what exactly does a polarizer do?

brian hendry · November 17, 2004

just wondering what a polarizer does and an N.9 filter does exactly?

...and if anyone has any websites that show what a bunch of different filters do to the stocks, that would be great....thanks a bunch.

DavidSloan · November 17, 2004

Just google it, there are a lot of sites.

David Mullen ASC · November 17, 2004

Ever worn a pair of polarizing sunglasses and cocked your head from side-to-side and noticed how glare is selectively removed off of shiny cars, roadways, or in the sky, etc.? Or how a Pola can remove reflections off of shiny surfaces?

An ND.90 is a gray filter (ND = Neutral Density -- i.e. no color) that cuts the light level coming through the filter by three f-stops (ND.30 loses one-stop; ND.60 loses two-stops.) It has no affect on colors and doesn't cut glare or reflections like a Pola does, just overall light. A Pola cuts almost two stops, depending on the filter, maybe a stop and a half.

The Tiffen website explains some of this, as well as many others.

Chris Keth · November 17, 2004

The polarizer cuts down on glare on nonmetallic surfaces. Shooting through a windshield of a car would be a situation where a polarizer would be common.

The ND.9 is a neutral density filter. It cuts down on all wavelengths of light evenly so color will not change. The .9 part means it cuts down light by 9 third stops or 3 full stops. An ND.1 would cut it by a third of a stop, ND.2 cuts by 2/3 of a stop, etc. Shooting outside on a very bright day, but wanting shallow depth of field might require this type of filter so you could shoot at a fairly open aperture.

Liek the post above says, there are a lot of books and website that can give you this sort of information. It's pretty easy to find, too.

Greg Gross · November 18, 2004

Hello xsorryx,

Please do not feel bad about asking any question on this forum. To

learn photography you need to ask questions and make mistakes.

If no one wants to answer you, post to me pd170user or e-mail me

at pd170user@yahoo.com and I will find an answer for you,anytime!

I am committed to sharing the knowledge that I have of photography.

Visualize a lake with a rocky coast line,the way the water approaches

the shore,notice you can see rocks under the water up close to the shore.

One thing I like to do with a 4X5 camera is to setup the camera with

a good angle,proper height above water and shore line. Visualize look-

ing on the ground glass at the shoreline where the water meets it. You

will see some beach type surface,rocky,sandy,muddy and then the water

starts out toward the expanse of the lake etc.. Up close to the shore you

will see rocks under the water. For this shot I put a polarizer filter on the

lens which will eliminate glare and reflections off of the water and allow

me to see the rocks under the water. In a sense you are looking through

the water. Just like David Mullen said,sunglasses(polarized) will let you

look in to the water. I like to do a wide angle shot like this and maintain

maximum depth of field. At a certain point in the frame you will no longer

see the rocks,just water,sky,clouds etc. . Greg

Jon Jost · November 18, 2004

New here. I am a very experienced director/cinematographer in 16, 35, Panavision, and in last years, DV.

Polarizing filters (linear ones) cut out a given angle of reflected light - for example the ones in sunglasses cut out horizontal light, and take the glare off of highways. Polarizing filters usually come in a holder that lets you turn it, to cut out the angle of reflected light you want to cut out. If you mess with one you will see that it can change the image considerably. I more or less ALWAYS have on on my camera, as used intelligently they give you more or less as much control over the image as lighting does (and I almost never use lights). When the light is overhead, midday, polarizing filters have little effect since the reflected light is minimal (unless, say, you were flying over the landscape). Polarizing filters are also NDs for some reason I never understood, and eat up about 1.3 to 1.5 in light. When in dark situations I sometimes take it off.

ND filters simply cut down on light without altering the color balance; they are used so as to open up the diaphragm, usually to get less depth of field, or sometimes to aim for the optimum f 5.6- f 8 lens setting. The differing numbers on ND filters have to do with how much light they absorb.

Jon Jost · November 18, 2004

PS:

If you are interested in wide-ranging discussions of art, politics, cinema, technical matters, etc. you might be interested in checking the site www.cinemaelectronica.com where we have such talks.

if interested in my work www.jonjost.com

Chris Keth · November 18, 2004

Oh, one other thing. ND filters kind of have two ways of being numbered for sale. There is the ND.3, ND.6, etc where every .1 reduces light by one third of a stop. I've also seen them sold as ND 2, ND 4, ND 8, etc. Where the number is the denominator of a fraction of the light it lets through. So ND 2 lets 1/2 the light through (and is equivalent to ND.3). ND 4 lets 1/4 of the light through and is equal to a 2 stop reduction (ND.6).

I'm not sure why there are two different ways but I've seen both used by manufacturers.

