# cir-polarizing filter question

Started Mar 23, 2003 | Discussions thread
Re: cir-polarizing filter question

Greg --

"Circular" in this context is, in lay terms, an unfortunately misleading description, and has nothing at all to do with the concept of rotating the filter to get the desired effect. Both linear and circular polarizing filters are used in exactly the same way; i.e. both have their polarizing element fitted in a ring that is free to rotate.

Unfiltered light comprises (in part) a transverse wave motion, vibrating in an infinite number of planes perpendicular to its path of propagation. Certain things such as non-metallic reflecting surfaces (e.g. glass and water) reduce this to (largely) a single plane -- i.e. they polarize the light that they reflect. If we introduce a filter which also polarizes light (during transmission), we reduce ALL the ight transmitted through it to a single polarization plane. By selectively rotating this plane until it is at 90 degrees to the one relating to the undesired reflections, we can cancel them out without undue detriment to the remainder for purposes of exposing film or a CCD. That's the role of a linear polarising filter, and the front element of a circular one.

What "circular" means is this: after the polarization plane has been set (by rotating the filter) to minimise reflections etc., an additional element (called a quarter-wave plate) re-creates a second polarization plane at 90 degrees to the first. Mathematically these two planes resolve to, effectively, the full 360 degrees (i.e. "circular") -- analogous to non-polarized light as far as the autofocus system is concerned.

Exactly the same principle is used with most FM public radio transmitters. Two antenna arrays are used, one radiating a horizontally polarized signal and the other vertical. The chief purpose of this is to permit good reception in cars (whose receiving antennas are aligned vertically in the main) as well as in homes (most commonly using horizontally aligned antennas) and anything in between. It doesn't come cheap, as you need to double the transmitter power with this arrangement; but hey, they have to first reach you in order to assail you with advertisements!

The type of AF system (contrast detection) used in consumer digicams works perfectly well with polarized light, so the basic linear polarizer is fine. The combination of a mirror and phase detection AF in SLRs is when problems arise, and the light must be effectively depolarized for satisfactory operation.

Mike

Gregj wrote:

I would like a straight answer on this myself. I had an old Nikon
F2 with circular polarizer and it was a very visible effect through
the viewfinder when I turned the outer ring of the filter. I
thought I would get a circular for my 717, but now I am confused.
I cannot understand how a linear would work, if turning the outer
ring on a circular has such a dramatic effect.

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