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Old May 18th, 2010, 06:07 AM   #1
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Polarising Filters

I have been reading up on polarising filters - mainly to do with still photography. I have an ND on order (waiting forever from Culumnet in the UK).

I was wondering if anyone uses a Polarising Filter for video - whether they create any strange patterns on the image (ie: circular versus linear) or whether the ND does the job well - including trapping water reflections?

If you do or have used a Polarising Filter which one please?

With regards

Jeff
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Old May 18th, 2010, 02:52 PM   #2
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I wouldn't leave home without one. The PL filters are essential for a lot of my work - for both stills and video.

I mainly use B+W, Tiffen, Hoya, Sigma EX, Heliopan and Rodenstock filters.
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Old May 18th, 2010, 05:30 PM   #3
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For a single sensor camera, a linear polarizer should be fine.

A circular polarizer (nothing to do with the physical shape of the thing BTW) is usually intended for a 3 chip camera, and especially one with any sort of auto iris, etc...so consumer 3 chips would want a circular for sure.

The issue that the circular polarizer compensates for is the color separation apparatus in a multi-sensor camera. This assembly used to be called a "prism" in old-video guy vernacular because the assembly split out colors to send to each sensor...(all of which are B&W only BTW, they simply take in photons).

This series of mirrors and transparent filters can act like a polarizer in itself. When a linear polarizer goes to work on the light coming into the lens, it's basically taking the light, which can be modulating in any number of "Phases"

(Think of a garden hose laying out on the lawn...now take one end and whip it up and down...the "wave" travels down the length of the hose...now try side to side...it would likely travel further if you put it on the driveway...you get the idea. These two "waves" were modulating 90 degrees out of phase from each other. Light comes into your lens in almost limitless angles around the axis of its travel.)

The polarizer acts as a bit of a 'coin slot'...like trying to create the hose wave through a picket fence...only one choice of angles. By rotating the polarizer, you are knocking out all the "phases" you don't want...reflections on the surface of the pond, or obscuring the view through a store window (you may want to research "Brewster's angle" for some insight into angles)

...or maybe just to remove some amount of distracting glare from a chrome-encrusted hot rod or motorcycle.

(I've even shot stills where I'll take two of a car...one with the polarizer rotated to remove glare from the side windows...then one to remove glare from the windshield...then it's time for photoshop.)

Every time light is reflected off something, like a store window, it gets polarized and now you can take the polarizer on your camera and knock it out cleanly...but you need to be at an angle, it won't work straight on, or at too extreme an angle.

The same principle would apply if linearly polarized light comes into your camera and hits a reflectived surface to be filtered/rerouted to its proper sensor...the light coming in is already polarized and further polarization will diminish it further, altering the color of your image rather substantially. I remember several owners of Sony EX1s ended up with linear polarizers and got on a forum several years ago saying that EX1s responded strangely to polarizers. Once they switched to a circular polarizer, they were fine.

A circular polarizer works exactly like a linear does...but on the back side, it puts a "twist' on the light being allowed to enter the lens...think of the garden hose now corkscrewing a little bit with each 'wave' you put into it. This allows the light to travel properly through the color separation mechanism, maintaining full strength in all three segments of the spectrum as the wave oscillation is not hitting each sensor with a different amount of 'fixed' polarization since the rotation keeps the light moving so different reflective surfaces at different angles aren't re-mixing your primary colors for you.

Hopefully something in there makes sense...I'm on a cram edit and I'm a little short of shut-eye.
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Old May 18th, 2010, 05:48 PM   #4
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In simple terms, Jeff, buy a circular polariser filter and you will not go wrong.
Also buy an ND filter, and some Grad ND filters if you intend to do landscape work.
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Old May 18th, 2010, 05:54 PM   #5
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Quote:
Originally Posted by Tony Davies-Patrick View Post
In simple terms, Jeff, buy a circular polariser filter and you will not go wrong.
Or...what Tony said.

(Though if you're running 4x4s in a matte box, an unecessary circular polarizer can cost significantly more than a linear... On a DSLR with a single sensor, there really isn't a need for a circular pola unless all your cameras, 3 chip video and DSLRs have the same front lens thread or matte box and you only want to own one polarizer.)
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Old May 19th, 2010, 02:52 AM   #6
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Good advice from Tony and Tim. I'll add, just be careful when using a polar with an extreme wide lens as the polarizing effect will be variable across the field of view.
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Old May 19th, 2010, 04:02 AM   #7
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Thank you for the helpful advice

Thanks all for the helpful advice. Tim, I think we're reading the same book and the explanation is very helpful. The part that threw me in the book was where is said linear polarisers have to be rotated to get the right effect in the part of the shot you want. I have seen this when wearing polarised sunglasses and I was wondering if this somehow was noticeable on a video shot - especially if panning or the like.

I will try a circular polariser as you say and will be mindful of the effect on a wide angle.

Again, thanks all for the comments, time for some shopping and experimentation.

With regards

Jeff
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