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11/05/2007 07:34:27 PM · #1 |
| I love using a polarizer and I find many times I need to put on my 2 stop ND filter as well to get the shutter slow enough to accomplish my desired effect. Instead of stacking things, do polarizers come in 4-stop or 6-stop versions? If so, has anybody actually used one? |
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11/05/2007 07:42:33 PM · #2 |
| AFAIK, they do not. I can see why not. Normally, the loss of light is considered a negative side-effect of a polarizer. Also, making polarizers with varying degrees of additional ND would result in numerous part numbers. Stacking isn't desirable either, though :-P |
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11/05/2007 07:45:49 PM · #3 |
| Out of curiousity (dummy question here), do they come in less than 2 stops? Like is there such a thing as a transparent polarizer? |
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11/05/2007 08:06:09 PM · #4 |
Originally posted by ursula:
Out of curiousity (dummy question here), do they come in less than 2 stops? Like is there such a thing as a transparent polarizer? |
No on that one too. ;-) |
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11/05/2007 08:07:43 PM · #5 |
Originally posted by scarbrd:
Originally posted by ursula:
Out of curiousity (dummy question here), do they come in less than 2 stops? Like is there such a thing as a transparent polarizer? |
No on that one too. ;-) |
But better brands eat less light. I think Nikon one is 1.5 stops |
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11/05/2007 08:12:22 PM · #6 |
| Again out of curiosity (and because I can't find the answer right now), why do you have to have the colour in polarizer filters for cameras? Why couldn't they be transparent? |
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11/05/2007 08:22:52 PM · #7 |
Originally posted by ursula:
Again out of curiosity (and because I can't find the answer right now), why do you have to have the colour in polarizer filters for cameras? Why couldn't they be transparent? |
Good question!
A simple linear polarizer just passes light of one polarization (vertical or horizontal, for example) and rejects light of the other polarization (a polarization 90° different from the one it passes). So in a scene with random polarization (equal amounts of all directions of polarization) you'll lose half the light.
So in the end, the least light you can theoretically lose is 50%, or 1 stop. Most polarizers wind up around 1.5 stops on average, but the amount actually depends a lot on just how much polarized light there is in a given scene.
A circular polarizer is just a linear polarizer with an added 1/4-wave plate behind it. The 1/4-wave plate takes 50% of the incoming light and rotates its polarization 90°, thereby giving the AF sensor an approximation of unpolarized light to work with. You tend to lose a little more light from this extra surface, but the effect is minimal compared to the linear polarizing element. |
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11/05/2007 08:26:11 PM · #8 |
| I have heard you can put two polarizers on and dial them to almost black out the light. Maybe that is something you should try to reduce the light. I don't know what other issues may arise though, I have never tried it. |
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11/05/2007 08:32:56 PM · #9 |
Originally posted by kawesttex:
I have heard you can put two polarizers on and dial them to almost black out the light. Maybe that is something you should try to reduce the light. I don't know what other issues may arise though, I have never tried it. |
I think front one has to be linear to do this. |
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11/05/2007 08:41:45 PM · #10 |
Last time I saw two circular polarizers stacked, it resulted in wild color shifts as the outer one rotated. It was like going nuts with the Hue/Sat slider; the image took on different casts.
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11/06/2007 07:49:44 AM · #11 |
Originally posted by Nikolai1024:
Originally posted by kawesttex:
I have heard you can put two polarizers on and dial them to almost black out the light. Maybe that is something you should try to reduce the light. I don't know what other issues may arise though, I have never tried it. |
I think front one has to be linear to do this. |
Yes, it does. The 1/4-wave plate on the back of a circular polarizer would make it less effective for this purpose. You want a linear polarizer in front, to knock out one polarization, and a circular polarizer in back, to knock out as much of the other polarization as desired *plus* put back some controlled depolarization so AF will still function.
If AF function is not a goal (and it may not be with this much ND) then two linear polarizers may be used. |
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