Where is the mark on the polarizer supposed to be?

Started 3 months ago | Questions
Dem Bell Regular Member • Posts: 333
Where is the mark on the polarizer supposed to be?

Most polarizers, either linear or circular, have a mark on their rim that is supposed to tell which orientation produces maximum effect. The sun is in the plane of the filter, the mark on the rim points towards the sun - bang, maximum effect. That's what I thought.

I understand that you don't always want the maximum effect, that not all polarizers have a mark, that you can see the effect of the polarizer in the EVF, etc.. etc... My question is not about how to use a polarizer but about the meaning of the mark on its rotatable part. Surely it has something to do with the way the tiny parallel wire grid is oriented in the filter, right?

There are some nice on-line tutorials like this:

https://learn.zoner.com/learn-to-use-a-polarizing-filter/

that confirm that and show the following example:

Three polarizing filters. All the filters have a mark on their rotatable part that marks the polarization plane. (Two have a line; the one on the right has a small triangle.) [from https://learn.zoner.com/learn-to-use-a-polarizing-filter/ ]

What they say makes perfect sense, except this does not seem to be the case in practice. It is easy enough to determine which orientation of the polarizer completely removes reflections from a window or blocks all light coming from an LCD screen. You don't even need to attach the filter to a camera.

As it happens, I had three random old Hoya polarizers in the draw, and I checked if the mark on the rim of these filters corresponds to the orientation in which the polarized light is blocked. No, it does not! It is all over the place! For two filters it was off by about 15-20 degrees, the third one was off by about 60 degrees. The actual position of the mark has nothing to do with the orientation of the polarizer grid and its maximum effect.

This begs the question: why bother putting a mark on the rim if it does not mean anything? Is this mark just a visual cue to tell you by how much the polarizer is being rotated? I am confused now. Should the tutorials be rewritten? Can my three filters be duds?

Three polarizing filters. The orientation that removes all polarized light (this is the orientation perpendicular to the direction of the polarizer grid) is marked by white putty (UHU White Tack). For all three filters, this is not the orientation marked on the filter.

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smithim Senior Member • Posts: 1,320
Re: Where is the mark on the polarizer supposed to be?
1

I confess, I've never thought about it, but always assumed that it was just a reference point. I don't see how it can be used to indicate maximum effect, as surely each filter will sit differently on each lens - AFAIA, filter threads are not that precise.

I just look through the viewfinder and turn the ring until I see the maximum effect (or whatever else I want).

Gerry Winterbourne Forum Pro • Posts: 17,526
Re: Where is the mark on the polarizer supposed to be?

Dem Bell wrote:

Most polarizers, either linear or circular, have a mark on their rim that is supposed to tell which orientation produces maximum effect.

Correct. Natural light vibrates at all angles; polarising tries to remove all light but a particular angle. (I say "tries" because if it really cut out all light except one specific angle it would darken things by 359/360 - almost pitch black.) The mark on the filter shows how that angle is aligned.

The sun is in the plane of the filter, the mark on the rim points towards the sun - bang, maximum effect. That's what I thought.

It's not just the light from the sun that is polarised - so is light from all over the sky, reflections from the scenery (and remember that unless you put the light source in the frame, all the light reaching the camera is reflected).

In practice, therefore, there's unlikely to be a single angle of maximum effect. The index mark is just a guide to your starting point.

I understand that you don't always want the maximum effect, that not all polarizers have a mark, that you can see the effect of the polarizer in the EVF, etc.. etc... My question is not about how to use a polarizer but about the meaning of the mark on its rotatable part. Surely it has something to do with the way the tiny parallel wire grid is oriented in the filter, right?

Well, there's no wire grid so in that sense you're wrong. But substitute "angle of polarisation" for wire grid and you're right.

There are some nice on-line tutorials like this:

https://learn.zoner.com/learn-to-use-a-polarizing-filter/

that confirm that and show the following example:

Three polarizing filters. All the filters have a mark on their rotatable part that marks the polarization plane. (Two have a line; the one on the right has a small triangle.) [from https://learn.zoner.com/learn-to-use-a-polarizing-filter/ ]

What they say makes perfect sense, except this does not seem to be the case in practice. It is easy enough to determine which orientation of the polarizer completely removes reflections from a window or blocks all light coming from an LCD screen. You don't even need to attach the filter to a camera.

As it happens, I had three random old Hoya polarizers in the draw, and I checked if the mark on the rim of these filters corresponds to the orientation in which the polarized light is blocked. No, it does not! It is all over the place! For two filters it was off by about 15-20 degrees, the third one was off by about 60 degrees. The actual position of the mark has nothing to do with the orientation of the polarizer grid and its maximum effect.

