At 02:09 AM 3/14/2003 -0800, Moose you wrote:
Piers is correct that what kind of polarizer you use won't affect the
exposure in Auto. You won't know just what it is going to be before hand,
but it will be an accurate center weighted exposure.
However, you are both assuming that the filter factor of a polarizer is
constant and that is incorrect.
Yes, I am assuming this.
The nominal filter factor is correct where there is no polarized light to
filter. Any polarized light removed by the filter increases the filter
factor by an unknown amount. Think about it. Metering without the filter,
your meter measures all that nasty glare off the water (or whatever). With
the polarizer on and adjusted to minimize the glare, one of the brightest
sources in the scene is gone and the adjustment for correct exposure can
easily be over 2 stops.
I used to expose with PL using a filter factor all the time (because St.
Ansel told me to). I got tired of the extra step of removing the filter
and putting it back on and started just metering through it. I've gotten
good and bad exposures either way, but in general I think my experience
probably confirms what you are saying about a non-constant filter
factor. However you are the first person to say authoritatively that a
filter factor for a PL is only nominal. So I guess the next step is Test
and Believe (maybe that's two steps).
Put a polarizer on a camera without a partially silvered mirror exposure
system or over a manual meter, rotate it while watching the meter with
different scenes and you'll see the effect.
I'm aware of that effect but hadn't realized that meant the filter factor
was not constant. Since polarized light is not a constant, the effect of
removing it wouldn't be constant, but I had not realized that this meant
the filter factor was not constant.
You can meter with a manual meter through the polarizer, I suppose, but
the whole thing gets pretty vague, esp. if you like spot metering. With
negative film, you can probably just adjust 2-3 stops depending on how
much you guess is polarized light and depend on the film's inherent
latitude, particularly on the over side, to carry the day. If you are
shooting slides and they are important, you need to shoot on auto,
practice a lot beforehand or break down and buy a C-Pol. Or take an OM-1,
2, 10 or 20(G) as backup and for metering with the polarizer.
Moose
If I continue to need the 77mm size, I will eventually get a circular
polarizer, as I want to use an OM-4, spot metering, and chromes. I need to
make do in the meantime. I'll run some tests this weekend. I guess the
safest thing may be to shoot in auto/OTF mode with this particular
polarizer and bracket as usual.
Thanks Piers and Moose.
Joel W.
Piers Hemy wrote:
Not sure if you will be using the OM spot meter or a handheld Joel. If
the latter, you are correct. If the former, then you really need a circular
polarizer with an OM-4, otherwise you can't be sure whether the spot
meter has already made the correction or not, due to the semi-silvered
mirror in
the (2SP, 3 and) 4 which conflicts with a linear polarizer. You get the
same effect with the viewfinder display using averaging metering but actual
exposure is unaffected. With spot metering, the exposure is
affected. Or are you proposing to meter without the polarizer, then
adjust? That would
work.
On Behalf Of Joel Wilcox
I'm going to be using a 77mm linear polarizer soon, likely with an OM-4
most of the time. Since I prefer to do multispot averaging, I believe that
I am going to need to meter the scene and then open up by the appropriate
filter factor for a polarizer, which is about 1 and 1/3 stops.
I have decided that if the multispot exposure setting matches the
center-weighted setting, I'll just put the filter on the lens and shoot in
straight auto mode. If the multispot setting is different, I'll probably
go with it and record the setting to memory, put the PL on the lens, and
dial in 1 and 1/3 stops of + exposure compensation.
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