Use polarizer to reduce flash glare ?

Discussion in 'Digital Photography' started by Wdflannery, Nov 12, 2003.

  1. Wdflannery

    Wdflannery Guest

    Well, I just bought one for that purpose .... I'm taking pics of objects, using
    off camera flash, and if the object is smooth it will usually produce some
    flash glare in the pic. I bought the polarizer to attempt to reduce it ....
    but can't think of any way to use it except trial and error ..... Will it
    help? Are there any guidelines for using a polarizer?
    Wdflannery, Nov 12, 2003
    #1
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  2. A polarizer is only effective if used at an approximate 30-degree angle
    to glare...only darkens skies 90-degrees from the sun. It's not magic.
    Randall Ainsworth, Nov 12, 2003
    #2
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  3. for best results try to use polariser both on camera AND flash, this called
    cross-polarization. i used cheap plastic polarizer for flash cut off
    Polaroid-made computer antiglare screen. better explanation is in Canon
    Macro book.
    "Wdflannery" <> wrote in message
    news:...
    > Well, I just bought one for that purpose .... I'm taking pics of objects,

    using
    > off camera flash, and if the object is smooth it will usually produce

    some
    > flash glare in the pic. I bought the polarizer to attempt to reduce it

    .....
    > but can't think of any way to use it except trial and error ..... Will it
    > help? Are there any guidelines for using a polarizer?
    Roman Marutov, Nov 12, 2003
    #3
  4. Wdflannery

    PhotoMan Guest

    "Randall Ainsworth" <> wrote in message
    news:111120032151220930%...
    > A polarizer is only effective if used at an approximate 30-degree angle
    > to glare...only darkens skies 90-degrees from the sun. It's not magic.


    90º is the maximum angle to the sun - it diminishes gradually from that
    angle to 0º and 180º
    PhotoMan, Nov 12, 2003
    #4
  5. Wdflannery

    jam Guest

    Unless you have a large camera-flash separation on the order of 110°,
    the angle between the outgoing and returning flash beams will be too
    narrow (too far off Brewster's angle) for a single polarizer to do
    much good. Try the cross-polarization trick mentioned in another
    reply. If the glare's coming from bare metal surfaces,
    cross-polarization is your only hope. If your subjects' surfaces
    aren't too rounded, you might be able to come up with a flash positon
    that directs the glare away from your lens, but I assume you've tried
    that.
    --
    Jeremy McCreary
    Denver, CO
    www.cliffshade.com/dpfwiw/
    -------------------------------------------

    "Wdflannery" <> wrote in message
    news:...
    | Well, I just bought one for that purpose .... I'm taking pics of
    objects, using
    | off camera flash, and if the object is smooth it will usually
    produce some
    | flash glare in the pic. I bought the polarizer to attempt to reduce
    it ....
    | but can't think of any way to use it except trial and error .....
    Will it
    | help? Are there any guidelines for using a polarizer?
    jam, Nov 13, 2003
    #5
  6. Wdflannery

    Wdflannery Guest

    Thanks for the info ..... so ....
    when light strikes a smooth object some light is reflected ... some scattered
    ..... and if the angle at which the light hits the object is small (0 to 70
    deg.) the reflected light will be polarized ????? Is that the situation ???
    Wdflannery, Nov 16, 2003
    #6
  7. Wdflannery

    jam Guest

    Short answer: Metallic surfaces don't impart polarization on the
    light they reflect. All other smooth surfaces impart some polarization
    which varies in =degree= with the angle of reflection. For any given
    reflecting material, maximum polarization occurs at Brewster's angle,
    which depends on the indices of refraction at the reflecting surface
    but for glass, water, and most materials photographers encounter falls
    very close to 55° off the normal to the reflecting surface. (That's
    the origin of the 110° figure in my post.) The =direction= of induced
    polarization is always perpendicular to the plane of reflection at the
    point of reflection.

