Polarizers

This thread is to explain some of the mystery behind Polarized Light, and Polarizing Filters. So, clear your schedule, sit back and enjoy. Some of you may find this FAQ a bit verbose, and a bit over analytical. But I tire of reading articles about photographic gear that pretty much tell the reader that such and such works because of PFM (Pure Frigging Magic). You can add your own middle word there if it suits you, but I want to keep this clean for the kids. I believe nothing works due to PFM, and I believe that if you have an understanding of why something does what it does, it makes you better from a photography standpoint, cause you can now employ your gear to optimize the conditions you photograph in.

What is Polarization??

Polarization: is a property of certain types of waves (light or electricity) that describes the orientation of their oscillations. (Think how Sine Waves are oriented)

Polarization of light is described by the specifying the orientaion of the sine wave as the light travels from point to point.

Usually the polarization is perpindicular to the direction the light is traveling. It may oriented in single direction (Linear) or may rotate while traveling (circular or eliptical)

So to explain this in terms of the sine waves:

Linear Polarization:

The perpindicular components (Red and Green) are in phase with each other, but the amplitudes (measured peak to peak by how tall the peaks are) are different. So basically stated the two components frequency is the same, (measured peak to peak, by how far the peaks are apart along the direction of travel) The amplitude dicates which direction the light is polarized towards (The purple line). If you don’t understand it, the light will align closest to the component with the most amplitude.
Mostly the amplitude will be the same, and cause the polarization to equally split the two compnents which are at right (90) degrees from each other. So the split is 45 degress, exactly half of 90 degrees.



Circular Polarization:

In this type of polarization, the amplitudes are the same, but the red and green components are 90 Degrees out of phase. So when one component is at null (the spot between the two peaks on a sine wave), the other component is at it’s maximum or minimum amplitude. This causes the light to “rotate”. Think of it like rifling in the gun barrel, the rifling causes the bullet to rotate. Here, the phase shift of the perpindicular components cause the light to percievably rotate, as one component is there at any given time, and the other isn’t. This can also be thought of in the way an DC electric motor spins. If you apply a electricity to the motor stators (these don’t spin, and are meant to make the motor spin by reversing their polairity continuously to get the comutator to rotate) to a wound motor comutator (the thing that spins), there will be a resultant magnetic field (electromagnet), that then causes the motor to spin towards it’s opposite pole, depending on the polarity of the, and the comutator. The direction (or handedness) the light rotates depends on which component is ahead or behind the other



Eliptical Polarization: When the amlitudes are either not the same, or not 90 degrees out of phase, but the amplitude ratio and phase offset is constant, you get eliptical polarization. Bascially think of it as unbalanced polarization, as the amplitude won’t align it in a linear fashion, and the “point” ends of the elipse will align based on the amplitude and the phase offset



You following this so far? Good.

Anyway if you followed me though that, and you now know how light aligns itself, you can understand how the filters we use as photographers to influence it, work to do what they do.

Linear Polarizer:

There are a couple types of Linear Polarizers. The one we care about is a absorptive polarizer. Linear Polarizers are made with something called Polarized Film or Polaroid Film (crazy huh?) They use Poly-Vinyl Alcohol Plastic (PVA) which is stretched during manufacture to align the PVA chains in one direction (remember the perpindicular components) Then they “dope” the film with iodine. The iodine electrons are then able to travel along the PVA chains, ensuring the perpindicular (to the PVA chains) light components are allowed to be transmitted, and the parallel components cannot be transmitted.

A basic drawing of this effect is seen here:




The above picture is a wire grid linear polarizer, but is the same concept of a PVA Linear Polarizer, just think of the wire grid as the PVA chains.

So it will allow the light we want to photograph through, and kill the reflected light which is parallel to the PVA chains, cutting reflection in your pictures, be it from water, or glass, or leaves on a plant for example. Reflection from metal cannot be eliminated with either linear or cicular polarizers, as it is not polarized due to the electromagnetic nature of it (Think Chrome) which is often the parallel component. It will darken skies, since most of the light from the sky is polarized, so the polarizer either linear or circular will darken sky by not allowing this light to pass. Since that light is eliminated, the sky consequently darkens since the polarized component is eliminated.

Before we talk about the circular polarizer, we need to know something about their construction. Inside a circular polarizer is something called a wave plate, which causes the light to be polarized circularly.

