DIY ND filter - use a linear polarizer?

Z

zm15

Well-known member
I've read the forum over and seem to find different things about what the expensive fader's use.

Most DIY say to use 2 CPL's, but yet i've seen some say the LCW Fader ND uses a CPL and a Linear polarizer.

What's the difference? Does it matter?
 
There's no difference between a circular polariser and a linear polariser, except that a linear polariser can be used with the auto focus on DSLRs (that's excluding the GH1/2 because they AREN'T DSLRs). So, for the GHx two circular polarisers are ideal.

Edit: it's a CIRCULAR polariser, not a LINEAR one, that can be used with DSLRs. Sorry for any confusion.
 
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it is a long time now that circular polarizer are no more required on most camera. but it does not hurt to check.
a circular polarizer is a linear polarizer with an additional layer to turn light back to a more "normal" behavior.
so if you can go with a linear polarizer it is a good point since they are oprtically better.
the problem with circular polarizer is to find a pair that is matched (it is never indicated on a circ-pol if it turns clockwise or counter clockwise).
and due to the additional layer, circ-polc also need to be mounted the right direction because they are one way.

if you assemble the wrong polarizer (or wrong order), you will get all sort of effect like color shift, artifacts (cross) etc...
 
nosys70 said:
it is a long time now that circular polarizer are no more required on most camera. but it does not hurt to check.
a circular polarizer is a linear polarizer with an additional layer to turn light back to a more "normal" behavior.
so if you can go with a linear polarizer it is a good point since they are oprtically better.
the problem with circular polarizer is to find a pair that is matched (it is never indicated on a circ-pol if it turns clockwise or counter clockwise).
and due to the additional layer, circ-polc also need to be mounted the right direction because they are one way.
Click to expand...


So if i were to use a Circular Polarizer and a Linear polarizer, would I need to flip the glass?
 
circular must be first from camera and linear second, that allows work AF as it will be only with circular polrizer.
linear with linear makes fade, and quarter wave plate makes circular polarization.
 
DaLiV said:
circular must be first from camera and linear second, that allows work AF as it will be only with circular polrizer.
linear with linear makes fade, and quarter wave plate makes circular polarization.
Click to expand...

what happens with 2 circulars? If i were to use the Circular first then linear, so i need to flip the linear glass?
 
[camera] <- [circular] <- [linear] <- in that case camera will see circular polarized light - so AF works as best possible with circular polarizer
[camera] <- [linear] <- [linear] <- in that case camera will see linear polarized light - so AF works as it can with linear polarizer
[camera] <- [circular] <- [circular] <- what actually will be between two circular polarizer - no data, it's act in chain: light source -> linear polarization -> quarter wave rotation (right or left) -> linear polarization -> quarter wave rotation (right or left) ... most interesting part are between quarter wave and next linear what will be passed - no information ... possible will be non linear fading effect = on small angle change big fading, and further rotation will be with very small light passing changes ...
[camera] <- [linear] <- [circular] <- in that case camera will see linear polarized light - so AF works as it can with linear polarizer, but notes from previous point also need to be checked ...

(sorry currently have only one circular filter - can't check physically) - so if anyone currenty have circular and linear - they can report what there occurs with light changing on change of angle - if there normal changing on angle change - then simple first from camera filter decide how works AF and light rotation direction by quarter wave plate even wil not be matter....
 
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DaLiV said:
[camera] <- [circular] <- [linear] <- in that case camera will see circular polarized light - so AF works as best possible with circular polarizer
[camera] <- [linear] <- [linear] <- in that case camera will see linear polarized light - so AF works as it can with linear polarizer
[camera] <- [circular] <- [circular] <- what actually will be between two circular polarizer - no data, it's act in chain: light source -> linear polarization -> quarter wave rotation (right or left) -> linear polarization -> quarter wave rotation (right or left) ... most interesting part are between quarter wave and next linear what will be passed - no information ... possible will be non linear fading effect = on small angle change big fading, and further rotation will be with very small light passing changes ...
[camera] <- [linear] <- [circular] <- in that case camera will see linear polarized light - so AF works as it can with linear polarizer, but notes from previous point also need to be checked ...

(sorry currently have only one circular filter - can't check physically) - so if anyone currenty have circular and linear - they can report what there occurs with light changing on change of angle - if there normal changing on angle change - then simple first from camera filter decide how works AF and light rotation direction by quarter wave plate even wil not be matter....
Click to expand...

Thanks for that!

So basically what i'm taking away from this is I should use Camera->Circular Polarizer -> Linear Polarizer. And tape down the circular polarizer?
 
You can flip a linear polarizer that has no different effect.
If you flip a CPL, the quarter-wave plate is now in the front, and you have something like a linear polarizer

To get that ND effect you can use:
2 Linear (some DSLR might get into trouble here. m4/3rd will work).
CPL and linear polarizer in front (will work with any camera)
With 2 CPL you should flip the front CPL. And you will get the same results as with CPL and linear polarizer in front.
 
wocag said:
You can flip a linear polarizer that has no different effect.

With 2 CPL you should flip the front CPL. And you will get the same results as with CPL and linear polarizer in front.
Click to expand...

That's exactly what I was looking for, thanks!
 
The biggest lens i'm using is the 14-140 which has a 62mm filter thread size. I also have 2 other lenses with smaller filter threads - a 20mm panny with 46mm and a Canon FD 28mm with 49mm filter thread.

Since I will be using at least 1 or 2 step up rings, plus the CPL and LPL filters, should I get bigger CPL and LPL filters to prevent vignetting? Still learning how all these things work!
 
You're welcome. I'm using 4X4 filters... flipping the CPL is an issue here...XD so i'm using linears as often as possible...
 
Just in case you're going to take apart a filter and flip it inside its threaded mount: use a pen and a firm grip to turn the retaining ring counter-clockwise until it unscrews. Use a lens cloth to catch the glass and retaining ring as you turn it over.
 
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