    Long answer: www.cliffshade.com/dpfwiw/polarizer.htm
    --
    Jeremy McCreary
    Denver, CO
    www.cliffshade.com/dpfwiw/
    -------------------------------------------


    "Wdflannery" <> wrote in message
    news:...
    | Thanks for the info ..... so ....
    | when light strikes a smooth object some light is reflected ... some
    scattered
    | .... and if the angle at which the light hits the object is small (0
    to 70
    | deg.) the reflected light will be polarized ????? Is that the
    situation ???
    jam, Nov 16, 2003
    #7
  8. Wdflannery

    PhotoMan Guest

    "jam" <> wrote in message
    news:rMAtb.208540$Fm2.204671@attbi_s04...
    > Short answer: Metallic surfaces don't impart polarization on the
    > light they reflect. All other smooth surfaces impart some polarization
    > which varies in =degree= with the angle of reflection. For any given
    > reflecting material, maximum polarization occurs at Brewster's angle,
    > which depends on the indices of refraction at the reflecting surface
    > but for glass, water, and most materials photographers encounter falls
    > very close to 55° off the normal to the reflecting surface. (That's
    > the origin of the 110° figure in my post.) The =direction= of induced
    > polarization is always perpendicular to the plane of reflection at the
    > point of reflection.


    What he said.
    PhotoMan, Nov 16, 2003
    #8
  9. Wdflannery

    Wdflannery Guest

    Cool.....

    So, if the sun is due west of me, and I'm shooting due west at a lake say, and
    the sun is at 55 deg to the west of the lake, then my shooting (vertical)
    angle doesn't make any difference, and I'll see the maximum polarization and it
    will be horizontal ???? Still, where is the 110 deg ????
    Wdflannery, Nov 16, 2003
    #9
  10. Wdflannery

    Wdflannery Guest

    Oops ... now I get it .... if the lake is perfectly flat .... I won't be able
    to see the reflection unless I"m at 55 deg. in the opposite direction ....hence
    the 55 + 55 = 110 angle. And I'm assuming that the 'plane of the reflection'
    is the plane containing the two lines source to object and object to viewer.
    Wdflannery, Nov 17, 2003
    #10
  11. Wdflannery

    jam Guest

    "Wdflannery" <> wrote in message
    news:...
    | Oops ... now I get it .... if the lake is perfectly flat .... I
    won't be able
    | to see the reflection unless I"m at 55 deg. in the opposite
    direction ....hence
    | the 55 + 55 = 110 angle.

    Well, you're getting closer. When the sun-lake-camera angle is 110°,
    and your camera angle down to the lake and the sun's angle above the
    lake both equal 35°, say early morning or late afternoon, then and
    only then will the sun's reflection off the lake will be maximally
    polarized at your position. If you change your camera angle down to
    the lake by more than ±15° from this "sweet spot", the reflection will
    be much less polarized at your position, and your polarizer will be
    much less effective in suppressing it.

    Note that the polarization of the water reflection will =always=
    parallel the lake surface. To suppress such a reflection, you'd simply
    turn your polarizer so that its blocking direction also parallels the
    lake. This geometry is so predictable that there's no need to use TTL
    control to quash water reflections with a polarizer once you know the
    polarizer's blocking direction.

    With some 3D imagination, this geometry can also be applied to curved
    surfaces. That's where the plane of reflection comes in. The direction
    of polarization is always perpendicular to the plane of reflection at
    the point of reflection. Again, TTL polarizer control becomes
    unnecessary once you've trained yourself to "eyeball" these
    relationships.

    | And I'm assuming that the 'plane of the reflection' is the plane
    | containing the two lines source to object and object to viewer.

    Exactly.


    Not to open another can of worms, but it's worth noting that the
    geometry of polarization due to reflection differs from the geometry
    of polarized skylight, because the polarization comes about via two
    very different physical processes.
    --
    Jeremy McCreary
    Denver, CO
    www.cliffshade.com/dpfwiw/
    -------------------------------------------
    jam, Nov 17, 2003
    #11
  12. Wdflannery

    Wdflannery Guest

    Thanks!
    Wdflannery, Nov 17, 2003
    #12
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