Photographic 'Circular Polarisers': what are they?

[sophiecentaur]

For years, I have been using a rotatable linear polariser to improve landscape photographs.
They work very well at reducing the reflections from water, reduce the effects of haze and generally sex-up pictures.
I thought the reason was quite simply that they reject on particular polarisation plane.
The latest thing, on Photo Forums, seems to be the 'Circular Polariser', which is claimed to have a similar effect and which is marketed by Hoya, for example. These filters are also rotatable.
I queried what they did and was told that they do the same enhancement of landscape pictures and that one could be used in conjunction with linear polariser to get cancellation (as with crossed polaroids) at a particular angle.

You can produce circularly polarised light fairly easily with a quarter wave plate system but I don't see how that can select / reject one particular polarisation - because the E vector rotates to every angular value.

Am I missing something here? Is it just an advertiser's way of renaming a Round, Rotatable Polariser as a Circular Polariser or is there some genuine effect here?

 

[mgb_phys]

Circular polarizer filters are a linear polarizer (to select one polarization) followed by a quarter wave plate to generate circular polarization again.
The only reason to use a circular is if you have a camera where the autofocus uses a beam splitter to take off part of the light (which is pretty much all of them)

 

[sophiecentaur]

Thanks. That make sense.
So, yet again, I must dig deep in my pockets!

 

[Andy Resnick]

I'd be surprised if those circular polarizers work that well- the polarizer may be very achromatic, but the retarder is likely not. In addition, the acceptance angle of a retarder is usually small. At least, the achromatic retarders I have seen (Fresnel rhombs) are very large and have small aceptance angles.

 

[sophiecentaur]

I just checked. My pentax k10 focusses perfectly well with an old fashioned polariser. So I just don't care! So there, Mr. Hoya.