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class="style5" Circular Polarizer

>Published : 27th Sept. 2005

>I was just reading about the circular polarizer and its recommended use when using a video tap on a film camera. I wondered if there is any difference between a normal polarizer and a circular polarizer when used simply on a film or video camera?

>Regards

>Michael Sweeney csc


>Michael Sweeney wrote :

class="Paragraph">>I was just reading about the circular polarizer and its recommended >use when using a video tap on a film camera.

>Some prisms have a polarizing tendency, so the optical paths to some video taps and viewfinders can cross polarize creating a very dark image on the video tap and/or through the eyepiece. The circular Pola prevents this.

>Best,

>Anders Uhl
cinematographer
ICG, New York


>A I understand it some video taps have a polarizer integrated in their system so to avoid cross-polarization(image blackout) a circular polarizer is recommended since it’s a "weaker" type of polarizer.

>On a related note this cross polarization effect is sometimes used on set: I actually developed a motor driven filter holder for Keslow Camera that takes two 138mm polarisers and by driving one of them creates a transition(image darkening type) on film.

>Jacek Zakowicz,
Lens technician
Los Angeles


>Jacek Zakowicz wrote :

class="Paragraph">>I actually developed a motor driven filter holder for Keslow Camera that >takes two 138mm polarisers and by driving one of them creates a >transition(image darkening type) on film.

>Ah, everything old is new again.

>Companies use to make "faders" for 8mm cameras that consisted of two polarisers mounted together -- one was fixed to the camera lens, the other rotated with a stick, for in-camera fades on cameras without variable shutters. Works as a powerful variable ND, too.

>Jeff "but there's a little light loss" Kreines


>Jacek Zakowicz wrote :

class="Paragraph">>"a circular polarizer is recommended since it’s a "weaker" type of >polarizer."

>That is the question. Is the circular polarizer a weaker or stronger type of polarizing filter compared to a normal polarizer? Would a circular polarizer be recommend for shooting with a three chip video camera because a prism is use to split the image?

>Michael Sweeney csc


>Jeff Kreines writes :

class="Paragraph">>Companies use to make "faders" for 8mm cameras that consisted of >two polarisers mounted together -- one was fixed to the camera lens, >the other rotated with a stick, for in-camera fades on cameras without >variable shutters.

>Perhaps worth noting that the effect is not the same as, say, an ND wedge disk or variable shutter would be, since you've got the effect of polarization, reflections disappearing, sky maybe intensifying, etc. as well. Not good or bad, just a choice.

>Tim Sassoon
SFD Vfx & creative post
Santa Monica, CA 90405


>Jacek Zakowicz wrote :

class="Paragraph">>"a circular polarizer is recommended since it’s a "weaker" type of >polarizer."

>Michael Sweeney csc wrote :

class="Paragraph">>That is the question. Is the circular polarizer a weaker or stronger type of >polarizing filter compared to a normal polarizer?

>You have a two fold answer to this question! As the quality of the polarization is down the efficiency of the Pola Foil in the laminate of the glass filter. For this answer I have only stuck with Standard Filters which are used in Matte Boxes and not gone into Round Screw In Filters, that's another kettle of fish !

Linear Polas are manufactured in four different strength's by Broadcast manufactures. In Circular, you have two available. manufactured by Schneider & Tiffen, both of which are High Efficiency. Top of the Line !

>Lower Efficiency Pola. Schneider One Stop Pola. Manufactured for the Fraser lens initially as light transmission was paramount, a lower efficient Pola was manufactured in 40.5 & 48mm I believe. The filter was then introduced to the standard filter range size on my request as I thought DoP’s may need to polarize in a Low Light situation, and the high & standard Polas would be to strong. Schneider agreed and the "One Stop Pola" hit the market....

>Standard Efficient Pola’s. These would be your normal Formatt, Harrison & Harrison & Tiffen.

>The difference is that even these have various strength's ! The strongest out of these three is the Formatt.

>High Efficient Polas. Introduced by Schneider, known as the "True Pola", and once they became so popular as DoP’s started to notice the difference, Tiffen introduced the "Ultra Pola" to gain back some of the lost market share !

>Now the question about strength is dependant on what you are comparing a circular against ?

>Circulars are manufactured for the reasons previously stated in other mails (below). They are however in the High Efficient Family of Polas.

class="Paragraph">>Some prisms have a polarizing tendency, so the optical paths to some >video taps and viewfinders can cross polarize creating a very dark >image on the video tap and/or through the eyepiece.

>Now when shooting on a Broadcast 3 Chip camera a Linear Pola will do just fine, which one you use is up to you ! Circulars are recommended for use on 35mm Motion Picture Cameras using a Video Tap, I have also heard of DoP’s complaining on some 16mm cameras with a Video Tap as well. They are also recommended on Mini DV cameras in AUTO EXPOSURE (PD's DVX, XL's)

>One very important thing to remember as a guide is, you pay for what you get ! ( except with the one stop Schneider, that's the same price as a Schneider Circular !)

>I hope this helps you all.

>Best

>Carey Duffy
South London Filter Ltd
http://www.camerafilters.co.uk


>Jacek Zakowicz wrote :

class="Paragraph">>a circular polarizer is recommended since it’s a "weaker" type of >polarizer."

>Carey Duffy gives a very detailed account of the various polarization efficiencies available. One thing to add, though, is that the actual difference between a linear and a circular polarizer has nothing to do with efficiency. A circular polarizer is simply a linear polarizer to which has been added, on one side (which should be mounted facing the camera), a clear layer known as a quarter-wave retarder.

>This effectively de-polarizes the light, allowing the linear polariser layer to do the job of a polarizer, darkening blue skies and reducing certain reflections, and then removing the polarization characteristics that would adversely affect the camera optics downstream, such as a partially polarizing beam splitter on a camera prism.

>The efficiency, or "strength" of a circular polarizer is directly related to that of the linear polarizer used to make it.

>Ira Tiffen
LA, CA


>Carey Duffy wrote :

class="Paragraph">>Now when shooting on a Broadcast 3 Chip camera a Linear Pola will >do just fine, which one you use is up to you !

>It was my understanding that using a linear Pola on a 3 CCD broadcast camera would add a subtle color cast on the image (normally greenish when polarizing in one direction, magenta on other), due to the fact that polarized light (that goes through the prism blocks) looses some color wavelengths.

>At least a 4x4 linear Pola I have does introduce this very subtle color cast. Is this a fault of this particular linear Pola or is the above assumption true? The Schneider true-Polarising circular Pola does not introduce any color cast at all.

I mean, if 3 CCD video cameras have prism blocks (as video tap on 35mm motion picture cameras) shouldn’t you in theory use a circular Pola instead of a linear Pola?

Cheers,

>Pedro Emauz
Videographer
Lisbon, Portugal


>Pedro Emauz wrote :

class="Paragraph">> It was my understanding that using a linear Pola on a 3 CCD broadcast >camera would add a subtle color cast on the image (normally greenish >when polarizing in one direction, magenta on other), due to the fact that >polarized light (that goes through the prism blocks) looses some color >wave lengths.

>I haven't seen this in a video camera, but it is common in some prism blocks. The prism itself will polarize to some degree with a magenta cast and a polarizer in front of the lens can boost the effect or make it visible where it was not before. I would guess that these cameras will bias polarized light toward magenta without a filter as well.

>best,

>Anders Uhl
Cinematographer
ICG, New York