polorizing filter

Charles Papert · October 9th, 2002, 10:00 PM

Jeff, I am certainly surprised to hear this...I have NEVER seen a circular pola in use on a film set & if in fact it did provide a more significant amount of polarization, that would be a powerful discovery. I would personally be more interested in the application of flare reduction (backlit sun on street etc) and deepening the effect on skies that are not exactly 90 degrees from the sun, and if a circular pola delivers, cool.

As it is, I'm sort of confused why most of the users here (on the DVinfo.net) who post about polarizers have brought up the circular version. The only thing I can imagine is that camera stores who are used to recommending them to their still camera users take it upon themselves to urge their video camera buyers to go with that version as well, when a linear pola should suffice. Anyone care to comment on where they have "heard" that they should be using circular polas?

Jeff Donald · October 10th, 2002, 06:05 AM

Some motion picture cameras would require circular polarizers. Cameras that use beam splitters for video assist would create polarized light. A linear polarizer would darken part of the video image. So, in theory any camera that uses beam splitters or semi-transparent mirrors would require circular polarizers.

I know there is an efficiency rating for polarizers and I believe it is based on the amount of polarized light (reflection?) blocked. Circular polarizers, depending on quality, have higher efficiency ratings. Polarizers also work on certain frequencies (color) of light. I seem to recall that polarizers for photography work mostly in the yellows. If I'm remembering this right, it might affect the metering of scenes with certain predominant colors.

Bryan Beasleigh · October 11th, 2002, 07:26 PM

Acording to the Tiffen site, a circular polarizer is a linear polarizer with a 1/4 wave retarder tacked onto it's back side.

Jeff Donald · October 11th, 2002, 09:04 PM

Bryan, you are correct. Circular polarizers contain a linear polarizer and the 1/4 wave retarder (Germans call them foils). The retarder is oriented 45 degrees in respect to the linear polarizers axis. The retarder causes the linear polarized light to rotate. Depending on the construction of the retarder, light will rotate either left or right in a circular motion.

Linear polarizers (part of the circular design) have slots or palings that allow light that is parallel to the slot to pass through. Other light is rejected. The more slots (narrower construction) the higher the efficiency and the less reflection. Of course the less light that passes through the greater the filter factor.

Circular polarizers may aid in two ways. Several manufactures claim the glass used in CP are planned flatter for use with telephotos. This flatter glass does not change the focal length of the lens (which would change DOF charts) or cause increased optical defects (more noticeable with a telephoto). The retarder helps reduce internal reflection (flare) and prevents a reduction in contrast.

Most manufactures don't rate the efficiency of their polarizers (too embarrassed?) but those few that do, show a slightly higher efficiency for the CP. Is it enough to see a difference in performance? I think so based on previous tests (film based). I'm not sure if the differences would show in video. I may try testing this in the next few months.

Jeff

Andre De Clercq · October 16th, 2002, 01:54 PM

The best way to comparatively test the ER (extinction ratio) of polars is to take two of them, put them over each other on a (strong) white lightsource (if circ polars, the retarders have to be at the outer sides) and veryfy (measure) how dark you can get the crossed polars. Good polars will get uniformly very dark and remain colorless. It can be verified that the very high attenuations are very, very rotation angle dependant (fractions of degrees) Checking/comparing the performances on a camera would be very difficult for getting reliable results. The ER and suppression angle is a very big issue in LCD projector technology for getting high contrast ratio's.
Beamsplitters or other parts in videocams don't polarize in their active zones so they don't need circ polars.

Dan Holly · October 24th, 2002, 01:44 PM

Back to the original question……..

For those few of us who stand in rivers, or have the tripod’s setup just out of the water while shooting footage all day……

You never will see the circular polarized filter being taken off the end of my telephoto lens during daylight hours where the sun is above the horizon. In the Alaskan summers, that can be quite a long time <;~)

Water shots of fish are absolutely amazing if the rivers are “gin clear” to start with. Some of the best non-action type of footage we shot this past summer, is of various types of Salmon in their natural habitat.

You’ll also generally notice that any serious fisherman has them on their head mounted lens………in the form of polarized sunglasses. If your not into fishing, watch a fishing show on Saturday mornings and observe the plethora of polarization.