A wiki description of a wave plate states:
A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it. A wave plate works by shifting the phase between two perpendicular polarization components of the light wave. A typical wave plate is simply a birefringent crystal with a carefully chosen orientation and thickness. The crystal is cut so that the extraordinary axis or "optic axis" is parallel to the surfaces of the plate. Light polarized along this axis travels through the crystal at a different speed than light with the perpendicular polarization, creating a phase difference. When the extraordinary index is smaller than the ordinary index, as in calcite, the extraordinary axis is called the "fast axis" and the perpendicular direction in the plane of the surfaces is called the "slow axis".
A graphic description of a wave plate is below:



If you look at the graphic above, by the time the light components get through the plate, one component is one quarter of a oscillation behind the other. Hence the name.

Remember how that wave plate works, and we will describe how a cirular polarizer works.

Cicular Polarizer:

A circular polarizer actually creates circularly polarized light, then the photographer chooses to absorb or pass clockwise or counter-clockwise light through it. The most common way to do this is to place a quarter wave plate (aka a retarder) after a linear polarizer. Remember earlier when we were describing what circularly polarized light was? We talked about how there was a phase offset associated with cirular polarization. So that linear polarized light comes through the linear polarizer first, which ideally should align the perpindicular light at a 45 degree angle between the to axes (X and Y or Fast and Slow, the phase offset right?) Once through the linear polarizer, it strikes the quarter wave plate. If you read above, you see that the quarter wave plate causes one of the perpindicular components to slow, causing a phase offset, therby causing the circular polarizing effect, due to the phase offset, the light components start to rotate.



The description of the graphic above is this: To understand this conversion process in more detail imagine a quarter wave plate and a linear polarizer laminated together. As illustrated in the diagram to the right, the transmission axis of the linear polarizer is at the negative 45° angle relative to the right horizontal. The slow axis of the quarter wave plate is horizontal and the fast axis is vertical. When one attempts to pass un-polarized light through the linear polarizer, only light that has its electric field at the negative 45° angle leaves the linear polarizer and enters the quarter wave plate.

So ultimately what we are doing is creating circularly polarized light, then either passing TTL (Through The Lens) or absorbing it, by spinning the ring on the circular-polarizer, (C-PL). When you spin a Linear Polarizer, it is aligning that “wire grid” which only runs in one direction, so it can absorb the polarized reflections from objects that are parralel to the wire grid. It allows the perpindicular light to pass.

What you are doing with the circular polarizer is the same essentially, but as you spin a circular polarizer, you are spinning that quarter wave plate, which will ultimately decide whether you pass or absorb the light.

So, how does this all apply to photography?

Well, use of a polarizer either Linear or Circular can do a lot of things. Darken blue skies, increase or decrease saturation depending on how it is oriented, and increase or decrease contrast in a scene based on orientation.

Simply put, reflection control with a linear polarizer works like this:

In a linear polarizer, it either eliminates the horizontally, vertically or diagonally polarized light, by spinning the filter until the PVA chains line up so that they are parallel to the type of polarization, and then elminating it, as only the perpindicular light gets through we said.

A Circular polarizer is a bit more complex. A circular polarizer has two elements. First a linear polarizer (yeah just what we talked about), and a quarter wave plate element mounted after the linear polarizer. As the light comes in, it becomes linearly polarized, as it hits the linear element. From there it is passed to the quarter wave plate where it is polarized in a circular fashion. Here is where the handedness (right or left) of the light rotation comes into play. The handedness will determine what light is absorbed, and which light passes. To create counter-clockwise circularly polarized light or vice versa one simply rotates the axis of the quarter wave plate 90° relative to the linear polarizer. This reverses the fast and slow axes of the wave plate relative to the transmission axis of the linear polarizer causing the vertical component to lag the horizontal, or vice versa. Causing the light to rotate clockwise or counter clockwise. Depending on how the circular polarizer and the linear polarizer are oriented relative to each other will determine whether the counter-clockwise or clockwise light will pass. To get one to pass and not the other, one simply needs to turn the polarizer 90°, and if you turn it another 90°, the opposite will occur. If you think of it in terms of polarity, if you reverse polarity or electricity or light, there will be 180° phase change. So if you change 180°, it will take the light completely through a polarity change, denying counter clockwise light during the first 90° for example, then denying clockwise light on the second 90°.

VARI ND or Fader Polarizers.

