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From: "Richard Prentiss" 
To: "photoecho" 
Subject: ersatz ND filters
Date: Mon, 31 Mar 2003 21:29:56 -0700
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I have lost the origin of the thread, but here's
my take on polarizers and other ways to
get a dark filter(not to be confused with a
dark slide). If you happen to have a pair of
circular polarizers, they can be used as a
variable ND filter if one of them is reversed,
so they are "face-to-face". Linear polarizers
suffer no such complexities, two of the same
size will stack perfectly and the mounts are
made to rotate. Another possibility is using
welders plates. These are available in various
"shades" at quite reasonable prices. NOTE
that they are not especially flat, and most
are a very dark green color. Some may also
show localized surface defects. My attempts
to cut them have been quite interesting.
All such plates are tempered to some degree.
Some have fractured in slightly unpredicatable
fashion, and some have just shattered into
unusable shards sharper than needles.
These are best used uncut. Buy several
while you're there.=20
All that said, they offer over 10 STOPS
of attenuation for a few bucks. Technical
specifications have proved difficult to find,
good thing we have light meters. At worst,
you could have a square piece of black glass
epoxied to an old filter ring, and the photo
effect you want. When I have a print small
enough to scan, will upload an example.
Rich

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I have lost the origin of the thread, =
but=20
here's
my take on polarizers and other ways=20
to
get a dark filter(not to be confused =
with=20
a
dark slide). If you happen to have a =
pair=20
of
circular polarizers, they can be used =
as=20
a
variable ND filter if one of them is=20
reversed,
so they are
"face-to-face". Linear=20
polarizers
suffer no such complexities,
two of the =

same
size will stack perfectly and the =
mounts=20
are
made to rotate. Another
possibility is=20
using
welders plates. These are
available in=20
various
"shades" at quite
reasonable prices.=20
NOTE
that they are not especially
flat, and=20
most
are a very dark green color. Some may=20
also
show localized surface defects. My=20
attempts
to cut them have been quite=20
interesting.
All such plates are tempered to some=20
degree.
Some have fractured in slightly=20
unpredicatable
fashion, and some have just shattered=20
into
unusable shards sharper than =
needles.
These are best used
uncut. Buy=20
several
while you're
there. 
All that said, they offer over 10=20
STOPS
of attenuation for a few bucks.=20
Technical
specifications have proved
difficult to =

find,
good thing we have light meters. At=20
worst,
you could have a square piece
of black=20
glass
epoxied to an old filter ring,
and the=20
photo
effect you want. When I have a print=20
small
enough to scan, will upload an=20
example.
Rich
 

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