Could you use two polarizing filters as a variable ND filter?

BetterSense · Dec 30, 2008

With 800 speed film and cameras that only go to 1000, bright days tend to force small apertures, if you can get a shot off at all. You can use a ND filter, but who wants to carry a filter around with their everyday snapshot camera?

Suppose you installed a non-circularly polarized filter on top of a circularly polarizing filter....my experience suggests that turning the filters at different angles to each other will make the combo more opaque. Does this actually work with camera filters? Would it be neutral-density enough?

Do you really need circularly polarizing filters with manual-focus cameras? I thought polarization only confused autofocus, but my friend says that it effects exposure controls too.

Bob-D659 · Dec 30, 2008

Yes it works, but filter quality determines if there is a colour shift. It's been used on some types of microscopes for decades, but they also use very high quality filters as well.

Pol filters will effect some types of TTL metering, but not all.

Photo Engineer · Dec 30, 2008

Yes, but they must both have the same type of polarization. One cannot be linear and the other cannot be circular.

Also, this method cannot be used well for digital cameras due to their inherent sensitivity to IR. You end up with some very nice IR color photos.

PE

nickandre · Dec 30, 2008

You won't be able to go very low in the density. The minimum would be something less than (or possibly equal to)2x the filter factor for a single polarizer (I think, if it works). I don't know how efficient they would be, because half the light should be blocked by a polarizer yet it is significantly more. So theoretically you could reach the 1 stop mark with two stacked filters but in practice I don't think that will happen. You could try, or you could find slower film like Ektar.

Better yet by using just a 4 stop removable density filter you could achieve an effective ISO of 50, and remove it when needed. I don't know if variable will help, especially if it's not that variable. Why not just get slower film though?

Q.G. · Dec 30, 2008

The 1 stop mark is easily reached by one filter alone.

You should, theoretically, be able to completely block all light. But that is indeed too much to hope for in practice.
A problem is also that the light that manages to come through is coloured (blue).

You can indeed use a linear and a circular polarizer. As long as the linear polarizer is outermost.
Two circular polarizers will not work.

Photo Engineer · Dec 30, 2008

Well, I've gotten moire patterns with different style polarizers. I may have had them in the wrong position.

PE

Nick Merritt · Dec 30, 2008

If the camera has a semi-silvered reflex mirror so that the meter cell is actually reading through the mirror then you'll need to use a circular polarizer. The Leicaflexes, Miranda Sensorex, and Nikon F3, to name three different cameras I know of, have this requirement.

BetterSense · Dec 31, 2008

Why not just get slower film though?

My everyday carryaround snapshot camera typically goes a week or more on a roll. I need low-light and indoor performance because I don't carry a flash, so when I venture outside I run into trouble if it's bright out.

David A. Goldfarb · Dec 31, 2008

I think Spiratone actually used to sell a "variable ND filter" that was just two polarizers, one stationary and one rotating, mounted to be used in this way.

Cameras that use beamsplitters for metering, like Canons from about the mid-1970s need circular polarizers.

The thing is, if you're going to carry a second filter anyway, and it's really no big deal to carry two or three filters, you might as well make it an ND and do it properly than to try to use two polarizers, because a continuously variable ND filter is rarely necessary.

If you find yourself in this situation often, then just don't shoot 800 speed film as your standard film.

2F/2F · Dec 31, 2008

BetterSense said:
With 800 speed film and cameras that only go to 1000, bright days tend to force small apertures, if you can get a shot off at all. You can use a ND filter, but who wants to carry a filter around with their everyday snapshot camera?

Suppose you installed a non-circularly polarized filter on top of a circularly polarizing filter....my experience suggests that turning the filters at different angles to each other will make the combo more opaque. Does this actually work with camera filters? Would it be neutral-density enough?

Do you really need circularly polarizing filters with manual-focus cameras? I thought polarization only confused autofocus, but my friend says that it effects exposure controls too.