A relatively large deal has been made about Singh Ray polarizers and “Fader” Polarizers or ND filters. There is really no magic going on here, and Singh Ray is really almost scamming people out of 300+ dollars when they buy these things. Nothing more is going on here than putting another Linear Polarizer on a circular polarizer. So bascially you can cut up to 8-9 stops of light with these things. If you remember how a linear polarizer works, it absorbs the component of light parallel to the PVA chains. So if you stack a linear polarizer in front of a Circular Polarizer (which we remember has a L-PL in front of it already). And then you spin the Linear polarizers 90° relative to each other, you can absorb essentially both the “parallel components” effectively cutting most of the light out of the scene. So if you really have that $300 burning a hole in your pocket for a Singh Ray Vari ND or Polarizer, my advice is to go buy a el-cheapo C-PL, and a L-PL which are already cheap, and stack them and experiment. If the results are good, you can save a lot of money. Maybe upgrade the C-PL a bit to get a better quality one, and get better results.

So why do they say “Don’t put a L-PL on a DSLR"?

The first component allows to block or highly reduce light scattered from a sky at 90 deg. and reflections from shiny surfaces such as glass or water. The second component is needed for most of the modern cameras to work with the resulting light correctly. If your camera uses beam-splitter reflecting all or part of light at 90 deg. into auto-focusing device and into an exposure-metering system (e.g. if you have am SLR or DSLR camera), then it utilizes only one linear polarization out of incoming light. Both systems are designed to work with light which is not linearly polarized. However, if we place a linear polarizer in front of a camera, this light might be totally blocked by 90° reflection, and both exposure meter and auto focus will not work properly. So, to make the camera work after passing the light through a linear polarizer, we need to convert this light into a state which contains both linear polarizations in equal amounts. This is done with the use of a quarter-wave plate, which produces a circular polarized beam, which allows your SLR camera to function properly.

Here are a few other things to keep in mind when using polarizing filters:
• When stacking filters, the polarizer should always be the last (outermost, if possible), filter on the stack. Passing through a polarizing filter further down the chain may alter the results of other filters you had used.
• Lenses with rotating front elements will mess up your polarizing filter's rotation. If your lens has a rotating front element, you should adjust the polarizing filter after you've set the focus.
• The effect of polarization (outdoors) is most prominent at a 90-degree angle to the sun.
• The angle of polarization varies continuously with the angle from the sun. With a lens wider than about 28mm (in 35mm film terms) the sky will be unevenly polarized. If you like the effect, go ahead and use it. The reason for this is when you get below 28mm, you get close to a 90° field of view, and like we talked about earlier, polarization is very sensitive to 90° changes in view. And we said that the polarization is at max with a 90° angle to the sun. So if you have composed with the camera facing away 90° from the bearing to the sun, and you have a lens that has a field of view of greater than 90°, you will see that the part of the sky the furthest from the sun will be darker, and the sky will get lighter as it gets closer to the bearing of the sun.

What to Buy????

Many companies make polarizers. Tiffen, Cokin, B+W, Hoya, etc. If you simply do a search for a linear or circular polarizer on the internet, you will find every major, reputable filter company makes them. Now what are the differences between polarizer A from one company, and polarizer B from another company? Things like flare control via muli-coating quality, and light transmittance are the big things to worry about. Other things to consider, how wide you lens is, or filter setup is, meaning will the filter you put on there vignette, either by itself, or if stacked with other filters. So these are all things to consider. Cost is another, and this is where some test results would be helpful, to find out if you are getting what you pay for. Well here is a good test of polarizers that was done in Europe a while back.
http://www.lenstip.com/115.1-article...ters_test.html
I am not going to tell you what polarizer is best to use. Every filter is different from performance, price, and ring thickness. So my advice at this point is to do a lot of reading. Also if you buy one, make sure the place you buy from will allow you to return it if you are unhappy with it’s performance. Also make sure of their return policy on open items, and how long you have to return it. This could maybe let you test a few out until you find one you like.
I personally use www.2filter.com to buy all of my filters. They offer great customer service, great return policy, and have a very diverse stock of filters. There are many others, that a google search will find.

By no means is this a one stop place for all info about this subject, so please add to this thread, and make it better. That being said, here are some links to some other great articles about polarizers:
http://www.camerapedia.org/wiki/Polarizer
http://www.bobatkins.com/photography...olarizers.html
http://www.luminous-landscape.com/tu...larizers.shtml
http://www.fas.harvard.edu/~scidemos...arization.html
http://www.mat.uc.pt/~rps/photos/filters_uv_pol/
http://en.wikipedia.org/wiki/Polarizer

Here are some differences in shots taken with a polarizer that is dialed all the way to cut reflection (max polarization), and dialed for minimum polarization:

Minimum Polarization:


Max Polarization of the same composition, less than a minute later


Minimum Polarization (shows rock reflection and water reflection)


Max Polarization:


So what do we get from all this???

