Polarizing filter acting as ND filter

SloYerRoll · January 21, 2008

So I've gone through light science & magic several times and have a strong grasp of what polarized light is and how to control it.

Question:
If there is no polarized light in the shot your composing. Does the polarizing filter act only as a ND filter? I understand there's a certain amount of polarized light practically everywhere and it can't be escaped. I'm more interested in hearing the big picture of polarizing filters and if they only act as ND's in a non-polarized light environment. Or is there another impact to having this filter in front of your glass?

Van Isle · January 21, 2008

:lurk

Good question...bump

gus · January 21, 2008

I agree...often wondered that myself & my call is yes it does. The shutter is still slowed from what i have noticed.

SloYerRoll · January 21, 2008

gus wrote:

I agree...often wondered that myself & my call is yes it does. The shutter is still slowed from what i have noticed.

I see your point and understand I have to go down two stops (on my particular filter) in order to keep a properly exposed shot. I'm wondering w/ exposure settings aside. Is there any reason I should take this filter off?

Is this a "poor man's" ND filter as long as you don't have any polarized reflections?

Manfr3d · January 21, 2008

Why dont you make the test and hold a polfilter in front of a laser?

SloYerRoll · January 21, 2008

Sorry, I didn't get any presents from Brookstone this year. No lasers.

I'm not sure how that would answer my question anyway. All I want to know is:

If there is no polarized light in the shot your composing. Does the polarizing filter act only as a ND filter?

Manfr3d · January 21, 2008

SloYerRoll wrote:

Sorry, I didn't get any presents from Brookstone this year. No lasers.

I'm not sure how that would answer my question anyway. All I want to know is:

Light from a laser is unpolarized thus you can use it to answer your question.
Simply point a laser to a white sheet of paper and take a picture of the spot
on the paper with your camera. Then hold the polfilter betwen paper and laser
and repeat the picure (maunal mode, tripod mount). The laser must be the
only lightsource in the room. Then compare the two images on computer
and detect the difference in brightness. If the 2nd image is darker then
the polfilter acts as an ND filter on unpolarized light.

pathfinder · January 21, 2008

A 1 stop ND is almost, but not quite, useless, and most polarizers cost 1 or 2 stops of slight at their max.

A good ND for waterfalls, will need to be 4 - 8 stops usually.

So, laser light is in phase, but not polarized??

SloYerRoll · January 21, 2008

I understand I need a ND filter. But I can only get so many toys at a time.
I'm asking if there's no polarized reflections (or the change in reflections are acceptable to the shot). Does the polarizer filter only act as an ND filter or are there other drawbacks to it that I don't know about?

Icebear · January 21, 2008

For all practical purposes, if there is no polarized light in the scene (?) a polarizer acts only as a weak ND filter.

SloYerRoll · January 21, 2008

Thanks John.
I was pretty sure this was the case. But it helps to know someone that can definitively answer the question.

It may be a weak ND filter. But it's two stops I didn't have before!

I'm so stoked I got my new camera bag. It's like the T.A.R.T.U.S. if your a "Dr. Who" fan (fits allot of stuff).

I'm still a bit interested in hearing about this laser thing though. I'm not buying one anytime soon since I need an ND filter now

D

Manfr3d · January 21, 2008

pathfinder wrote:

A 1 stop ND is almost, but not quite, useless, and most polarizers cost 1 or 2 stops of slight at their max.

A good ND for waterfalls, will need to be 4 - 8 stops usually.
So, laser light is in phase, but not polarized??

Err doesn't seem to make sense. Light from a light bulb however
is unpolarized because it is produced by radiative emission
from the electrons in the bulb's wire and since there is no
preferred direction along which they could produce their
electric field the emmitted light is unpolarized. You could
also shoot into the sun and test how much light the filter eats.

OsirisPhoto · January 21, 2008

SloYerRoll wrote:

Thanks John.
I'm so stoked I got my new camera bag. It's like the T.A.R.T.U.S. if your a "Dr. Who" fan (fits allot of stuff).

