Maybe silly question - Circ Polrz Filters

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.

 

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).

 

Actually, the use of C-PL is not limited to digital cameras. From
http://www.luminous-landscape.com/tutorials/polarizers.shtml:

"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:
http://www.mat.uc.pt/~rps/photos/filters_uv_pol/#polq2


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Thank you, that helped a lot.