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. 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. 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.
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
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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