This begs the question: why bother putting a mark on the rim if it does not mean anything? Is this mark just a visual cue to tell you by how much the polarizer is being rotated? I am confused now. Should the tutorials be rewritten? Can my three filters be duds?

Three polarizing filters. The orientation that removes all polarized light (this is the orientation perpendicular to the direction of the polarizer grid) is marked by white putty (UHU White Tack). For all three filters, this is not the orientation marked on the filter.

No filter removes all polarised light. First, if it did the image would be black; second, polarisation works at more than one angle. The effect you see depends on many things; unless the scene is identical each time - which means that clouds don't move, no time passes, foliage is perfectly stationary etc - you wouldn't expect to see the same effect every time.

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Gerry
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OP Dem Bell Regular Member • Posts: 333
Re: Where is the mark on the polarizer supposed to be?
1

smithim wrote:

I confess, I've never thought about it, but always assumed that it was just a reference point. I don't see how it can be used to indicate maximum effect, as surely each filter will sit differently on each lens - AFAIA, filter threads are not that precise.

That's the back part of the polarizer that will sit differently on different lenses depending on their filter threads. This part is just a metal ring with no optics in it and its orientation does not affect the image. Its job is to sit firmly on the lens and hold the rest of the filter in front of the lens.

The actual optics is firmly fixed in the front part of the filter that is rotated to control the effect of the filter. The orientation of the front part of the filter is completely decoupled from the orientation of the back part.

I just look through the viewfinder and turn the ring until I see the maximum effect (or whatever else I want).

Alex Ethridge
Alex Ethridge Veteran Member • Posts: 4,994
The lack of an accurate mark is a disservice to the user.
1

The first polarizer I bought was in 1965 and it had more than just a mark; it was an actual, physical handle about a half-inch long sticking out to the side from the rotating rim. It was accurate. It was ALWAYS accurate. A couple I bought in the 60s and 70s had a mark instead; I missed the handle but at least the mark was accurate. Just point it toward the sun for maximum effect, no time-consuming spinning to figure out where maximum effect was.

The lack of an accurate mark is a disservice to the user.

I've often thought of gluing my own marker on but I guess the reason I haven't is I don't do near the outdoor shooting I once did.

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BBbuilder467 Veteran Member • Posts: 4,515
Re: Where is the mark on the polarizer supposed to be?
1

The linear ones that I've used seem to go darkest held up to a monitor when the pip mark is about the 12 o'clock position. That's how I mark the CPL s that aren't marked and use that for reference. I try to mimic the old reference with the lever. You knew where you were just by feel.

smithim Senior Member • Posts: 1,320
Re: Where is the mark on the polarizer supposed to be?
1

Dem Bell wrote:

smithim wrote:

I confess, I've never thought about it, but always assumed that it was just a reference point. I don't see how it can be used to indicate maximum effect, as surely each filter will sit differently on each lens - AFAIA, filter threads are not that precise.

That's the back part of the polarizer that will sit differently on different lenses depending on their filter threads. This part is just a metal ring with no optics in it and its orientation does not affect the image. Its job is to sit firmly on the lens and hold the rest of the filter in front of the lens.

The actual optics is firmly fixed in the front part of the filter that is rotated to control the effect of the filter. The orientation of the front part of the filter is completely decoupled from the orientation of the back part.

I just look through the viewfinder and turn the ring until I see the maximum effect (or whatever else I want).

I sit corrected 

Bob
Bob Senior Member • Posts: 2,232
Why do we need a mark?
5

I just turn it until I like the result.

BBbuilder467 Veteran Member • Posts: 4,515
Re: Why do we need a mark?
1

Bob wrote:

I just turn it until I like the result.

It was mainly a reference point. It gave you a good starting point and was easier to hold that location if you stacked ND filters. You probably knew where it went before you put the polarizer on.

The levers were nice. You could feel them.

I don't notice it so much hand-held, but still like them marked with a tripod.

petrochemist Senior Member • Posts: 2,651
Re: Why do we need a mark?
2

Bob wrote:

I just turn it until I like the result.

IMO this is the only way.

Polarisers effect many aspects the colour of blue skies, reflections, saturation of foliage...

In many cases these will be different for different parts of the image, or the effect wants to be controlled subtly such as boosting the reflections on the water without getting glare from the windows.

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OP Dem Bell Regular Member • Posts: 333
Re: Where is the mark on the polarizer supposed to be?

