Re: Circular Polarizers, A Filter You Need- not so- linear vs. circular

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