Leon Rodriguez · November 19, 2004

http://www.cs.mtu.edu/~shene/DigiCam/User-.../polarizer.html

http://www.geocities.com/cokinfiltersystem/polarizer.htm

http://www.camerafilters.com/pages/cpl.aspx

http://www.codixx.de/polarizer.html

This will get you started. David's right though, go to Tiffen, if you have any further questions e-mail Ira Tiffen right from the website. He'll probably answer you personally. No such thing as an unworthy question in the search for knowledge, bro. As Greg said, you're in the right place to ask any question concerning cinematography.

David Mullen ASC · November 19, 2004

Actually, Ira Tiffen is no longer at Tiffen, but he's always more than willing to talk about filters.

November 19, 2004

Please do not feel bad about asking any question on this forum. To
learn photography you need to ask questions and make mistakes.

Yes.. sort of. But people have to try putting in a bit of effort into making internet searches first. As opposed to getting other people to do it for them all the time. I'm not directing that at anyone, I?m just saying generally.

Rachel Oliver · November 19, 2004

PS:

If you are interested in wide-ranging discussions of art, politics, cinema, technical matters, etc. you might be interested in checking the site www.cinemaelectronica.com where we have such talks.

if interested in my work www.jonjost.com

<{POST_SNAPBACK}>

Hi;

Wow! Nice to see you dropped in Mr Jost, I'm a real fan of some of your films, during my final year in my film degree I routinely viewed "all the vermeers in new york" and have recently seen "Sure Fire" It's wonderful to view films shot and operated by the director themselves, I find your camera work to be highly original and inspirational too (very human?) Anyway welcome to the forum, your knowledge and artistry will be appreciated......

Olly

brian hendry · November 22, 2004

Hi;

Wow! Nice to see you dropped in Mr Jost, I'm a real fan of some of your films, during my final year in my film degree I routinely viewed "all the vermeers in new york" and have recently seen "Sure Fire" It's wonderful to view films shot and operated by the director themselves, I find your camera work to be highly original and inspirational too (very human?) Anyway welcome to the forum, your knowledge and artistry will be appreciated......

Olly

<{POST_SNAPBACK}>

thank you very much everyone. its really nice to hear i can come here and ask questions. ive learned so much so far from just reading other threads. thank you so much again!

Dominic Case · November 22, 2004

Polarizing filters are also NDs for some reason I never understood,

Think of light waves like waves in a long rope that you vibrate up and down, or from side to side.

Most light is unpolarised - that is, the waves are aligned in all directions. Polarising filters only let in the waves in one direction: therefore, they reject some of the light, so the effect is exactly the same as an ND. IN theory a polariser would be 0.30ND to unpolarised light, but in practice it's a little more.

The polarising filter acts like a railing fence would act if that waving rope was strung up through it. The rope can still wave vertically, but not from side to side.

Some surfaces reflect light in the same way - they only reflect light polarised in one direction. By rotating the filter until its angle is across the light's angle, it stops all of that reflected light, while still passing 50% of the normal, unreflected light.

Laurent Andrieux · November 23, 2004

it's not exactly one stop, because it's not exactly 50 % percent. There is a direction where it cuts most of the light, end then it cuts less when light comes from a slightly different direction.. it's not a binary effect, there is more like a gaussian curve.

In fact, the Brewster angle is arctang(n) where n is the refraction index of the reflecting surface (glass : n=1.5, A=56°, water : n=1.33, A = 53°).

As the surface of a non conducting (dielectric) reflecting surface - or the clouds - does polarize the light first, the filter will then estinguish the reflections from this surface if correctly set (round setting) and if the incoming light makes this angle after reflection with the direction to the camera.

EDIT> It doesnt work with metal surfaces because they conduce electricity. (the energy of the light is in it's electrical field component). They don't polarize the light, so it doesn't work. But if you light the metalic surface with a polarized light (you have polarizing gel that you can put on your sources) then, you can cut the reflection from this light...

As for density numbers : density is by definition, log (Opacity) where Op = incident (incoming) flux/transmitted flux.

If a filter absorbs one stop, it then divides the flux by 2 and log (2) = 0.3. This is why.

Edited November 24, 2004 by laurent.a

Chris Keth · November 25, 2004

Wow, that's pretty cool. I never knew there was a correlation between polarization and the refractive index of the surface. I'm from a family of physics majors so I like that kind of stuff :)

Laurent Andrieux · November 25, 2004

as for history, it is said that Brewster had the intuition of such a relationship looking at the Senat windows in Paris at the Luxembourg Park...

As clouds polarize the light and the windows' glass do too, he saw the clouds'reflexion would light off if observed from a certain angle. People who know the place - by the fountain where kids play with wooden boats - can figure out there is enough room fot this experiment, there.