circular vs standard polarizing filter

Started Dec 11, 2003 | Discussions thread
Ching-Kuang Shene Veteran Member • Posts: 5,724
Re: circular vs standard polarizing filter

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
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