Re: Circular Polarizers, A Filter You Need

"Matt Clara" <hey.wood.y@[EMAIL PROTECTED]
> wrote:

>"Fat Sam" <samandjanet@[EMAIL PROTECTED]
> wrote in message 
>news:G4WdnRPxh-gJRiTbRVnyugA@[EMAIL PROTECTED]
>> Wayne J. Cosshall wrote:
>>> Hi All,
>>>
>>> I've written the first of a series of articles on the use of real
>>> camera filters with digital photography. The first is on the circular
>>> polarizer filter:
>>> http://www.dimagemaker.com/article.php?articleID=1063
>>>
>>> Cheers,
>>>
>>> Wayne
>>
>> I have a circular and a linear polariser. I was told that I would be 
>> better to use the circular polariser on my digital camera, as it would 
>> give better results.
>> But I've used both the linear and the circular polarisers on my digital

>> and I honestly can't see any difference in the results when I compare
the 
>> two.
>>
>
>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.


Not so, Matt.  

The circular polariser has two elements.  The first is a linear
polariser, which has exactly the same blocking effect as any linear
polariser.  The second element is a "quarter wave retarder" which
effectively unpolarises the polarised light, making it acceptable to
autofocus systems and some light metering systems, for example that of
the Nikon F3.

From the B+W web site:
"B+W pol-filters are made in various types and dimensions, both in the
form of linear as well as circular pol-filters. Basically, both
consist of a linearly polarizing film. With a circular polarizing
filter, an additional retardation film is added, which places the
linearly polarized light into rotation, thereby preventing erroneous
measurements in optical measurement systems; hence, for cameras with
internal devices for light meters (TTL) and/or autofocus, only
circular polarizing filters can be used! The same applies to digital
cameras and camcorders."  

B+W are being cautious here, because only a very few manual focus SLRs
ever needed a circular polarising filter.  Most TTL meters are
perfectly accurate when linear polarisers are used.