A Modest Proposal I
Being a recommendation that logic, however flawed a tool in many human
eneavours, may be usefully applied to technical problems in photography.
1. Polarizing filters have been used in photography for many decades to
achieve certain desirable effects.
2. The advent of TTL metering using partially silvered SLR mirrors led
to a problem with polarizers. Light passing through the partially
silvered mirrors is itself polarized. That is, only one plane of
polarization passes through the mirror, with the rest being reflected.
The simple result is that placing a polarizer in front of the lens will
result in metering errors, the amount varying with the angle of the
polarizer.
3. The solution was a so-called circular polarizing filter, which has a
second active layer that 'unpolarizes' the light that has assed through
the primary filter. Works great for metering and there are no complaints
during the film era.
4. Digital sensors largely replace film. Users complain that polarizers
give subtle, but annoying problems with their digital cameras. It turns
out that the 1/4 wave plates used to re-scramble polarized light aren't
linear with wavelength, so the effect varies with color. Didn't bother
film, causes problems with digital.
So what's the solution? As Mike G pointed out, it appears some filters
work better than others, the expensive ones being mostly better.
1. So one solution is filter testing and purchase of new filters, not an
inconsiderable expenditure of time and money.
The next three solutions differ in that they take advantage of the
advent of digital capture and processing, rather than assuming nothing
has changed other than a new weakness. Remember, the problem is with
metering, not polarizer and film/sensor - AND that's an area where
digital is VERY different!
Unlike film, one may determine instantly whether an exposure is accurate
on a DSLR. Hmmmmm. So why not attack the problem where it lives, with
exposure, using those free, old, LINEAR polarizers we still have lying
around?
2. What if I simply take some test shots to determine the angle of the
linear polarizer with the least effect on metering and mark that angle
on the polarizer. Then I start any shot with polarizer at that position,
set manual exposure, and rotate to my heart's content.
That may seem wrong at first, but think it through. Rotation affects all
unpolarized light the same regardless of the setting of the filter. The
whole point of using the filter is to vary the balance between polarized
and polarized light reflected from the subject(s). So the only change to
the basic exposure as the filter is rotated is to the elements intended
to become darker.
3. I can do a more through testing to determine the compensation to the
meter reading needed at marked angles of polarizer rotation. I could
even mark the polarizer in stops, rather than degrees. This should get
exposure close with the first shot. Then, remember chimping and
histograms? Check exposure and adjust as necessary.
4. Oly started it and now many have followed - Live View. Well guess
what? The mirror is up out of the light path during LV, and metering is
done using the image sensor itself, not some separate thingie through a
polarizing mirror. LV metering is unaffected by linear vs. circular
polarizers*.
Last, but not least, for those of us relatively adept in the digital
darkroom:
5. Eschew polarizers for everything but "seeing through" reflective
surfaces like glass and water and perhaps shots with large amounts of
specular reflections. Those are the only effects that aren't easily
duplicated in an image editor.
One of the big uses of polarizers is for darkening skis and bringing out
clouds. This is easily done in post, and actually better in most cases
with WA and super WA lenses, where the polarizer effect varies across
the sky. The increases in saturation and mid-large scale contrast are
also easily duplicated in editing.
Moose
* An assumption on my part, as I don't have a LV DSLR to test it on. I
can't see why it wouldn't be true, though. I'm not sure testing on a
non-DLSR digicam would convince some people. Somebody with a LV DSLR up
for a test?
Ken Norton wrote:
> Rambling Joel wrote:
>
>> ... I was actually kind of disappointed with a couple shots on digital with
>> PL that I think will be OK on film with PL. I just think that's kind of
>> weird, but I guess we have to remember that a sensor is not the same as
>> film. In practical terms, I'm still learning what that amounts to. I see
>> it working in a more nuanced (i.e., trickier) way with ND grad split filters
>> as well as PLs.
> I'm equally dismayed with my results of polarized digital images. It's
> something that I've fought and remain flummoxed. Maybe we can figure this out
> this weekend.
>
--
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