Matt Clara wrote:




The circular will tend to give slightly _poorer_ results, as it's designed
to let some polarized light through, as some camera's autofocus systems (and
ttl metering) depends upon it to get the job done.

--
www.mattclara.com



Both the circular and linear devices do exactly the same thing at the
first step- the light goes through a linear polaroid. That is where all
the action happens, all the filtering of the scene. For the simple
linear device, the light exits the filter essentially fully linearly
polarized and then enters the camera. Since reflections off surfaces can
polarize light, if there are any reflections inside the camera- say to
the viewfinder or to the light meter- there is the potential for
interactions, crossings, of this polarization with the incoming
polarization, which could affect the results in a negative way. If there
are no reflections in the camera, if the light goes straight through to
the focal plane and never is reflected, then linear polarized light
inside the camera should be irrelvant.

The circular device simply takes the linearly polarized light from the
first filter and converts it to circularly-polarized light by passing it
through a second filter, a quarter-wave retarder. If the filter is well
made, this second filter should result in no significant loss of light
or any other degradation of image quality. After exiting the filter,
circularly polarized light will pass though a camera just the same as
unpolarized light, in other words, just fine. The critical thing is that
it won't make any difference at what angle you set that first linear
polaroid.

There should be absolutely no difference in image quality between the
two filters if they are of comparable quality. For straight-through
cameras, there should be no performances differences. For cameras with
internal reflections, however, the circular polarizer will give more
reliable results, as the circularly polarized light it produces will be
immune to polarizing effects of the internal relections.

Since linear polarizers tend to be cheaper than circular ones, if you
have a straight through camera, you can probably save some money by
buying a linear device.

I hope this makes it a little clearer.

Joe