Calling analytical: more polarizer mysteries

Thanks Kelly, and I look forward to seeing your images.

Yes, I am planning on participating in the Studio Lighting event. However, again, I will probably not shoot the models like everybody else but am thinking of bringing my 3D rig to take advantage of the studio setting, the lighting (and: the crowd!) Not sure on that yet, though.
It should be packed!

The photograph is the perfect answer!

I'm not certain why the exposure meter doesn't work. I've read that meters don't always react well to light that is linearly polarized. I think they use a beam splitter, which can't always distinguish the correct brightness of linearly polarized light. This is why most manufacturers suggest using a circular polarizer with modern cameras. The camera works fine w/ a CPL, which is just a linear polarizer with a quarter wave plate. But the way you've described your set-up, the light that makes it through should be circularly polarized. Hmmm...

So, let's look at the ND filter you created. (BTW, I tried this myself, and the pictures were... interesting... I'll post them at some time). If light passes through a linear polarizer first, then passes through a CPL, it's basically passing through two linear polarizers, then a quarter-wave plate. If the linear polarizers are perfectly mis-aligned, no light should pass through. If they two are perfectly aligned, then you won't lose any more light than with a single linear polarizer, in theory. In both cases, any light that does make it through will be circularly polarized, so from what I understand, the camera should be able to auto-exposure from it.

I Googled this, and others seem to suggest that their auto-exposure did work using this method. I can't remember if it worked when I tried it. (See http://forums.dpreview.com/forums/readflat.asp?forum=1029&message=21676259&changemode=1)

If it's not working: (a) Maybe I'm wrong. (b) Maybe it's too dark and the camera is having a hard time finding the correct exposure. (c) Is it possible that you attached the filters in the wrong order, or you used two CPLs or linear polarizers. You could easily use two linear polarizers and get the same variable ND effect, and the camera wouldn't be able to correctly auto-meter the linear polarized light.

For the variable neutral density setup to work correctly, in theory it should look like:
light   ---> 1st passes through linear pol ---> 2nd passes through CPL ---> finally passes through the lens and into camera.

When this thread first came out, I think the person who posted the question had the filters in the wrong order. If this isn't the case for you, and you are using the setup above, then I'm at a loss.
Did this help anything?

I just tested this. You might try repeating this to see what you find:

1. Put CPL on camera.
2. Put Linear pol on camera. Tested to make sure "variable ND" filter was working. It was.
Note: the combo will never make a completely dark image. There will always be a bit of light that makes it through both polarizers if it's oriented correctly.
3. Tested on Manual mode. As I twisted the filter, the image went from lighter to darker. Etc.
4. Tested on P mode. (I shoot w/ an A33, so P mode sets exposure value automatically). For part of the time, I could tell it was adjusting exposure correctly because the shutter speed slowly changed. However, after a point, the camera couldn't compensate and the image became increasingly dark. I'm not sure if it became too dark for the auto exposure mode to work or if it had a tough time auto adjusting to the circularly polarized light.
5. I then tested the exposure system w/ just the individual filters, with the lens stopped down to F22 to dim the images as much as possible. The camera struggled with both, but more with the linear polarizer (no surprise).
6. What does this mean? Does it struggle just because there is so little light coming through that it's having a hard time adjusting? Maybe. But, like you, I think it's worse with the polarized light, even though it's circularly polarized. I don't understand the exposure system well enough to know why. But it seems to me that if you're going to try to use a "variable ND filter" you need to set white balance and exposure settings manually. I think the auto-exposure works well enough for some of the time, but when the light becomes too polarized, it continues to adjust, but it doesn't adjust enough.

Hmm... that's all I know right now. As much as I love the theory behind the "variable ND filter," in practice it seems terrible. In particular, the colors are terrible.