You can get a low-end second body and keep another film in it to solve your problem. There is no good reason not to have one these days, as they are literally cheaper than dirt. With what you will spend on all your filters, you could get a second body. It would probably come with a normal lens for under 100 bucks; perhaps *well* under. As an example, I have four Canon FD bodies (two F-1s and two FTbs). They are all either pro bodies or the next level down. I have spent a total of $550 on them! That includes the following lenses in superb condition: 17mm f/4 S.S.C., 28-90 Vivitar Series 1, 50mm f/1.4 S.S.C., 55mm f/1.2 S.S.C. , 200mm f/2.8 IF, not to mention a mint Speed Finder with case, Low Light Booster FT, two Viv. 283s, Power Winder F, Motor Dive MF, all in outstanding condition, and a Haliburton case for it all.

I have spent at least half that amount on filters just in the past few years!

Q.G. · Dec 31, 2008

You can use a ND filter, but who wants to carry a filter around with their everyday snapshot camera?

Just noticed that:
How, BetterSense, is not wanting to carry one filter to be reconciled with the idea of carrying two, to do the same job the single one would do better?

You can get a low-end second body and keep another film in it to solve your problem.

Indeed. Even more to carry though.

Simplest and cheapest would indeed be to get a good ND filter.

BetterSense · Dec 31, 2008

How, BetterSense, is not wanting to carry one filter to be reconciled with the idea of carrying two, to do the same job the single one would do better?

The two polarizing filters could stay on the lens if they were adjustable, so I wouldn't have to 'carry them around' separate from the camera.

I already have different bodies but I only bring one with me every day. A lot of people think dragging one camera around is weird enough.

Rol_Lei Nut · Dec 31, 2008

BetterSense said:
The two polarizing filters could stay on the lens if they were adjustable, so I wouldn't have to 'carry them around' separate from the camera.

You'd be losing at least a stop per filter, plus you'd always have to check their alignment.

Possible vignetting problems, esp. with wide-angle lenses.

Possible extra flare, reflections & quality degradation due to 4 extra air-glass surfaces.

Not a good idea IMHO....

Swapping out the fast film & replacing it with slow film when needed seems to be a much better option (you just need to note how many exposures you took, so you can advance it to that point - +2 - later).

Katier · Dec 31, 2008

I have to agree on the idea of a second body. That said surely 400ISO is plenty for most indoor work but if you find yourself going in and out all the time then the ideal solution most definitly would be to get yourself a second body.

PhotoJim · Dec 31, 2008

One other thing to bear in mind is that colour negative film can handle some overexposure.

Rate your 800 speed film at 400 or even 200 and experiment. I wouldn't be surprised if the results were more than acceptable.

You can even do this mid-roll but you may want to warn your lab as the density of the negatives will vary.

FilmLives! · Dec 31, 2008

Photo Engineer said:
Also, this method cannot be used well for digital cameras due to their inherent sensitivity to IR. You end up with some very nice IR color photos

Not understading that one, Photo Engineer. If Polarizing filters pass all visible wavelengths, why would stacking two together create issues with infrared? Thanks,

Jim

Photo Engineer · Dec 31, 2008

Jim;

Digital cameras are sensitive to visible and IR light. They have a blocking filter for IR to enhance visible sensitivity ratio.

If you block visible light with a polarizing overlay, you allow IR to increase in proportion to visible wavelengths. Since film is not ordinarily IR sensitive it has no bad effect on film, but enhances the Digital camera sensitivity to IR light. Therefore, you can take some quite good IR B&W and color pix with digital this way, albeit at very low speeds, about ISO 25 or lower.

Crossed polarizers pass low levels of visible light but almost all IR.

PE

FilmLives! · Jan 1, 2009

Photo Engineer said:
Jim;

If you block visible light with a polarizing overlay, you allow IR to increase in proportion to visible wavelengths. Since film is not ordinarily IR sensitive it has no bad effect on film, but enhances the Digital camera sensitivity to IR light. Therefore, you can take some quite good IR B&W and color pix with digital this way, albeit at very low speeds, about ISO 25 or lower.

Crossed polarizers pass low levels of visible light but almost all IR.

