Vari ND filters act like normal polarizers!

[g.l]

We all know that variable Neutral Density filters (FaderND etc) are made up of two polarizers. But I was shocked to find that many (all?) commercial ones also act like normal polarizers, filtering out polarized skylight and reflections! In this video, I'm rotating a 7dayshop Variable ND filter - the entire filter, not just the front element. It removes the water reflections just like a normal polarizer! You can try this simple test (post your results): - hold the fader up to an LCD screen (must be LCD) - rotate the whole thing - if it gradually blocks out the LCD screen, your filter acts like a normal polarizer! It sucks! You probably don't want a fader to filter out reflections - even if you do, you can't adjust the filter angle (like with a normal pola) - once it's mounted you can't rotate it! Worse, as you fade the image, you are actually changing the pola filter angle! It sucks even harder for 3D shooting with 2 bodies - each fader usually screws on in a different rotation, so you may get different pola filtering on the sky & reflections! Why does it happen? Faders are made from two polarizers. If they are made using two circular polarizers - the problem goes away! (see tech details below). Instead it seems that most (all?) commercial faders are made with a circular at the back (for camera metering compatibility), and a linear at the front - the linear is causing the problem. Details Linear polarizers filter out polarized light, and change how they do it as you rotate them. Two linear layers do the actual fading. A Circular polarizer is made up of a linear layer, and a quarter-wave retarder layer. The retarder effectively unpolarizes polarized light - this is needed because some cameras' metering cannot cope with polarized light. So with a standard CPL, a linear layer is at the front and does the work, and the retarder at the back makes the filtered light unpolarized again. So a fader made with two circular filters (facing each other so the linear layers are on the inside) does not act like a pola filter - incoming light is unpolarized by the 1st retarder, then the two linears fade the image, then the last retarder unpolarizes it again. I've tested this and it works as it should. So, does anyone have a commercial fader that doesn't act like a pola filter?? Or can you recommend cheap CPL filters for a DIY fader that works without colour shifts or other issues?

 

[QuickHitRecord]

Just finding this now. Nobody had anything to say?!

This makes sense, however I wonder if there is a fader ND out there with two rotating elements so that you can first rotate the polarizer to the desired orientation, and then correct your exposure with the second ring...

 

[g.l]

Just finding this now. Nobody had anything to say?!

Guess not .

This makes sense, however I wonder if there is a fader ND out there with two rotating elements so that you can first rotate the polarizer to the desired orientation, and then correct your exposure with the second ring...

Interesting idea, not seen anything like it.

I did end up finding an ultra-cheap vari-ND from China (via Ebay) that didn't act as a polarizer - circular at the front, linear at the back. Absolutely required for my 3D rig where both images have to look identical (although you have to avoid closing them down too much or you get different cross patterns which ruins shots).

 

[QuickHitRecord]

I am about to go down the 4x4 ND + polarizer path, but the cost is just killing me.

Interesting idea, not seen anything like it.

Maybe I can buy two cheap faders, knock out the glass in one, and then glue and screw them together. What do you think?

I did end up finding an ultra-cheap vari-ND from China (via Ebay) that didn't act as a polarizer - circular at the front, linear at the back. Absolutely required for my 3D rig where both images have to look identical (although you have to avoid closing them down too much or you get different cross patterns which ruins shots).

Good solution. Which one was it?