[JeanChris]

Hi all

I've just noticed some strange interference when staking a grad filter and a circular polarizer, and wonder if this is correct, known, and might degrade a picture (I haven't noticed this in pictures yet)

First, I noticed looking through a CPL that I could darken (nearly completely) my computer screen, and so I assume that flat lcd screens produce polarized light.
Second, staking the CPL with a Kood grad (gg2h), the screen appears quite brightly in the transition area of the grad. Turning the CPL brightens the dark areas, but doesn't darken the bright ones.

My amateur-scientific conclusion is that the transition zone of the grad de-polarizes light. Is this a known fact ?

I've read that staking filters should be avoided without anything but the best quality stuff, but still... I don't feel ready to invest in Lee filters. Btw could anyone with Lee grads (or other good brand) verify this too ?

thanks

[JeanChris]

I did some further testing. Pictured my grads against the computer screen with my polarizer on different positions. The result is quite scrary to say the least !
Perhaps I shouldn't have bought my grads on eBay (even though the box says the larger one is Kood) ?

Minimum polarization


Maximum polarization

[SpecialK]

I don't know about the quality, but you are putting 3 pieces of glass in front of your lens.

[JeanChris]

Just two at a time (CPL + 1 grad). The smaller grad is from before I upgraded to Cokin P system. I think the benefits in dynamic range outweight the loss in IQ.

note : perhaps I should have posted on "pentax-camera-field-accessories" thread, sorry about that.

[ytterbium]

It's because of organic glass (plexiglass). You can see tensions in it, when placed between two polarizers. If you bend the ND Grad slightly the bent place would change brightness.
Thats a property of polarized light in plastic, they show such demo in physics class.

A most likely remedy would be mineral glass based ND grad. I doubt you can make plexiglass with variable density component without creating some tension inside it.

Actually it is not depolarizing light. The plastic just rotates polarization plane of light passing trough it. Rotation angle depends on tension. So it's not like you could not polarize some parts of frame at all, they will be simply polarized at different angle.

[urje]

I guess it would be a little annoying (or maybe impossible) in practice, but if you moved the two around, you probably wouldn't have a problem... (so subject-polariser-grad-camera)