Using Crossed Polarizers for a Variable ND Filter

Strong neutral density filters are great for landscape photography when you want to allow motion blur to occur. But they can be expensive and hard to find.
Lately I’ve been exploring using longer exposures in my landscape photography to produce motion blur in waves, trees, clouds and flowing water. I’ve been testing a number of heavy neutral density filters (more about this in some other articles). For those who do not shoot this way, let me explain. Some movements, such as a waterfall or water cascading over rocks in a stream may require a shutter speed of 1/2 a second or longer to create that pleasing blur that you often see. Things like turning the ocean flat or making clouds into streaks in the sky can require exposures in the 30 second to two minute range, or longer and so on. Such exposures can’t be obtained simply by stopping the camera lens right down when the light is bright ad we cannot always return when it is late in the day. Also using a small aperture forces a large depth of field, which you may not actually want. The solution is to use a strong neutral density filter to cut the light levels so that you can use a combination of ISO, aperture and shutter speed that you want to achieve the effect you desire.

In researching all this I found the Singh Ray Vari-ND filter, that goes from 2 to 8 stops of exposure effect as you rotate the filter. Sadly it is a fairly expensive filter and only available in 77 and 82mm sizes. So I set out to work out how this was done. The hint was on their site when it said that you could use it with other filters but not polarizers. Photographers have known about the two-polarizer effect for ages and I guessed that this was what they were doing. Indeed it is used in photo-microscopy to photograph certain types of specimen against a nice black background. A polarizer only allows through light whose rays are vibrating in a certain direction. It blocks others. Put two polarizers together and rotate them so their transmission directions are 90 degrees apart and all the light is blocked. In practice it massively attenuates the light but may allow a little through because they are not perfect.

So I grabbed two polarizers I had and tried it. No result. The reason was because they were circular polarizers. A circular polarizer has a normal linear polarizer at the front and a rotating plate closest to the camera that redistributes the ray vibration directions so that the light is effectively no longer polarized. This is done because linear polarized light can interfere with some camera autofocus and exposure systems. Two circular polarizers didn’t work because the light was “de-polarized” by the first one.

Next step was to buy two linear polarizers and sure enough they work. But this could cause problems with some cameras. So the ideal answer is this: stack a linear polarizer with a circular polarizer, just make sure the circular polarizer is the one closest to the camera lens. The first, linear polarizer will polarize the light. The second, circular polarizer, will use its linear polarizer component to create the variable neutral density effect and then its “de-polarizer” will prepare the light before it goes into the camera lens. To make life hopefully easier I used the same brand filter for all the tests, Hoya.

Using two polarizers for variable ND filter

So I now had a working arrangement that would function on my camera with no dramas. The issue now was to test by shooting and see how it went.

Without the polarizers, the exposure was f6.3 and 1/320 second. With both polarizers attached and set for minimum absorption the exposure was f6.3 and 1/40 second, giving us a 3 stop reduction to start with. I taped the back polarizer so it could not rotate and then started turning and shooting as the filter combination got darker. As you can see from the shots below, all made at the same f6.3 f-stop, we went from a 3-stop reduction to a 10+ stop reduction in light.

No filter, 1/320 second
Using two polarizers for variable ND filter

Minimum 3 stops, 1/40 sec
Using two polarizers for variable ND filter

Approx. 4 stops, 1/30 sec
Using two polarizers for variable ND filter

5 stops, 1/10 sec
Using two polarizers for variable ND filter

7 stops, 1/2 sec
Using two polarizers for variable ND filter

8 stops, 1 sec
Using two polarizers for variable ND filter

9 stops, 2 sec
Using two polarizers for variable ND filter

9+ stops, 2.5 sec
Using two polarizers for variable ND filter

10+ stops, 5 sec
Using two polarizers for variable ND filter

10++ stops, 6 sec
Using two polarizers for variable ND filter

You will notice that at 9 stops there is a color shift starting to occur. By 10 stops this is extreme. So what I found was that these polarizer’s are not perfect, which makes sense because Hoya would not have chosen the polarizing material with this application in mind. The filters are obviously not blocking across all visible wavelengths evenly when crossed and is letting through more at the blue/violet end. Singh Ray have obviously chosen polarizing material to make their filters that are not as strong polarizer’s (hence the two stop minimum) and only get to eight stops when at 90 degrees with no color shift. Incidentally I also tried this on my IR converted camera and it did not work really there, but more on this in a separate article.

The Singh Ray Vari-ND is a perfect solution for those using wideangle lenses, since the single filter thickness minimizes vignetting
issues. But you can also work quite well by stacking two polarizers as I have, especially if you use larger filters
and a step-up ring or work primarily with only moderate wide to telephoto lenses. The exact result you get will depend on the polarizers you use and you may well find no color shift at the most extreme end. Some may be concerned about the effect on image quality. The answer is that if you use good filters and they are clean the effect will be minimal. Given the cost of polarizing filters is not high I would recommend dedicating two filters to this if you like this effect. That way you need not separate the filters in the field and thus can keep in the inner surfaces spotless.

