Dual Polarizer

[orpheus]

Hi everyone,

 

Just a mindless stupid question. I was just fooling around with my

extra polarizer that I got from a lens package, and happen to

remember my high school Physics lessons on optics. I've been looking

for some good deals on neutral density filters, but just happen to

think of using two polarizers to make a variable ND filter, since you

can control the light intensity coming through. Of course, I know

the intended use for polarizers was not for that, but do you guys

think that would be a feasible way of creating a ND filter of any f-

stop? Perhaps I'm overlooking some higher optics theories of

polarizers...Aside from the problems of vignetting or removing WANTED

reflections, would that cause any major problems in any other respect?

 

Peter

[duane_kucheran]

Yes this works. I tried it out once. The combination is sensitive to the polarizations of the various light sources in the image so it may take a bit of fiddling. If memory serves it also had a limited attenuation of maybe 3-4 stops (ND 1.0).

 

B+W used to make a variable ND filter based on this principle.

 

BTW, make sure the 1st polarizer is a linear type. If it is the circular type (with 1/4 wave plate) then you will not see a variation in attenuation as it is rotated.

 

Cheers,

[jbq]

Yes, it works, but you need the front filter to be a linear polarizer.

[jiayao_zhao]

If you use AF camera, LPL may change your AF results.

[jbq]

Jiayao: what matters for those cameras is that the rear polarizer should be a CPL, so that the light hitting the sensors is not linearly polarized. But you still want the front polarizer to be a linear polarizer (if you use a circular polarizer for the front filter, the light hitting the rear filter will be linearly polarized, and you won't get the "variable ND" effect, the rear polarizer will simply act as a pure ND filter, regardless of the relative orientation of the filters).

[david_h._hartman]

Duel polarizers make a poor excuse for a neutral density

filter. Each polarizing filter has two glass disks for a total of

four. In between is a gel filter, a circular polarizer has a

quarter wave plate combined with the polarizing gel. The total

number of layers will be six or seven. Do you want all that in

front of your lens? Besides all the layers you may have a color

shift.<br>

<br>

If you have two polarizing filters (at least one being linear)

play with it and have fun, otherwise buy a quality ND filter,

there are plenty to chose from. Here is a sample of what is

available in 62mm...<br>

<br>

B&W -1, -2, -3, -6, -10, -13, -20<br>

Hoya -1, -2, -3, -9<br>

<br>

Regards,<br>

<br>

Dave.

[stevef]

These actually exist commercially although I don't know how common they are. Here is a link to one.

 

http://www.thecamerahunter.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=TCH&Category_Code=SFN

[kevin_kyyro]

I was screwing around with two circular polarizers and this is what happened:

http://www.flickr.com/photos/kevincairo/772137868/

I think it's pretty cool.

[ggcadc]

I did this and got some great shots this weekend, i saw many people recommending a "cheap" set of oplarizers to do this with, and they got alot of color shift.. i did this with some nicer filters and got a better result... check it out:

 

currently the first four photos are shot using this technique