BBbuilder467 wrote:

The linear ones that I've used seem to go darkest held up to a monitor when the pip mark is about the 12 o'clock position. That's how I mark the CPL s that aren't marked and use that for reference. I try to mimic the old reference with the lever. You knew where you were just by feel.

Thanks for your comment. I learnt today that, out of 4 LCD monitors we've got at home, two have vertical polarisation, one horizontal and one emits light polarised at 45 degrees to the horizon. It might be a good idea to stick with the same monitor when marking unmarked filters

OP Dem Bell Regular Member • Posts: 333
Re: Why do we need a mark?
1

petrochemist wrote:

Bob wrote:

I just turn it until I like the result.

IMO this is the only way.

Most manufactures seem to agree with this. Looking at the B&H website, it is mainly Hoya and Marumi who are using this mark. Tiffen, B+W, Singh-Ray, Sirui and many others do not bother.

Heliopan is an interesting one - they put lots of numbers around the rim: 1 2 3 4 5 6 ... so you can note the orientation of the filter just by looking at it.

ken_in_nh Contributing Member • Posts: 855
Re: Why do we need a mark?
3

I've used polarizers for at least 50 years - all kinds of brands and sizes.  Never noticed marks, and find it hard to understand how they can be useful.  I've always used the viewfinder to judge the effect, and sometimes shoot different rotations, especially with water reflections - sometimes the reflections and specular highlights themselves are nice; sometimes the darker water is.

Bob
Bob Senior Member • Posts: 2,232
Re: Why do we need a mark?

Dem Bell wrote:

petrochemist wrote:

Bob wrote:

I just turn it until I like the result.

IMO this is the only way.

Most manufactures seem to agree with this. Looking at the B&H website, it is mainly Hoya and Marumi who are using this mark. Tiffen, B+W, Singh-Ray, Sirui and many others do not bother.

Heliopan is an interesting one - they put lots of numbers around the rim: 1 2 3 4 5 6 ... so you can note the orientation of the filter just by looking at it.

That's a variable ND filter, not a polarizer, so the marks indicate light reduction, not orientation.

https://www.bhphotovideo.com/c/product/829300-REG/Heliopan_708290_82mm_Variable_Gray_ND.html

ThrillaMozilla Veteran Member • Posts: 4,959
Re: Where is the mark on the polarizer supposed to be?
2

I didn't measure with lab equipment, but my Hoya filter seems to be right on the mark. It's easy to measure, so I assume that generally the marks are quite accurate.

I think it's possible that you are confused about the direction of polarization. It doesn't always "point to the sun". Reflected light is (partly or totally) polarized perpendicular to the plane of the incident and reflected rays.

For reflection from a surface, the direction of polarization is oriented relative to the surface, not to the sun. In the sky, polarization is at a maximum at a scattering angle of 90 degrees.

And just to make the explanation more complicated, on photo filters a straight line passing through the center and the mark is perpendicular to the plane of polarization, so the mark does indeed point to the sun for maximum effect.  In scientific instruments the mark is generally rotated by 90 degrees relative to that in photo filters.

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ThrillaMozilla Veteran Member • Posts: 4,959
Re: Where is the mark on the polarizer supposed to be?

Gerry Winterbourne wrote:

No filter removes all polarised light.

In the right orientation they do remove virtually all polarized light.  Photographic polarizing filters are almost perfect in that respect.  (Footnote:  someone once tested a horrible, cheap filter that transmitted a few percent of polarized light, but that was an exception.)

However, most light that you see in nature is not 100% polarized.

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BBbuilder467 Veteran Member • Posts: 4,515
Re: Where is the mark on the polarizer supposed to be?

ThrillaMozilla wrote:

I didn't measure with lab equipment, but my Hoya filter seems to be right on the mark. It's easy to measure, so I assume that generally the marks are quite accurate.

I think it's possible that you are confused about the direction of polarization. It doesn't always "point to the sun". Reflected light is (partly or totally) polarized perpendicular to the plane of the incident and reflected rays.

For reflection from a surface, the direction of polarization is oriented relative to the surface, not to the sun. In the sky, polarization is at a maximum at a scattering angle of 90 degrees.

And just to make the explanation more complicated, on photo filters a straight line passing through the center and the mark is perpendicular to the plane of polarization, so the mark does indeed point to the sun for maximum effect. In scientific instruments the mark is generally rotated by 90 degrees relative to that in photo filters.

