Re^2: Polarizer: linear o
W.J.MARKERINK
I received your message (written on 08.08.95) on 08.08.95 about
[Re: Polarizer: linear or]:
B><Without getting technical, the linear is cheaper and works just fine if
B>you don't use autofocus. >
B>
B>Talk about mis information! !
B>
B>Circular polarizers have nothing to do with AF. They are REQUIRED for all
B>cameras that utilize a beam splitter in the camera optical path. This
B>could be because the cameras meter is on or behind the mirror. This could
Bob, you mentioned this 'meter *on* the mirror' also for the Rollei
(6008,?): could you explain how this exactly works? I don't assume the
meter flips up and down with the mirror, or am I wrong? I never heard of a
meter on the mirror.... You don't mean the secondary (smaller) mirror, do
you?
B>be because the AF system is behind the lens. These cameras must have a
B>circular polarizer as they partially polarize some of the light entering
B>the camera to use it for metering or AF. Since some of the light is
B>already polarized a linear polarizer would, at some positions, effect the
B>light being used for metering or AF. A circular polarizer has a second
B>element behind the foil that allows some unpolarized light to pass through
As far as I understand it, this explanation is _not true_.
Since a linear and a circular polarizer use exactly the same polarizing
foil (assumed that the filters are of the same manufacturer), there is no
way that any *extra* amount of unpolarized light is allowed to pass
through by a second filter. The second filter is called a 'retarder', or
quarter-wave plate. It reconstructs/simulates the 50/50 balance/mix of
horizontal and vertical polarized lightwaves, but is does not change the
already polarized light; the (lack of) reflections and colorsaturation
remain exactly the same.
This 50/50 mix is also the reason why the combination of two circulars
don't act as a complete block filter when you rotate them against each
other. You need to position the retarder first and last in the lightpath,
otherwise the next polarizer has an 50/50 mix of light to begin with, and
since only a 0/50 or 50/0 mix can be blocked by the next polarizer, the
blocking will not work.
For example, combining two circular Cokins this way will not work, the
first filter has to be reversed! Combining a circular and a linear is
easier: just make sure that the linear goes up front! A retarder inbetween
two polarizers will definately prohibit any 100% blocking effect....
BTW, your above explanation gives fuel to the rumour that circulars don't
give as good polarizing effects as linears do! This is, as you very well
know, not true, both filters give the same polarizing result on film! Only
differences that people can find, is between different manufacturers,
since the foil that is used differs from manufacturer to manufacturer.
B>so the meter and AF systems will work properly at any position of the
B>polarizer.
B>
B><I recommend getting a good quality filter though, as all polarizers are
B>not cut from the same block of glass>
B>
B>No polarizer is made from solid glass. They are all foils, some simply
B>laminated between glass, some edge sealed between glass and some used as
Don't forget to mention the stretched optical quality of the sealed
(Kaesemann) version!....;-)
B>plain foils.
B>
bye,
_/ _/ _/_/_/_/_/ _/_/_/_/_/
_/ _/ _/ _/ _/ _/ _/
_/ _/ illem _/ _/ an _/ _/ _/ arkerink
_/_/_/
The desire to understand
is sometimes far less intelligent than
the inability to understand
Fotobank Nederland (r) +31 80 441009 w.j.ma...@fotobank.nl
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BobS31
the system used for the profi III? is
it a lever, a knob, or what, and is it captive
or free to wobble out?
also, could you explain a little more about the
tripped lever cam arrangement? do you mean
that one doesn't need to tighten the clamp
my manually screwing it in?>
What it really is is a semi-silvered area on the mirror that lets light
through to the cells. It is very obvious if you look into the body at the
mirror with the lens removed.
Bob Salomon, HP Marketing Corp., Giottos, Gepe, Heliopan, Kaiser
Fototechnik, Linhof, Minox, Rimowa, Rodenstock, Rollei, Silvestri