Uhh, here I am again. I actually didn't mean to send that
version of the email -- I wrote most of that to convince
myself that I still remembered things correctly, then saved
that to a safe place and rewrote the email into about two
paragraphs
What I meant to say:
Just a slight correction to your very nice explanation of
polarization and metering: circularly polarized light is,
in fact, affected by a linear polarizer. The light that
passes through will be linearly polarized just the same
as unpolarized light would be. It happens that, along
any given linear polarization direction, circularly
polarized light looks the same as unpolarized light (or,
more accurately, randomly polarized light). Thus, the
quarter-wave plate serves to unpolarize the light as
far as the semi-silvered mirror is concerned.
As for using a reversed circular polarizer as an ND filter,
the linear polarizer is still a linear polarizer. The light
coming out of it will still be linearly polarized, even with
the quarter-wave plate in front of it. A quarter-wave plate
essentially does not affect unpolarized light.
Urr. I should know not to send email before noon.
But, I did want to add an explanation of why the polarizers
look different in front of a mirror. I didn't mean to burn
this much list traffic doing that, but the additional bandwidth
now will be pretty much lost in my own noise :-/
When you look at your face through a linear polarizer in front
of a mirror, the light starts at your face, unpolarized. Half
the amplitude is lost on a pass through the polarizer and this
continues to the mirror. The reflection does not affect the
polarization direction, so it returns to the polarizer and
nearly 100 0s transmitted back. So 1/2 the amplitude (or 1/4
the original power) reaches your eye. A linear polarizer backwards
is another linear polarizer, so the same thing happens.
With a circular polarizer looked at from the rear (quarter-wave
plate toward your eye), unpolarized light leaves your face, is
bobbled around but not really affected by the quarter-wave plate,
then polarized (losing 1/2 amplitude) by the polarizer. Proceeds
to the mirror, and back, exactly as before, except the light
that comes to your eye is circularly polarized, which you can't
detect [*].
However, when the polarizer is looked at from the front, after
the light passes through the polarizer, it is circularly polarized.
It bounces off the mirror and returns, is jumbled by the quarter-wave
plate [**], and faces the wrath of the linear polarizer once more. Only
in this case, the light is _not_ already polarized along the same
axis that the filter passes. So, approximately 1/2 of the amplitude
is passed once more. With the original 1/2 amplitude loss, we're
now at 1/4 the original wave amplitude, or 1/16 the original power.
Compared to backwards, the light is linearly polarized and about
two stops dimmer than when looking through the polarizer the other way.
Anyway, I hope you don't mind my rambling. I found it interesting
to think about again. Not strictly OM content, but at least related
to optics. :-)
joey
[*] Modulo the polarization sensitivity discussed in another current
thread. :-)
[**] If you happen to hold the circular polarizer exactly n/2 wavelengths
away from mirror, where n is a [very large] integer, then something
funny should happen -- the quarter-wave plate will actually re-
polarize the reflected light along the axis of the linear polarizer
and you won't be able to tell the difference between the circular
polarizer and the linear polarizer. If you can do this, you should
also have no trouble hand-holding a long telephoto at 1 sec, though.
700 nanometers is not very far.
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