That was basically the logic. Rotate the pip mark toward the sun, rather than away from it. It wasn't meant to be precise, but it made a simple reference point.

ThrillaMozilla Veteran Member • Posts: 4,959
Re: Where is the mark on the polarizer supposed to be?

BBbuilder467 wrote:

That was basically the logic. Rotate the pip mark toward the sun, rather than away from it. It wasn't meant to be precise, but it made a simple reference point.

At the risk of restating the obvious, that only applies to the sky. And the mark is precise.

If one understands polarization and knows what one is doing, one can rotate the mark to the plane of the incident and reflected/scattered ray, or perpendicular to it.  Otherwise, you can simply rotate it according to the appearance that you are trying to achieve.  You don't have to understand it or use it, but it has a purpose.

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OP Dem Bell Regular Member • Posts: 333
Re: Why do we need a mark?

Bob wrote:

Dem Bell wrote:

petrochemist wrote:

Bob wrote:

I just turn it until I like the result.

IMO this is the only way.

Most manufactures seem to agree with this. Looking at the B&H website, it is mainly Hoya and Marumi who are using this mark. Tiffen, B+W, Singh-Ray, Sirui and many others do not bother.

Heliopan is an interesting one - they put lots of numbers around the rim: 1 2 3 4 5 6 ... so you can note the orientation of the filter just by looking at it.

That's a variable ND filter, not a polarizer, so the marks indicate light reduction, not orientation.

https://www.bhphotovideo.com/c/product/829300-REG/Heliopan_708290_82mm_Variable_Gray_ND.html

No, I am talking about Heliopan polarizers. This one has numbers running from 1 to 26:

https://www.talamas.com/rentals/product/heliopan-105mm-circular-polarizer-filter

petrochemist Senior Member • Posts: 2,651
Some technical info on polarization.

ThrillaMozilla wrote:

Gerry Winterbourne wrote:

No filter removes all polarised light.

In the right orientation they do remove virtually all polarized light. Photographic polarizing filters are almost perfect in that respect. (Footnote: someone once tested a horrible, cheap filter that transmitted a few percent of polarized light, but that was an exception.)

However, most light that you see in nature is not 100% polarized.

Virtually all is IMO a rather unhelpful term. In some cases 0.002% transmission could be significant, in many the few percent transmitted by that cheap filter will not matter.

Polarizers will indeed remove the bulk of visual light that is polarized in the opposite orientation, but very few are effective with Near infrared which many digital cameras have some sensitivity to. You can buy polarizers that are effective over the entire wavelength range silicon sensor can record, sadly they are quite expensive so I haven't got any.

I don't know of any natural source where light is 100% polarized & I think lasers may be the only man made sources that actually achieve 100% (even the very best polarizers leak a few ppm of the polarization they are designed to block). Most natural sources have photons of random polarization but fortunately this can be treated as vectors the proportion in the diracton of the polarizer being blocked & the remainder passed. Refection etc will only polarize a portion of the reflected light with the maximum portion when the light is incident at the materials brewster angle (63 degrees for water IIRC).

A theoretically ideal polarizer would block 100% of the light that matches it's polarization and transmit 100% of that of the perpendicular polarization. Where light is of an intermediate polarization it. In real life filters don't manage this. There are filters available now in both 'high extinction' & 'high transmission' variants. I believe this is a case of choosing between blocking a little of the light the filter should pass to ensure none of the light it should block gets through (high extinction) versus only blocking most of the polarized light it should, while passing all that of the opposite polarization. Normal filters will be somewhere in between these extremes.

There are also some odd old polarizers that are colour dependent, some just blocking polarized light in a specific colour band & passing other colours irrespective of polarization and a few others (probably made up of two of these) that let through red light polarized one way & blue light of the opposite polarization. I don't think these are made any more, certainly all those I've collected are at least 30 years old.

Using two crossed  normal polarisers gives a fairly good indication of how much light of each polarization is removed it not hard to get down to around 0.5% transmission in the visual with this approach. With all of my polarisers this raises to around 50% transmission by about 750nm - easily seen with several of my unmodified cameras and not dissimilar to a R72 filter on my modified bodies. To achieve the effect of crossed polarizers the front one must either be a linear polarizer or it must be turned around to be face to face with the rear one. Circular polarizers have a quarter wave plate added behind the polarizer to throwing out the polarization of the emergent light.

BTW things can get weird if you have 3 linear polarizers. Use 2 at 90 degrees to each other & nearly all the light is blocked. When the 3rd in an intermediate alignment is added between these (without touching either so they remain crossed) light transmission increases!

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