On Mon, 21 May 2001, Dave Schneider wrote:
> I also need to find a fabric that is really light proof in a single
> layer that is more flexible.
My experience is that this is harder to do than one might think.
> On the subject of light tight, was my flash inside the fabric to
> severe a test?
That depends on how much of the light from the flash you could see. My
guess is that the attenuation was quite high? Perhaps a few micro-holes?
> Would the fabric from FreeStyle
> have worked just fine in outdoor sunshine even though the flash was visible
> through it?
Hard to say without seeing what your test results looked like? My guess is
that for normal use, it would probably be OK, but ...
Typically, what you find is a mostly light proof fabric, but with a few
pinhole defects. That usually isn't too big of a problem since there are
several things on your side:
- The bellows is usually constructed by sandwiching materials together,
and it is unlikely that minor defects in one layer will line up with
one in the next. This means it is unlikely you'll get a straight
through hole with two or more layers of material.
- The light path through the micro-hole is usually not on the axis of
the film plane. As such any light rays entering the camera through
this micro-hole can only reach the film plane from internal
reflections. Since we make the bellows interior as absorbing as
possible to minimize flare from light scattering from the lens these
reflections are highly attenuated. You will probably get more
scattered light in the camera due to lens defects and flare than from
a few rays from micro-defects in the bellows fabric layers.
- Film is actually fairly slow and, depending on the camera design,
is generally only exposed to the light in the camera for the duration
of the time that the shutter is open. Typically the amount of light
getting to the film from a micro-hole is quite small and would
require very long exposure times to record. Since the image forming
light should be more intense, you shouldn't notice anything.
Now, don't get me wrong. These arguments aren't intended to say that
people should use bellows fabric with pinholes - quite the contrary. The
above discussion assumes that the fabric used should be as opaque as
possible. I also deliberately use the term "micro-hole" to differentiate
between a much larger pinhole that could admit enough light to be a
problem.
All I'm saying is that we have several competing requests here. On one
hand we want a very light weight, thin, and flexible material. While at
the same time, we want this material to be perfectly opaque. These are
kind of mutually exclusive conditions, so you have to make some reasonable
trade offs. You will probably see the optical thin spots in an already
thin opaque material if you put a very bright flash behind it or look hold
it up to the sun.
- Wayde
([EMAIL PROTECTED])
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