Actually, it's TARDIS ([SIZE=-1]Time And Relative Dimension In Space)

[/SIZE]

SloYerRoll · January 21, 2008

Hyperbaric wrote:

Actually, it's TARDIS ([SIZE=-1]Time And Relative Dimension In Space) [/SIZE]

Good catch!

Scott_Quier · January 22, 2008

If there is no polarized light in the shot your composing. Does the polarizing filter act only as a ND filter?

Something to consider - in an absolute, anal retentive sense you have to know that any filter will do more than modify the amount of light passed. Even very high priced/quality ND filters are going to alter the color of light. At that level, it is fair to assume that a CP will also be altering the color of light.

In a practical sense, this isn't an issue as one can simple correct the WB in post.

Given the hypothetical scene with NO polarized light, I would say that the CP would behave, and could replace, an ND filter. But, like you say, finding a scene with no polarized light will be virtually impossible.

jfriend · January 25, 2008

SloYerRoll wrote:

So I've gone through light science & magic several times and have a strong grasp of what polarized light is and how to control it.

Question:
If there is no polarized light in the shot your composing. Does the polarizing filter act only as a ND filter? I understand there's a certain amount of polarized light practically everywhere and it can't be escaped. I'm more interested in hearing the big picture of polarizing filters and if they only act as ND's in a non-polarized light environment. Or is there another impact to having this filter in front of your glass?

I just came across this thread and I'm not sure if this question was really answered completely.

Regular light (with no source that causes it to have a polarized bias) is "randomly polarized". Every light photon has a plane of polarization. Random polarization just means that each photon of light has a random orientation of it's plane of polarization such that there is an even distribution of all planes of polarization in the light, leading to an average of no net polarization when you average them all. It's kind of like white light. There's no such thing as white light. White light is just an even distribution of colors that all adds up to what we see as white.

Now, when you put a polarizing filter in front of randomly polarized light, it will block all the rays of light that have a plane of polarization that line up with the orientation of the polarizing filter and block none of those that are 90 degrees to it. For all the rest that are somewhere in between, it partially blocks them in proportion to how closely they do or don't line up with the filter. The net result is that a polarizing filter blocks half of randomly polarized light.

The other effects of using a polarizing filter are pretty much the same as using any filter.

Imperfections in the filter glass can degrade the optical quality of your image.
Transmissivity or absorbtivity of the filter glass can vary by wavelength, causing a color shift.
Reflections off the filter glass surfaces can cause flaring or a loss of contrast.
Extreme wide angle can have more light-fall-off at the edges with large apertures if the filter causes any vignetting

The first three items are usually much less of an issue in the better made (and more expensive) filters. The better filters are ground flatter, have their optical qualities of the glass tested to a small tolerance in the manufacturing process, use higher quality glass that has more constant properties with different wavelengths, use more involved ant-reflection multi-layer coatings to eliminate reflections at all different wavelengths, etc...

If you have a high quality polarizer, I see no problem in using it as an ND filter. I've found that when I need an ND filter, a CP is usually not a strong enough ND filter (it's really only one stop in randomly polarized light). Usually, when I need an ND filter (like to blur a waterfall in bright light), I end up needing 3-4 stops. I've actually used a 3-stop ND + my CP to get a total of four stops.

SloYerRoll · January 25, 2008

Thanks a ton jfriend!

Everything you've said makes perfect sense. When said like that. I don't know how it could have ever been confusing.

Thanks for nailing this question!

Cheers,
-Jon

papajay · January 28, 2008

Polarizers "101"
I've only had a polarizer for a week or so, and really had no clue how it worked or how to use it. In fact, I'd gone to the camera shop intending to buy an ND filter...it was the salesman that convinced me that a polarizer would be a better use of my money for the type of picture taking I do (mostly family vacations, lake views, etc....nothing professional).

Frankly, I forgot to check Dgrin for some tips on the use of polarizers and filters in general, but found this article on the web to be quite educational:

http://dpfwiw.com/polarizer.htm

There's even a link to a little applet that lets you interactively play with the incident light beam and the corresponding polarizer orientation to see what the effect is...pretty cool.

Polarizing filter acting as ND filter

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