circular vs standard polarizing filter

Started Dec 11, 2003 | Discussions thread
OzRay
OP OzRay Forum Pro • Posts: 19,428
Re: circular vs standard polarizing filter

Ching-Kuang Shene wrote:

OzRay wrote:

That's interesting, because I've always used a standard one with my
SLRs. That's what I'm curious about, seeing as they sell the two
types for digital cameras. I've tried to do an Internet search, but
have not come up with an explanation. What's the difference between
the two (other than one costs more)?

If you search DPreview, you will come up with so many posts
answering this question, including a few of mine. A complete
answer is very simple, but technical. However, I'll try to answer
it is a non-technical way.

Some cameras have beamspliter or some device that can split the
light coming through the camera lens to various components such as
viewfinder, light meter and AF system. The splitting ratio has to
be constant for all components that receive the split light to work
properly. For example, just an example, the incoming light may be
split 50% to viewfinder, 20% to meter and 30% to the AF system.
The meter will measure this 20% incoming light for determining
exposure value (EV).

If a linear polarizer is used with this system, due to the "phase"
of the polarized light, the constant split ratio cannot be
maintained. In other word, 60% may go to the viewfinder, 15% to
meter and 25% to the AF system. Thus, the meter, which does not
know the split ratio has been changed, will use this 15% for
exposure value determination. Since the incoming light is 5% less
than the supposed amount, the metering system will believe the
illumination is 5% weaker and may increase EV to counter balance,
which means a larger aperture or slower shutter speed will be used.
Consequently, the scene will be over-exposed. On the other hand,
the AF system may need certain amount of light (e.g., F5.6) for
the AF mechanism to work properly. Now, the reduction of 5% (from
30% down to 25%) may cause the light that is available to the AF
system to use may drop below the minimum needed amount, and the AF
system may fail to operate.

A circular polarizer will readjust the polarized light by the
linear one so that the beamspliter will keep the splitting ratio
constant.

Since consumer digicams do not use beamspliter and its equivalent,
consumer digicams can use linear polarizers. Since SLR/DSLR
cameras do have beamspliters, they cannot use linear polarizers due
to the above mentioned reason.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 User Guide

Excellent. Thanks guys.

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