Doug Richardson wrote:
>Also why shade the hole in the body when you're about to cover it with a
>great hunk of glass which is going to let lots of light stream in for
>hours on end?  Let's suppose that having no lens in place admits 100
>times the light that a fast lens does.  If five seconds exposure with no
>lens in place is enough to fog the film, then why does 100 x 5 seconds
>with a fast lens uncapped not produce the same fogging?  The only
>hypothesis I can come up with is that an open mount admits the light at
>a far wider range of angles than a lens does, and that some if this
>light arriving at odd angles is sufficiently scattered from surface to
>surface within the camera that it eventually works its way to the film.
>But given that the camera can be carried outdoors for all day with the
>lens cap off, why doesn't enough of the light arriving through the lens
>follow the same route (perhaps after a few extra scattering operations
>to get it started) and eventually fog the film?

I doubt that you're aiming the camera directly at the sun when there's
a lens attached, otherwise you'd soon be needing new shutter curtains!

There may be a threshold effect here:  the shutter does its job under
normal circumstances, but it's a CLOTH shutter, after all, and it isn't
perfectly light-tight.  Normally what light gets through is so little
that it's probably at the level where the film is insensitive
(reciprocity failure).  But increase the light intensity enough and
after a while the film begins to notice.

I've been shooting with M's for over a year now, but usually with K-25
in sunlight.  Even so, I had noticed a few slightly fogged slides, which
I'd (incorrectly) attributed to faulty processing.  Guess I'll be a lot
more careful when changing lenses from now on.  I may even start using
lens caps on sunny days.

BTW, does anyone know if there is a lens cap that fits over the 12501
shade for the M 21mm/3.4 Super Angulon?  Any home-brew solutions?

- -- 
Pieter Bras