Difference between CPL and PL Filter

I have found a Polarizing Filter --- NOT Circular Polarizing Filter.

I was not even aware such an animal existed.

I had often thought that Polarized sunglasses and the like work no matter which way they are pointed and don't have any "Circular" adjustment, so why would not that same work for lens filters.

Can someone enlighten me as to the difference between CPL and plain old PL, and when one would be advantageous over the other.

Thanks for reading and all help is appreciated.

They do the same thing, but old linear polarizing filters can cause issues with metering and autofocus on modern cameras.

http://www.bobatkins.c​om …technical/polar​izers.html

gonzogolf wrote in post #19161427
http://www.bobatkins.c​om …technical/polar​izers.html

Thanks.
Quoted from the bobatkins link: " If a linear polarizer is rotated so as to block this reflected linear component"

I assumed that a non-circular polarizer would be like the lens on a pair of sunglasses and not rotate; however, after reading that line, I begin to wonder if linear polarizing filters also rotate, much like a graduated ND filter. --- Do they or do they not rotate ?

I also found this quoted from the same source: "There is no difference between how and when you use a linear and a circular polarizer. The fact that it is a circular polarizer matters only to the camera, not to you or to the final image on the film."

That is a most informative article; thanks for pointing me in that direction.

BuckSkin wrote in post #19161440
I also found this quoted from the same source: "There is no difference between how and when you use a linear and a circular polarizer. The fact that it is a circular polarizer matters only to the camera, not to you or to the final image on the film."

That is a most informative article; thanks for pointing me in that direction.

Yes, it was informative. It was probably written pre-mirrorless but if I understood it correctly, mirrorless cameras would not need a circular polariser; a linear polariser would do?

A Linear Polarizer has a single element which eliminates all light rays that are not parallel to the 'slits of the element. Reflection light is such a one-direction set of light rays, so they are blocked by the filter when they are not in the same orientation as the 'slit'. This single-directional light coming thru the filter can cause issues with ttl metering

The CPL has a linear element, just like the Linear Polarizer. That element is rotated just like a conventional linear polarizer.
But it also has a 'quarter wave plate', which takes the light that passes thrn the slit and 'scrambles' into a circular pattern all the light waves so they are not all parallel to the slit...the rescrambled light is properly detected by the ttl light meter in the camera.

Wilt wrote in post #19161455
A Linear Polarizer has a single element which eliminates all light rays that are not parallel to the 'slits of the element. Reflection light is such a one-direction set of light rays, so they are blocked by the filter when they are not in the same orientation as the 'slit'. This single-directional light coming thru the filter can cause issues with ttl metering

The CPL has a linear element, just like the Linear Polarizer. That element is rotated just like a conventional linear polarizer.
But it also has a 'quarter wave plate', which takes the light that passes thrn the slit and 'scrambles' into a circular pattern all the light waves so they are not all parallel to the slit...the rescrambled light is properly detected by the ttl light meter in the camera.

Yes, that's what the article described but I inferred from it (and may well have misunderstood it) that the issue was that the partially reflective mirror of a modern SLR or DSLR also polarises the light that passes through it on its way to the meter, so the amount of light being metered depends on the angle of the 'slits' of the linear polariser (degree of cancelling out by the two polarisers), but then when the film or sensor is exposed the mirror is out of the way so the light is only passing through the LP filter, allowing more light to hit the film or sensor than the meter had measured. I don't own a mirrorless camera that I could test this on. Please correct me if my understanding is flawed.

Your understanding is fundamentally correct...things inside the camera are inherently 'polarizing' and work inaccurately in the presence of a pure polarized waveform (thru linear polarizer) which is not in same orientation as the polarizing element.