Polarising filter puzzle

Discussion in 'Digital SLR' started by Jeff R, Apr 12, 2005.

  1. Jeff R

    Jeff R Guest

    Can someone explain this one to me?

    I have two polarising filters, both screw-ons with std filter threads and
    rotating mounts.

    One is a Hoya PL-CIR.
    The other, Suparon PL.

    Both, individually, do the usual "darken the sky" and "reduce the reflected
    glare" stuff; quite satisfactorily. (I could post pics, but take my word...)

    Ok, so today I held both of them in line and rotated them to observe the
    "darken to black" effect we all know and love when we cross two polarised
    filters. I did this with the filters off the lens - just held in my hand.

    Nowt!

    Well, at least, very little effect. The colour balance altered
    significantly, from bluish (not quite 80A blue, but blue at least) to yellow
    (tungsten yellow). No significant darkening at all.

    Ahhh... but it gets better.

    When I flipped the closest-to-my-eye (Hoya) filter so that it was now
    threads-facing-me, the two filters polarised perfectly, from almost clear to
    solid black.

    Flipping the Suparon had no effect.
    Flipping the Hoya altered completely its properties. By itself, it fully
    polarises the LCD screen I'm reading now. Flipped back to front, it just
    alters the colour balance.

    Is this a property of Hoya filters that's well-known? How does it work? I
    would have thought that a polariser would not be dependent on direction of
    the light source.

    Anyone?
     
    Jeff R, Apr 12, 2005
    #1
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  2. Jeff R

    Alan Browne Guest

    er, that's what polarizers do. Let light oriented one way pass,
    oriented the other way is blocked.
    LCD screens often have their own polarizer layer (for contrast and
    anti-glare I believe), so with a polarizer set to let everything pass,
    you can turn the polarizer 90 degrees to the LCD and it will block.

    (eg: set both pols "open" and repeat your experiment against the LCD
    screen. Rotate the filter in your hand while viewing the LCD. Both
    filters should have the same effect. Note: by rotate I mean the whole
    filter body, not one element v. the other)


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    Alan Browne, Apr 12, 2005
    #2
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  3. Jeff R

    Owamanga Guest

    Two PL's (not circular) would behave the same either way round (full
    black when correctly aligned). You can even buy sheets of the stuff
    for weird photographic effects.

    What you did was introduce a circular polarizer (the PL-CIR) which
    *additionally* passes the light through a quarter wave retarder so it
    is no longer polarized as it enters the lens.

    This is required so that TTL metering and AF systems used on modern
    cameras can work properly (which some can't when given polarized
    light).

    Reversing the hoya (PL-CIR) meant the light went through the circular
    (quarter wave) part first, and then *was* polarized ready for full
    cancellation by the second polarizer.
     
    Owamanga, Apr 12, 2005
    #3
  4. Jeff R

    Owamanga Guest

    Alan, read my other post on this. He's seeing it because one of his
    polarizers is circular. Two non-circular polarizers keep everything
    relatively neutral gray to black, regardless of which side
    (front-back/back-front) you put them.

    Introduce the CPL and you get weird color shifts (blue to
    orange/brown) when the CPL is turned backwards, but behaves normally
    forwards.

    Somewhere (and I've lost it I think) I have a blue/red filter that
    changes color based on rotation, using quarter-wave polarizing magic.
     
    Owamanga, Apr 12, 2005
    #4
  5. Jeff R

    Alan Browne Guest

    I didn't read his full post nor yours, so no surprise if I screwed up.
    Still, what I said about overlaying a pol over some LCD screens holds.
    Similar to the effect when wearing polarized sunglasses and looking at
    the rear window of cars with polarized skins on them (to reduce glare)
    and you see a moiré pattern... or looking at your LCD clock sideways.

    Cheers,
    Alan.

    --
    -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
    -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
    -- slr-systems FAQ project: http://tinyurl.com/6m9aw
    -- [SI] gallery & rulz: http://www.pbase.com/shootin
    -- e-meil: Remove FreeLunch.
     
    Alan Browne, Apr 12, 2005
    #5
  6. From: "Jeff R" <>

    | Can someone explain this one to me?
    |
    | I have two polarising filters, both screw-ons with std filter threads and
    | rotating mounts.
    |
    | One is a Hoya PL-CIR.
    | The other, Suparon PL.
    |
    | Both, individually, do the usual "darken the sky" and "reduce the reflected
    | glare" stuff; quite satisfactorily. (I could post pics, but take my word...)
    |
    | Ok, so today I held both of them in line and rotated them to observe the
    | "darken to black" effect we all know and love when we cross two polarised
    | filters. I did this with the filters off the lens - just held in my hand.
    |
    | Nowt!
    |
    | Well, at least, very little effect. The colour balance altered
    | significantly, from bluish (not quite 80A blue, but blue at least) to yellow
    | (tungsten yellow). No significant darkening at all.
    |
    | Ahhh... but it gets better.
    |
    | When I flipped the closest-to-my-eye (Hoya) filter so that it was now
    | threads-facing-me, the two filters polarised perfectly, from almost clear to
    | solid black.
    |
    | Flipping the Suparon had no effect.
    | Flipping the Hoya altered completely its properties. By itself, it fully
    | polarises the LCD screen I'm reading now. Flipped back to front, it just
    | alters the colour balance.
    |
    | Is this a property of Hoya filters that's well-known? How does it work? I
    | would have thought that a polariser would not be dependent on direction of
    | the light source.
    |
    | Anyone?
    |
    | --
    | Jeff R.
    |


    There are two type Horizontal/Vertical Polarizers and Circular Polarizers.

    The two work differently for similar effects.

    Verify which type is each filter you have.
     
    David H. Lipman, Apr 12, 2005
    #6
  7. Jeff R

    Owamanga Guest

    He's already done that and the effect he describes confirms it.

    The Suparon is a non-circular polarizer, hence the code 'PL'
    The Hoya is a circular polarizer, hence the code 'PL-CIR'

    Another common marking is 'CPL' for Circular PoLarizer.
     
    Owamanga, Apr 12, 2005
    #7
  8. From: "Owamanga" <owamanga(not-this-bit)@hotmail.com>

    | On Tue, 12 Apr 2005 17:10:25 GMT, "David H. Lipman"
    |
    | He's already done that and the effect he describes confirms it.
    |
    | The Suparon is a non-circular polarizer, hence the code 'PL'
    | The Hoya is a circular polarizer, hence the code 'PL-CIR'
    |
    | Another common marking is 'CPL' for Circular PoLarizer.
    |
    | --
    | Owamanga!
    | http://www.pbase.com/owamanga

    Thanx for the clarification.

    BTW: I couldn't think of the term when I stated "Horizontal/Vertical Polarizers".
    The terminology is Linear Polarization.
     
    David H. Lipman, Apr 12, 2005
    #8
  9. Jeff R

    Jeff R Guest

    Thanks Owamanga and others for the responses.

    My background in ham radio has me prepared for circular vs linear
    polarisation, but I confess this discussion has posed more questions than it
    has answered. I am particularly intrigued by the concept of the quarter
    wave retarder you mentioned. A bit of Googling turned up:

    http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/CircularPolarization/CircularPolarization.html

    which cleans up the whole mess (for me at least).

    I could conceive of how a filter could achieve linear polarisation, but
    circular? The link helps, as did your pointer.

    Thanks again.
     
    Jeff R, Apr 13, 2005
    #9
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