"Matt Clara" wrote:

"Fat Sam" wrote in message
...
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.