Now as you rotate the filters from minimum position you will get little effect to start with and much greater effect with small movements as you move closer to 90 degrees. You can see this on the graphs below, but especially on the second one calibrated in stops. These graphs are for the ideal polarizer.

Using two polarizers for variable ND filter

Using two polarizers for variable ND filter

What is really amazing, as the shots below show, is that if you set a moderate reduction when you then rotate the combination you can vary the polarization effect on, say, the sky or reflections. This is an additional creative bonus.

Using two polarizers for variable ND filter

So I now have a new creative possibility in my camera bag when I go out to shoot, a variable neutral density filter combination that also gives me polarizer effects.
 
 
 

Leave a Reply

  1. Great to see some more detail on this concept along with real world examples, many thanks.

    I’m interested in trying this, but the filters I have don’t have any marked graduations. Can you publish the calculations behind the 2 graphs so I can calculate the correlation between degrees and stops so I can mark up my filters to indicate the relative angle change between them to obtain the approximate desired effect?

  2. Tape one filter so it can’t move and then set the other for the minimum darkening of the sky 90 degrees from the sun. Put a mark on both filters to set the zero point. That’s your start point. Maximum darkening of the filter combo is 90 degrees from this setting.

    Note that how well it works depends a lot on the quality of your filters.

  3. Tape one filter so it can’t move and then set the other for the minimum darkening of the sky 90 degrees from the sun. Put a mark on both filters to set the zero point. That’s your start point. Maximum darkening of the filter combo is 90 degrees from this setting.

    Note that how well it works depends a lot on the quality of your filters.

  4. Hello,

    thank You for this nice article! It is the first article with examples of color shift, I have found after weeks of looking information about this theme. Two comments:

    1) Which Hoya filter line did You use? STANDARD, HMC, SUPER HMC, PRO1, PRO 1DIGITAL? These lines should differ by quality, although I have no information, that color shift is part of this difference.

    2) As far as I know, Singh Ray Vari ND added from 2 to 8 densities with no color shift. At these densities, there is no visible color shift also in Your case of PL and PL-CIR from Hoya, as You shown above. So it looks like PL and PL-CIR could be the same quality as Singh Ray one. Am I right?

    3) Moreover, Sing Ray offers “Mor Slo” ND 5 density filter (looks like renamed common ND filter) to achieve Vari ND density from 7 till 13. See “http://www.singh-ray.com/morslo.html”. So I guess, that Singh Ray is aware about color shift even in its Vari ND filter more that 8 density and it is solving it with additional common ND filter.

    4) Singh Ray offers also Vari ND Duo filter, which adds “warmer” colors to the photo. Could it be prevention to the blue color shift, aven hardly noticeable? See “http://www.singh-ray.com/varinduo.html”.

    Stepan

  5. Hello,

    thank You for this nice article! It is the first article with examples of color shift, I have found after weeks of looking information about this theme. Two comments:

    1) Which Hoya filter line did You use? STANDARD, HMC, SUPER HMC, PRO1, PRO 1DIGITAL? These lines should differ by quality, although I have no information, that color shift is part of this difference.

    2) As far as I know, Singh Ray Vari ND added from 2 to 8 densities with no color shift. At these densities, there is no visible color shift also in Your case of PL and PL-CIR from Hoya, as You shown above. So it looks like PL and PL-CIR could be the same quality as Singh Ray one. Am I right?

    3) Moreover, Sing Ray offers “Mor Slo” ND 5 density filter (looks like renamed common ND filter) to achieve Vari ND density from 7 till 13. See “http://www.singh-ray.com/morslo.html”. So I guess, that Singh Ray is aware about color shift even in its Vari ND filter more that 8 density and it is solving it with additional common ND filter.

    4) Singh Ray offers also Vari ND Duo filter, which adds “warmer” colors to the photo. Could it be prevention to the blue color shift, aven hardly noticeable? See “http://www.singh-ray.com/varinduo.html”.

    Stepan

  6. Can I ask You, what will happen, if You rotate these two filters to the minimum light transmission position = maximum light absorbtion (relative 90 degrees position between CPL a LPL)? Will it become Infra Red filter? Or is it pure theory? Or is it wrong theory? Which residual color it has? Unwanted blue one? Or wanted brown-yellow one? Thank You, Stepan

  7. Can I ask You, what will happen, if You rotate these two filters to the minimum light transmission position = maximum light absorbtion (relative 90 degrees position between CPL a LPL)? Will it become Infra Red filter? Or is it pure theory? Or is it wrong theory? Which residual color it has? Unwanted blue one? Or wanted brown-yellow one? Thank You, Stepan

