Pentax recommends that a circular polarizer be used.
The discriminating criteria isn't necessarily that a camera has auto
focus. The criteria you have to be aware of is whether the camera
uses an optical path to AF or metering sensors which involves a beam
splitter or auxiliary mirror.
The DS, like many SLR cameras since the beginning of the 1980s, has a
semisilvered section of the primary mirror that feeds light to the AF
and metering sensors through a small auxliary mirror behind the main
mirror. Each mirror and beamsplitter operates to polarize light
passing through it and reflecting off it to some degree. The issue
that can happen, not necessarily *does* happen, is that you are
sending doubly polarized light to a sensor ... If it happens to an AF
sensor, it can throw off the focusing. If it happens to a metering
sensor, it can throw off the exposure evaluation.
It doesn't *always* happen. A combination of maximum aperture, lens'
exit pupil, the specific optical path in a particular camera
sometimes conspires to cause errors; other combinations do not. Some
cameras' auxiliary light paths for sensors are particularly prone to
issues, others are not.
I don't know how sensitive the D/DS bodies are to these kinds of
problems. If a linear polarizer is working for you, heck, just use
it. But if you start getting AF focusing errors and/or exposure
errors with certain lenses, you might do some tests to see if its the
linear polarizer that's causing the problem.
Godfrey
On May 20, 2005, at 5:34 PM, Shel Belinkoff wrote:
Typical wisdom is that a circ pol should be used on AF cameras, and
that
such a pol should be used on the DS. Maybe you should compare both
and see
if there's a difference. Maybe the need to use a circ pol is just
some
internet legend that has spread over the years.
So should a circular be used on the DS? I tried my non circular
and it
seemed to work fine.
A circular works on any camera. A linear polarizer can cause
problems
on cameras that have complex light paths to the AF or exposure meter
sensor. Linear polarizers are cheaper and more efficient than
circular ones, that's why it's important to know the distinction and
when they can be used.
