Polariser filter

Hi all I understand that it is not ok to use an analogue polariser on a digital camera but is it ok to use a circular polariser on film ?.

Peter.

There's no law against it. The problem is in the metering not in some monstrous effect on the image.

clip from Popular Photography 2002-09

There are also some rules about stacking circular and linear polarizers -- just try things and see for yourself what happens.

WIth digital you don't have to wait for the film to come back..

Thank you for the reply, I am using a Mamiya RB67 pro SD and my metering is by handheld meter, I also use the Cokin filter system I already have a circular polariser and I have just found a linear polariser on ebay, so, I will be making some comparisons.

Thanks again. Peter.

Thank you for the reply, I am using a Mamiya RB67 pro SD and my metering is by handheld meter, I also use the Cokin filter system I already have a circular polariser and I have just found a linear polariser on ebay, so, I will be making some comparisons.

Thanks again. Peter.

Any polarising filter will be fine with your camera, but it is difficult to measure correctly with a handheld meter - because the light transmission depends on the angle of the filter.

A polarizing filter is most beneficial in that it mitigates reflections, increases saturation and contrast without impairing a color cast, makes clouds more vibrant, acts as a neutral density filter. It cuts haze because it filters UV. Early scientists trying to comprehend the double image they saw through a transparent Islandic Spar filter, wrongly assumed light had a north and south pole -- somewhat like a magnet. They labeled this action polarization; too late to change this name now.

Light photons travel following a wave motion pattern. Unlike water waves that ungulate up then down, light waves vibrate in all possible planes (unpolarized). These waves can become polarized (vibrating in restricted planes) after being reflected from some surfaces and/or traversing some transparent materials.

The polarizing filter acts like prison bars in that light waves can only navigate through, if the direction of vibration matches up with alignment of the bars. We adjust the polarizer for a desired effect by rotating it prior to snapping the shot.

Sorry to report that polarized rays can interfere with some of the automation of a modern camera (both digital and film). This interference is on a case-by-case basis; better to err on the side of caution. We mount a circular polarizer in lieu of a simple linear polarizer.

The circular polarizer is actually two filters sandwiched together. The upfront filter is a standard linear, the behind filter is called a “retarder”. The retarder undoes the polarization so that the exposing light has no impact on the camera’s automation. However, the upfront linear has done its deed, so we get the full influence of the polarization filter.

Any polarising filter will be fine with your camera, but it is difficult to measure correctly with a handheld meter - because the light transmission depends on the angle of the filter.

A polariser has a fixed correction factor, that provides correction for the light loss due to the material it is made of.

The polarising effect, selective (!) darkening of parts of the scene, is why you use the filter in the first place, and you do not need to correct that.

Some people do compensate for the overall effect of using a ploraiser (dulling of the scene) by adding a bit more brightness to it. But that is not required, and if you do not like it you should not use a polariser to begin with.

TTL meters do it automatically, because they do not understand that selectively losing light is what filters are used for. But that is a fault. Not something you should emulate when not using TTL-metering.

So it doesn't matter that it can be difficult (is it?) to measure with a handheld meter, because you do not need to. Just apply the filter factor that is on the rim of the filter.

A polariser has a fixed correction factor, that provides correction for the light loss due to the material it is made of.

The polarising effect, selective (!) darkening of parts of the scene, is why you use the filter in the first place, and you do not need to correct that.

 

Some people do compensate for the overall effect of using a ploraiser (dulling of the scene) by adding a bit more brightness to it. But that is not required, and if you do not like it you should not use a polariser to begin with.

TTL meters do it automatically, because they do not understand that selectively losing light is what filters are used for. But that is a fault. Not something you should emulate when not using TTL-metering.

 

So it doesn't matter that it can be difficult (is it?) to measure with a handheld meter, because you do not need to. Just apply the filter factor that is on the rim of the filter.

Doesn't the rotation angle setting affect the stops?

Using a circular polarizer with film is no problem.

AFAIK, TTL metering through a polarizing filter is no problem as well, regardless of the angle of the polarizer EXCEPT for Canon F-1, new F-1, FT, FTb, and FTbn. Because of their beam-splitter light meters, TTL metering is a PITA with a regular polarizer. The owner's manuals for these Canons recommend using a circular polarizer.

Doesn't the rotation angle setting affect the stops?

No. It affects the effect. Which is blocking light you want to have blocked.

And yes, if you, selectively,block more light,you get less light. But that's the entire purpose of the thing.