PE

Photo Engineer - Interesting. Looking at the spectral transmission graphs for a circular polarizer I agree, polarizers pass a lot of infrared light. The part I'm trying to wrap my brain around is why, with crossed polarizers, the reduction in visible light transmission through the filters would not also result in a similar decrease in the amount of infrared light passing through. I would expect that the reduction in light transmission would be about equal across the spectrum - therefore, wouldn't there also be about as much reduction in the amount of infrared passing through the filters as visible light? If not, why is that? Thanks,

Jim

Photo Engineer · Jan 1, 2009

Jim;

Crossed polarizers only act as ND filters in the region of the spectrum where they affect light. They do not affect light very much in the IR region as you have noted. The ratio of wavelengths blocked is important.

But, this is not a venue for discussing digital and I'm afraid that if we continue to discuss this it will pass the bounds of an analog forum. Sorry.

PE

Ray Rogers · Jan 1, 2009

FilmLives! said:
The part I'm trying to wrap my brain around is why, with crossed polarizers, the reduction in visible light transmission through the filters would not also result in a similar decrease in the amount of infrared light passing through.Jim

A good question.

What is the difference to a polarizer between visual light and IR ?

I have never looked into how a polarizer works, but I suspect that is the key.
There might be someone here who might has a working knowledge of how polarizers work... and can explain how wavelength is involved.

Is this a question for a physicist?

???

nickandre · Jan 1, 2009

Q.G. said:
Simplest and cheapest would indeed be to get a good ND filter.

Yes, but then you're shooting rather grainy film outside. Why not use a high resolution film like Ektar when you can? 35mm is grainy enough without 800 speed film.

Ralph Javins · Jan 5, 2009

Good morning, BetterSense;

In looking at the "filter factor" for a polarizing filter. usually it is about "2.5X," or a little more than one full "f stop." This is the amount you must open up your camera "exposure value" when using a polarizing filter, regardless of the speed of the film in the camera. You do that by opening up the lens one stop, or slowing down the shutter by one step, and you will be close enough for an "around 2X" filter factor. With really good linear polarizing filters, you can stack two of them together to make a variable neutral density filter that will run from about 20 % visible light transmission (a 5X filter factor) down to close to 1 % (a 100X filter factor). It is not easy to predict exactly how much you will get if you turn one filter relative to the other by "X" degrees. Maximum attenuation will be when the filters are rotated 90 degrees from each other. In any case, you will progably want to take the filters off the camera when you are shooting indoors or in low light.

Please note well that this is NOT ENOUGH to be used as a solar filter for looking at or photographng the sun. One of the reasons is the infrared or IR transmission characteristic of these filters; they do not attenuate very much IR.

Two additonal points:

(1.) I do not have any experience in stacking circular polarizing filters with linear polarizing filters, so I cannot comment on that subject.

(2.) If you are willing to carry two (2) polarizing filters, why have you not considered carrying one each only 4X or 8X neutral density filter that will be thinner and lighter than a single polarizing filter?

Steve Smith · Jan 5, 2009

During world war II Polaroid made variable density viewing goggles using this method. Two polarisers were used and a control varied the angle between the them to control the amount of light passing through.

Dead Link Removed

Steve.

BetterSense · Jan 6, 2009

(2.) If you are willing to carry two (2) polarizing filters, why have you not considered carrying one each only 4X or 8X neutral density filter that will be thinner and lighter than a single polarizing filter?

I had been assuming that they would be perfectly clear when aligned with each other, so that I could keep them on the camera all the time. But if they attenuate that much even by themselves, then I would have to take them off indoors and you are right, it makes no sense anymore. I might as well just carry an ND filter.

Which I cannot find anywhere BTW. First of all I can never seem to find ND filters, and then I have to choose which camera I want to buy it for, out of three. Is there some kind of universal filters where the plastic bit fits into different threads, so at least I could use it on all three of my cameras (all with 50mm focal lengths but varying threads).

Steve Smith · Jan 6, 2009

There are two options:

1. Buy the filters for the largest thread you have and buy step up adaptors for the other lenses.

2. Get a system like the Cokin or Lee filter holder which takes square filters. You will need an adaptor for the different thread sizes.

The Lee or Cokin systems are good for graduated density filters but if you just want a few neutral density filters, option 1 is simpler.

Steve.

Could you use two polarizing filters as a variable ND filter?

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