Well as you can see from those shots, at min polarization you can't see down into the water, because you get all the reflection, as the retarder (quarter wave plate) is oriented to let all the light through. Whereas the max polarizer shots you can see into the water, because the retarder (quarter wave plate) is only letting the direct light through, not the reflected light, that has been phase shifted due to striking an object and reflecting into the lens. The other benefits are it usually warms the light also, especially useful in the fall, or at sunrise/sunset. You are better able to control and increase contrast in the scene, and this typically boosts saturation.

Saved for future expansion

Damn dude, well done. This will be an article as soon as you're done filling in your blanks. I can move/edit this post if need be as well.

Yeah I still need to have bit of a discussion about Brewster's Angle.

Thanks a lot for the writeup, I really enjoyed it. I wish there were more things of this nature on NSOP. I know that my teeny grasp of photography sure could be helped by some of the skilled artists we are lucky to have around.

Glad you enjoyed it. I enjoy writing stuff like this, as it gives me an outlet for all the Physics of Sound stuff I had to learn to be a Sonar Tech, since sound wave theory is much the same as Electrical wave and light wave theory.

where's the sample pics?

Originally Posted by jacobsen1

where's the sample pics?

Working on it. I had to watch the USA lose yesterday.

so in the same place as mine then...

yeah, I had that, DURING our party (we had to hold off on singing happy bday for OT to end! ) then I had to deal with the party and we watched some closing ceremony crap then I passed out. I have a nice backlog now as well.

^ I use a polarizer on my wides a lot. Some times you get funky skies, some times you don't. It's worth trying if you have one that fits IMHO.

Originally Posted by Donkeypunch

Glad you enjoyed it. I enjoy writing stuff like this, as it gives me an outlet for all the Physics of Sound stuff I had to learn to be a Sonar Tech, since sound wave theory is much the same as Electrical wave and light wave theory.

Nice! On that note I would love to hear from some of you about how to be better with long exposure, night shots, and b&w.

Anybody? I have learned SO much from this forum, it's one of my favorite sites.

Originally Posted by Dave6265

Nice! On that note I would love to hear from some of you about how to be better with long exposure, night shots, and b&w.

Anybody? I have learned SO much from this forum, it's one of my favorite sites.

Well I suppose at some point, maybe all of us landscapers should get together and do a FAQ about Long Exposure, night and B&W??? But yeah we can probably arrange something like that.

Andy, thanks for writing this up, it was very informative.

Originally Posted by Donkeypunch

Well I suppose at some point, maybe all of us landscapers should get together and do a FAQ about Long Exposure, night and B&W??? But yeah we can probably arrange something like that.

:thumbs up: All in good time, but I'm sure it'd be appreciated by many, as there are some real talents on this site.


Looking forward to more

Ben,

I'm really not thrilled with the example shots I took. Sky was too muddy so I really didn't see a good difference between blue and blown out, and the reflection I got didn't really show enough difference. Gonna re-shoot next week. Eh, we got nothing but time, so this will get finished soon.

-Andy

Originally Posted by Kilonad

You had me at Brewster's Angle.

I've been meaning to throw together a workshop like this for photographers.

Thanks, I wanted to get this up as soon as I could, so right now it is a "working" FAQ. But yeah some other stuff like example shots, and Brewsters Angle needs to get in there yet. Man I have such a backlog of stuff to work through though.

Bump, added some example shots in post #4. Video's showing the effect while spinning the polarizer are coming shortly.

Andy,

I've got a couple of questions, I've read online that it might not be a good idea to use a polarizer on a ultra-wide angle (such as my 11-16). Is this only because I might get uneven polarization in the sky? I assume that for everything else it would be fine.

My other question is if I get a polarizer for my 11-16 can I use a step up ring and also use it on my 50-135? I would be going from 77 to 67.

subscribed for daytime reading

Nevermind, I guess I didn't read enough of the comments, obviously this question was already answered.

Originally Posted by brian1971

I've got a couple of questions, I've read online that it might not be a good idea to use a polarizer on a ultra-wide angle (such as my 11-16). Is this only because I might get uneven polarization in the sky? I assume that for everything else it would be fine.

yeah, it's just the sky issue. I have a 77mm CP, just ask to borrow it at a sunrise and you can see for yourself.