  8. Wayne,
    I just bought a Tiffen SR polarizer which is advertised as a linear polarizer to put over my Hoya circular polarizer. I get no change upon rotation. Any idea why? Is the Tiffen not a true linear polarizer??? I am familiar with what I should see (that’s how I check to see if a pair of sunglasses is really polaroid before I buy them). Also, I am curious if you have tried adjusting the color shift with a custom white balance adjustment using a gray card on a digital camera. Can WB adjustment compensate for the color shift? I was going to try this but, can’t make my polarizer stack work.
    John

  9. Wayne,
    I just bought a Tiffen SR polarizer which is advertised as a linear polarizer to put over my Hoya circular polarizer. I get no change upon rotation. Any idea why? Is the Tiffen not a true linear polarizer??? I am familiar with what I should see (that’s how I check to see if a pair of sunglasses is really polaroid before I buy them). Also, I am curious if you have tried adjusting the color shift with a custom white balance adjustment using a gray card on a digital camera. Can WB adjustment compensate for the color shift? I was going to try this but, can’t make my polarizer stack work.
    John

  10. 1. My filter is a standard Hoya. I am not sure that they make the R-72 in the other lines.

    2. Seems about right. I do know from some research that pol filters vary greatly in their effects at all wavelengths.

    3. Yes, it does seem like that.

    4. Yes, I wondered about that too. Probably they have combined a slight red filter in the pack to get the effect.

  11. 1. My filter is a standard Hoya. I am not sure that they make the R-72 in the other lines.

    2. Seems about right. I do know from some research that pol filters vary greatly in their effects at all wavelengths.

    3. Yes, it does seem like that.

    4. Yes, I wondered about that too. Probably they have combined a slight red filter in the pack to get the effect.

  12. I don’t consider Hoya’s as wrong polarizers. The point is if they are not chosen for the purpose, like the ones used in dedicated variable ND filters, then you may have this issue.

  13. I don’t consider Hoya’s as wrong polarizers. The point is if they are not chosen for the purpose, like the ones used in dedicated variable ND filters, then you may have this issue.

  14. I got an unwanted blue tone but my understanding is that this varies very much depending on the brand of polarizer you have.

  15. I got an unwanted blue tone but my understanding is that this varies very much depending on the brand of polarizer you have.

  16. HI John,
    If you are using a circular and a linear, the circular must be the one closest to the camera lens. I have found that to work.

    The custom white balance should work to get rid of the blue cast, but I haven’t tried it.

    Cheers,

    Wayne

  17. HI John,
    If you are using a circular and a linear, the circular must be the one closest to the camera lens. I have found that to work.

    The custom white balance should work to get rid of the blue cast, but I haven’t tried it.

    Cheers,

    Wayne

  18. Wayne,

    Thanks for the reply.

    No that’s not it, I am orienting the filters properly…with my Hoya, if put in front of an LCD display or looking through polaroid sunglasses, when rotated goes completely black. My Tiffen linear polarizer had no change during this test nor does it have any effect in front or behind the Hoya circular polarizer (I also have flipped the lenses just in cas Hoya had somehow been placed in the filter ring backwards). In other words I have tried every orientation combination I can with both lenses. Also my Nikon D60 seems to focus fine with the linear polarizer in place by itself (which I didn’t think should work but, maybe it just impairs autofocus accuracy). Anyway, all the tests I do tell me the Tiffen is not really a polarizer??? Perhaps I have a defective filter. Very strange.

    John

  19. Wayne,

    Thanks for the reply.

    No that’s not it, I am orienting the filters properly…with my Hoya, if put in front of an LCD display or looking through polaroid sunglasses, when rotated goes completely black. My Tiffen linear polarizer had no change during this test nor does it have any effect in front or behind the Hoya circular polarizer (I also have flipped the lenses just in cas Hoya had somehow been placed in the filter ring backwards). In other words I have tried every orientation combination I can with both lenses. Also my Nikon D60 seems to focus fine with the linear polarizer in place by itself (which I didn’t think should work but, maybe it just impairs autofocus accuracy). Anyway, all the tests I do tell me the Tiffen is not really a polarizer??? Perhaps I have a defective filter. Very strange.

    John

  20. Wayne,

    I just received a Hoya Linear Polarizer I ordered on Ebay….works just fine (darkens perfectly) with my Hoya Circular Polarizer. Don’t know what’s up with the Tiffen LP…I’d swear it’s an ND filter in the wrong ring!

    Thanks for the article … can’t wait to try this out on some moving water.

    John

  21. Wayne,

    I just received a Hoya Linear Polarizer I ordered on Ebay….works just fine (darkens perfectly) with my Hoya Circular Polarizer. Don’t know what’s up with the Tiffen LP…I’d swear it’s an ND filter in the wrong ring!

    Thanks for the article … can’t wait to try this out on some moving water.

    John

  22. I am curious do you have an intesert in astro-photograpy? In which infrared, radio and gamma ray photography is used to reveal beautiful things of the universe. A book called Visions of the Universe includes some stunning and beautiful photographs of the universe in other wavelenghths of the electromagnetic spectrum. It really is amazing that there is so much more to see beyond visible light!