When you compensate for a greater effect of the polariser, you will overexpose the bits in the scene not affected by the filter. You do not want that.

The bits that aren't touched by the polariser filter do not need compensation for anything except the constant, and non-selective absorption of light by the filter material. Apply that and you will be fine. But stop there.

Using a circular polarizer with film is no problem.

 

AFAIK, TTL metering through a polarizing filter is no problem as well, regardless of the angle of the polarizer EXCEPT for Canon F-1, new F-1, FT, FTb, and FTbn. Because of their beam-splitter light meters, TTL metering is a PITA with a regular polarizer. The owner's manuals for these Canons recommend using a circular polarizer.

It can (!) be a problem in all metering systems that have a reflecting surface in the path the light has to take to reach the sensor. And not just those Canon cameras direct light to the sensor using reflecting surfaces. Whether that is a problem depends on the nature of the reflecing surface.

A CPL filter, however, is always a safe choice. Whether needed or not, it does the job.

The main difference is that modern CPL filters tend to be multicoated and absorb slightly less light than old linear polarisers, which AFAIK haven't been in production for about 20 years.

I've used linear polarisers - simply because I had 'em - on several DSLRs and to be quite honest I could see no adverse affect on either the metering or AF. So don't believe all the advertising BS put about!

Nobody will die and nothing will catch fire from using a linear pol on a digital camera. :rolleyes:

Same goes for using a CPL on film - it'll work identically to a linear polariser, except maybe giving you +1/3rd stop more light, thanks to more efficient polariser foils and better AR coating.

For DSLRs with anti-aliasing filters, those are made with birefringent material, and so are affected by linear polarized light.

For most scenes, you might not notice, or might only notice looking very carefully.

Simpler digital cameras depend on the lens not to be able to resolve enough detail for aliasing,

and so don't have anti-aliasing filters. (They might also not have a way to hold filters.)

For DSLRs with anti-aliasing filters, those are made with birefringent material, and so are affected by linear polarized light.

Any polarising filter only has an effect on already polarised, or partially polarised light. Therefore, if an anti-aliasing filter was noticeably affected by polarised light, then that effect would also be noticeable with light that was naturally polarised; e.g. blue sky, reflections off water, glass, foliage etc.

Additionally, a Circular Polarising filter uses a bi-refringent 1/4 wave plate sandwiched with a linear polarising membrane. The nett result being almost identical to that of linear polarised light passing through a bi-refringent material in close proximity to the sensor.

In short, there should be no difference at the sensor between naturally polarised light, artificially (linear) polarised light, and that from a circular polarising filter consisting of a sandwich of linear polariser followed by a bi-refringent retardation plate. All will tend to nullify, or partially nullify, the beam-splitting effect of a singly bi-refingent anti-aliasing filter.

FWIW, Nikon's explanation of the action of their anti-aliasing filter is that it consists of two layers of bi-refringent material with their axes at right-angles to each other. This arrangement should result in a total 180 degree polarisation phase shift that's largely immune to incident polarisation effects, while simply producing four spatially shifted images.

No. It affects the effect. Which is blocking light you want to have blocked.

And yes, if you, selectively,block more light,you get less light. But that's the entire purpose of the thing.

 

When you compensate for a greater effect of the polariser, you will overexpose the bits in the scene not affected by the filter. You do not want that.

The bits that aren't touched by the polariser filter do not need compensation for anything except the constant, and non-selective absorption of light by the filter material. Apply that and you will be fine. But stop there.

Your post is very confusing to me. First you sayu NO than you say yes.

The instruction for a polarizing filter says to adjust let's say between 1 1/4 to 1 3/4 stops. How do you do that in the real world? What parameters do you use?

Your post is very confusing to me. First you sayu NO than you say yes.

 

The instruction for a polarizing filter says to adjust let's say between 1 1/4 to 1 3/4 stops. How do you do that in the real world? What parameters do you use?

I use the fixed correction factor that is usually printed on the rim, and nothing else.

And the answer is no. Do not compensate for the variable effect of the filter. See my earlier reply.

I use the fixed correction factor that is usually printed on the rim, and nothing else.

And the answer is no. Do not compensate for the variable effect of the filter. See my earlier reply.

Usually the effect of the filter is small enough that you don't need to allow for it, but if a significant amount of the incoming light is all polarised in the same direction you may need to compensate if not using TTL metering (self compensating)

On occasion I've had reflections from water that provided far more than half of the light. Arrange the polariser one way & expose for the reflection, twist it by 90 degrees & you have to expose for the dim pond floor...

