Maybe silly question - Circ Polrz Filters

Discussion in 'Digital Photography' started by TheNewsGuy(Mike), Apr 26, 2006.

  1. With a digital camera (D50) I know I have to buy a "circular" polarizing
    filter. But what does that mean exactly? Are the polarizing "lines"
    concentric circles? Does it mean that if I turn the filter there is no
    change in the effect on the reflections like my old film polarizing filter?

    Thanks in advance for any answer.
    TheNewsGuy(Mike), Apr 26, 2006
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  2. A circular polarising filter consists of a plain polariser (closer to your
    subject) followed by a quarter wave plate in contact with it. The latter
    "scrambles" the light coming out of the plain polariser so that your
    camera's light metering and AF sensors work correctly. PROVIDING you hold
    it the correct way round, a circular polarising filter will behave like a
    linear filter, and, at certain orientations, will reduce the intensity of
    polarised light e.g. reflections from non-metallic surfaces, or (some of)
    the blue of the sky. There's no effect if held the other way round, other
    than attenuation.
    (Can't think of a reason for this, but I "feel" that circular polarisers
    aren't as strong in action as linear polarisers.)

    The normal mounting threads should ensure that a circular polarising filter
    is mounted correctly (unless someone has incorrectly assembled the filter).
    Malcolm Stewart, Apr 27, 2006
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  3. TheNewsGuy(Mike)

    Matt Ion Guest

    Actually, the use of C-PL is not limited to digital cameras. From

    "There are two types of polarizing filters available — linear or
    circular. Linear polarizers are more effective and less expensive than
    circular ones. But circular polarizers are needed with just about any
    camera that has a through-the-lens metering system, or autofocus.

    "The reason for this is that both of these systems use semi-silvered
    mirrors to siphon off some of the light coming though the lens. If that
    light is linearly polarized it renders either the metering or the
    autofocus ineffective. This means that you're going to have to buy
    circular polarizers unless you're shooting with a pre-1970's camera, or
    a view camera."

    There's a pretty good technical description of the differences here:

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    Matt Ion, Apr 27, 2006
  4. TheNewsGuy(Mike), Apr 27, 2006
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