October 9th, 2002, 10:00 PM	#16
Charles Papert Wrangler Join Date: Jan 2002 Location: Los Angeles, CA Posts: 6,810	Jeff, I am certainly surprised to hear this...I have NEVER seen a circular pola in use on a film set & if in fact it did provide a more significant amount of polarization, that would be a powerful discovery. I would personally be more interested in the application of flare reduction (backlit sun on street etc) and deepening the effect on skies that are not exactly 90 degrees from the sun, and if a circular pola delivers, cool. As it is, I'm sort of confused why most of the users here (on the DVinfo.net) who post about polarizers have brought up the circular version. The only thing I can imagine is that camera stores who are used to recommending them to their still camera users take it upon themselves to urge their video camera buyers to go with that version as well, when a linear pola should suffice. Anyone care to comment on where they have "heard" that they should be using circular polas? __________________ Charles Papert www.charlespapert.com

October 24th, 2002, 01:44 PM	#21
Dan Holly Major Player Join Date: Jul 2002 Location: Anchorage, Alaska Posts: 336	continued...... Back to the original question…….. For those few of us who stand in rivers, or have the tripod’s setup just out of the water while shooting footage all day…… You never will see the circular polarized filter being taken off the end of my telephoto lens during daylight hours where the sun is above the horizon. In the Alaskan summers, that can be quite a long time <;~) Water shots of fish are absolutely amazing if the rivers are “gin clear” to start with. Some of the best non-action type of footage we shot this past summer, is of various types of Salmon in their natural habitat. You’ll also generally notice that any serious fisherman has them on their head mounted lens………in the form of polarized sunglasses. If your not into fishing, watch a fishing show on Saturday mornings and observe the plethora of polarization. __________________ Dan Holly Anchorage, Alaska

October 10th, 2002, 06:05 AM	#17
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	Some motion picture cameras would require circular polarizers. Cameras that use beam splitters for video assist would create polarized light. A linear polarizer would darken part of the video image. So, in theory any camera that uses beam splitters or semi-transparent mirrors would require circular polarizers. I know there is an efficiency rating for polarizers and I believe it is based on the amount of polarized light (reflection?) blocked. Circular polarizers, depending on quality, have higher efficiency ratings. Polarizers also work on certain frequencies (color) of light. I seem to recall that polarizers for photography work mostly in the yellows. If I'm remembering this right, it might affect the metering of scenes with certain predominant colors.

October 11th, 2002, 07:26 PM	#18
Bryan Beasleigh Trustee Join Date: May 2002 Location: Barrie, Ontario, Canada Posts: 1,922	Acording to the Tiffen site, a circular polarizer is a linear polarizer with a 1/4 wave retarder tacked onto it's back side.

October 11th, 2002, 09:04 PM	#19
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	Bryan, you are correct. Circular polarizers contain a linear polarizer and the 1/4 wave retarder (Germans call them foils). The retarder is oriented 45 degrees in respect to the linear polarizers axis. The retarder causes the linear polarized light to rotate. Depending on the construction of the retarder, light will rotate either left or right in a circular motion. Linear polarizers (part of the circular design) have slots or palings that allow light that is parallel to the slot to pass through. Other light is rejected. The more slots (narrower construction) the higher the efficiency and the less reflection. Of course the less light that passes through the greater the filter factor. Circular polarizers may aid in two ways. Several manufactures claim the glass used in CP are planned flatter for use with telephotos. This flatter glass does not change the focal length of the lens (which would change DOF charts) or cause increased optical defects (more noticeable with a telephoto). The retarder helps reduce internal reflection (flare) and prevents a reduction in contrast. Most manufactures don't rate the efficiency of their polarizers (too embarrassed?) but those few that do, show a slightly higher efficiency for the CP. Is it enough to see a difference in performance? I think so based on previous tests (film based). I'm not sure if the differences would show in video. I may try testing this in the next few months. Jeff

October 16th, 2002, 01:54 PM	#20
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	The best way to comparatively test the ER (extinction ratio) of polars is to take two of them, put them over each other on a (strong) white lightsource (if circ polars, the retarders have to be at the outer sides) and veryfy (measure) how dark you can get the crossed polars. Good polars will get uniformly very dark and remain colorless. It can be verified that the very high attenuations are very, very rotation angle dependant (fractions of degrees) Checking/comparing the performances on a camera would be very difficult for getting reliable results. The ER and suppression angle is a very big issue in LCD projector technology for getting high contrast ratio's. Beamsplitters or other parts in videocams don't polarize in their active zones so they don't need circ polars.