Hi all I understand that it is not ok to use an analogue polariser on a digital camera but is it ok to use a circular polariser on film ?.

Peter.

As far as I know there is no such thing as a digital polariser. Newer filters sold for digital may have better coatings but they still polarise light using real light before any digital to analogue conversion. Polarisers themselves can be high extinction or high transmission types, then there is a linear/circular choice depending on if a quarter wave plate is added. It doesn't work well combining two circular polarisers (at least unless you switch one round - two CPLs back to back give very weird effects, front to front they can work like a variable ND).

You can get perfectly good results using a 50 year old linear polariser on a digital camera, my grandfathers old 'pile of plates' polariser (probably over 100 years old) would also work if it was a bit wider - less than 1/2" diameter is just too small for my lenses.

FWIW circular polarisers came out long before digital photography was practical, yes they work fine with film as that's what they were originally made for! I think my first CPL was brought in the mid 1980's

Usually the effect of the filter is small enough that you don't need to allow for it, but if a significant amount of the incoming light is all polarised in the same direction you may need to compensate if not using TTL metering (self compensating)

On occasion I've had reflections from water that provided far more than half of the light. Arrange the polariser one way & expose for the reflection, twist it by 90 degrees & you have to expose for the dim pond floor...

And that is what you want the polarizer to do. If not, the choice is not to add exposure (and overexpose the unaffected part), but use less of the polarizing action. Or none of it.

And that is what you want the polarizer to do. If not, the choice is not to add exposure (and overexpose the unaffected part), but use less of the polarizing action. Or none of it.

Indeed it's what you what you use it for, but it's effect does require the effect to be compensated, or the pond floor will be invisible.

Indeed it's what you what you use it for, but it's effect does require the effect to be compensated, or the pond floor will be invisible.

No. Only the general/overall absorbance, unrelated to effect, needs compensation.

That different parts of a scene need different exposure to end up in the desired tone/brightness is a diffetent matter, completely separated from selective filtering.

When all theory has been expounded, do the friggin' experiment!

Here's a set of comparisons between a linear and circular polarising filter, on all the DSLRs I could lay my hands on:

1. An old Mk1 Canon 5D fitted with 50mm f/1.8 Canon 'plastic fantastic' lens -

2. Almost equally old Nikon D700 fitted with 50mm f/1.8 AF-D Nikkor -

3. Nikon D800 with same 50mm AF-D Nikkor -

4. Nikon 'DX' D7200 with 35mm f/1.8 AF-S lens -

5. Sony A7Riv with 50mm f/1.8 AF-D Nikkor lens via adapter (manual focus) -

All the above are SOOC JPEGs with no post work done apart from resizing and adding title text. All shot at f/4, ISO 100, Aperture priority, and AWB. Except the D7200, which was shot at f/2.8 to equalise the D-o-F and the D700 that was set to its base ISO of 200.

Surprisingly, there is no discernable difference between the linear and CPL filter shots taken with the same camera, despite there being a slight visual tint and density difference when looking directly through the filters. I guess AWB and auto-exposure really do work to level out any difference!

I was also surprised to find that the CPL filter was visually denser and gave an average 1/2 stop slower shutter speed, but as you can see the end results are difficult to tell apart.

WRT AF: None of the cameras struggled to find focus, or missed focus enough to worry about. Although focus speed seemed a bit slower with either polariser fitted. I put this down to having a dimmer, and maybe less contrasty image to focus on.

I deliberately threw the lens well OOF before each shot, and let the camera AF do its thing - except for the adapted lens on the Sony, which was manually focused.

Here are 100% crops of the AF focus area from each camera:

And to round things out the cropped focused area from the Sony MILC.

Conclusion: The older DSLRs with AA filters did show quite a dramatic colour shift when fitted with a polariser. However the type of polariser appears to make little to no difference.

Also, with the cameras tested, there were no severe side effects from using a linear polariser. In fact the extra half stop of light from the linear filter might be advantageous.

So based on the above results, it's my opinion that reports of the death of the linear polariser are grossly exaggerated. Further, the spread of fear and panic over using linear polariser on DSLRs and AF/AE film cameras was IMHO largely unfounded, and a cry of wolf to stimulate sales of 'upgraded' filters.

Maybe the above experiment should have been done years ago?