My other question is if I get a polarizer for my 11-16 can I use a step up ring and also use it on my 50-135? I would be going from 77 to 67.

yes again. I don't think I have a 67 ring, but ALL my filters and my lee holder are set for 77mm. I actually get step up rings and replacement lens caps for all my lenses for 77mm if they're NOT 77mm to make it easier. That way you leave the ring on the lens and you can swap caps across all your lenses.

Thanks. I ended up ordering both a polarizer & step-up ring from 2 filter last night. It shipped first thing this morning, I'm hoping it comes tomorrow. I'd love to be able to use it for sunset tomorrow & sunrise Saturday.

Is anyone familiar with Nikon's CPL-II slim polarizer?

nope!

Originally Posted by MK19

Is anyone familiar with Nikon's CPL-II slim polarizer?

I've never heard anything bad about nikon's filters - except that they're

oh, and I'll add, if you go slim, it's nice in that you can use them with really wide lenses, but it also means you CAN NOT add more filters to the front of them. That would vignette but there are times when I'd rather crop the corners and stack.

Originally Posted by jacobsen1

oh, and I'll add, if you go slim, it's nice in that you can use them with really wide lenses, but it also means you CAN NOT add more filters to the front of them. That would vignette but there are times when I'd rather crop the corners and stack.

I've never even thought of stacking a CPL; I just use the GND's for that

yeah, what about when you want a GND annnnd a CP?

or even a 10-stop and a ND?

It depends on the brand - my hoya hd is slim as **** and still has front threads.

OK. I have a few $ left on my Dad's B&H gift card and I researched a lot of the CPL's- this is what I am strongly looking at. I shoot almost 90% of the time outdoors and being in WA it gets hot, wet- you name it. I need a rugged filter.

B+W Kaeseman CPL

Have also heard nothing but great things about that line of CPs. But why get a 72mm? A 77mm will be much more useful down the line and adapter rings are cheap!

Originally Posted by MoLS

Have also heard nothing but great things about that line of CPs. But why get a 72mm? A 77mm will be much more useful down the line and adapter rings are cheap!

I do have a 67-77mm B+W adaptor, but every time I put a filter on there it is damn near impossible to get off if it goes on a bit too snug or gets tight as a result of rotating a lot. No more step-up adapters for me

Then I would opt for the filter that fit my landscaping lens. (Which I think yours is the same lens as mine!)

Originally Posted by MoLS

Then I would opt for the filter that fit my landscaping lens. (Which I think yours is the same lens as mine!)

Well, I admittedly don't use my 10-20 NEAR as much as I should (I have a 77mm CPL for that lens). This would be for the 18-200

Gotcha! Carry on then, good sir!

Let us know how it is when you get it. I'll inevitably have to replace my hoya when it comes apart and won't snap back together again anymore.

Oh! And if you can deal with not having a warranty: http://maxsaver.net/B-W-72mm-Kaesema...er-Filter.aspx I got my hoya from them and it is a legit site and a real hoya.

Originally Posted by MoLS

Gotcha! Carry on then, good sir!

Let us know how it is when you get it. I'll inevitably have to replace my hoya when it comes apart and won't snap back together again anymore.

Oh! And if you can deal with not having a warranty: http://maxsaver.net/B-W-72mm-Kaesema...er-Filter.aspx I got my hoya from them and it is a legit site and a real hoya.

Right-O old bean, right-o! I will prolly get it from B&H, as I still have a few $ on a gift card. I like Maxsaver a lot tho

Great write up!

Originally Posted by Whitford

Killer write up, thank you.

Your welcome.

The B+W Kaeseman should be in hand for Friday and this weekend is shaping up to be a sunny one, so a write-up on my results is in order

Originally Posted by astockwell

Your welcome.

You're even.

Anyone else only use circular polarizers for stream shots/wet foliage?

Originally Posted by Stime187

Anyone else only use circular polarizers for stream shots?

I do. It helps cut back glare, especially where water collects to run off (waterfalls, over rocks).

Originally Posted by MK19

I do. It helps cut back glare, especially where water collects to run off (waterfalls, over rocks).

Sorry, I meant that's the ONLY time I put a CPL on my lens. I don't use it for anything else, including blue skies. I was wondering if anyone else used it as sparingly as I do.

But yes, it works great for wet rocks/foliage.