Oh, P.S. The filters used were an old Nikon Linear polariser and a slightly newer Hoya CPL.

The D7200 seemed to be focused the best with the most contrast separation between sky and clouds. Maybe lower exposure saturated the colors more.

The D7200 is the only one of those DSLRs that doesn't have an AA filter Alan. And besides, there's a bit of white cloud behind the tree branches that's helping the contrast for focusing.

I was dogged by a stiff, blustery breeze that moved the clouds constantly and blew the tree branches about that I was using to Auto Focus on. So I wouldn't draw too many conclusions about absolute AF performance.

The point is that the AF on each camera worked equally well (or poorly) between no-filter and either flavour of polarising filter.

Myth busted - in my opinion.

(snip)

 

FWIW, Nikon's explanation of the action of their anti-aliasing filter is that it consists of two layers of bi-refringent material with their axes at right-angles to each other. This arrangement should result in a total 180 degree polarisation phase shift that's largely immune to incident polarisation effects, while simply producing four spatially shifted images.

OK, first, the big problem with aliasing is that it can interact with the Bayer filter.

It can generate false colors with high spatial frequencies. Monochrom digital cameras won't have that problem.

The explanation for the D800 and D800E is here:

https://cdn-10.nikon-cdn.com/Images/Learn-Explore/Camera-Technology/D-SLR/2012/Moire-D800-D800E/Media/OLPF_schematic.pdf

For the D800, the first birefringent sheet separates into two spots depending on polarization. If the incoming light is horizontally or vertically polarized, it will only get one spot.

(My thought would have been to do it at 45 degrees, but it doesn't say that.)

Then there is a λ/4 wave plate to convert those spots into circular polarization. The second birefringent sheet separates those into vertical spots.

If the first step doesn't separate the spots, you could get aliasing in that direction.

And yes naturally polarized sources will cause effects, though most aren't so strongly polarized as filters.

And maybe with a little luck, strong polarization and high spatial frequency don't tend to occur together.

While we all like the super high resolution, not so many subjects have those high spatial frequencies.

The D800E is interesting. One might guess that they just leave out the birefringent sheets, but I suspect that the rest of the system is based on their thickness.

So, they remove the λ/4 plate, and replace it with a plain glass sheet, and then put the second birefringent sheet to undo the first one.

There are then some recommendations to reduce aliasing, such as stop down the lens a few stops, so diffraction will increase the spot size.

OK, first, the big problem with aliasing is that it can interact with the Bayer filter.

It can generate false colors with high spatial frequencies. Monochrom digital cameras won't have that problem.

 

The explanation for the D800 and D800E is here:

 

https://cdn-10.nikon-cdn.com/Images/Learn-Explore/Camera-Technology/D-SLR/2012/Moire-D800-D800E/Media/OLPF_schematic.pdf

 

For the D800, the first birefringent sheet separates into two spots depending on polarization. If the incoming light is horizontally or vertically polarized, it will only get one spot.

(My thought would have been to do it at 45 degrees, but it doesn't say that.)

 

Then there is a λ/4 wave plate to convert those spots into circular polarization. The second birefringent sheet separates those into vertical spots.

 

If the first step doesn't separate the spots, you could get aliasing in that direction.

 

And yes naturally polarized sources will cause effects, though most aren't so strongly polarized as filters.

And maybe with a little luck, strong polarization and high spatial frequency don't tend to occur together.

While we all like the super high resolution, not so many subjects have those high spatial frequencies.

 

The D800E is interesting. One might guess that they just leave out the birefringent sheets, but I suspect that the rest of the system is based on their thickness.

So, they remove the λ/4 plate, and replace it with a plain glass sheet, and then put the second birefringent sheet to undo the first one.

 

There are then some recommendations to reduce aliasing, such as stop down the lens a few stops, so diffraction will increase the spot size.

That's all very well, but it's still possible to provoke moire effects with the D800. For example, by copying half-tone dots at a magnification where the dot spacing is close to the photosite spacing.

I suspect that the lack of efficacy of any AA filter that doesn't also totally negate a high pixel density, is the reason that manufacturers simply dropped the inclusion of AA filters altogether.

Whatever. This is wandering way off topic from the linear v circular polariser debate/mythology. The type of which, as shown above, seems to make very little difference to either camera operation or sensor image.

And I'm not about to buy a Canon Pellix or Sony a77 to find exceptions to the rule.

BTW, kudos for finding the Alt-graphics key combo to get the lambda symbol!