jerryc41 Loc: Catskill Mts of NY

A CPL is a circular polarizing filter. Who knows why it is called a CPL?

MT Shooter Loc: Montana

The polarizing filter used with SLR's is a circular polarizer. The first stage of the polarizer is a linear filter which filters out light that is linearly polarized in a specific direction. The second stage, for reasons related to the auto sensors within the camera, then circularly polarizes the light before it enters the camera.

Graphic

emrob62 Loc: NEPA

So based on MTs explanation the L stands for Light

jerryc41 Loc: Catskill Mts of NY

Right. I was surprised to read that the term "circular" does not refer to the shape of the filter. That's what I had always thought - round vs square. It's the way it polarizes that gives it the "circular" name.

ygelman Loc: new -- North of Poughkeepsie!

Admittedly, although I knew how circular polarization differs from linear polarization, I also thought that 'circular' in lens filters merely referred to the shape. But MT Shooter's information is new to me.

On that note, however, the following (also from Wikipedia) is very interesting: Linear polarizing filters can be easily distinguished from circular polarizers. In the former, the polarizing effect is the same regardless of which side of the filter the scene is viewed from. In the latter, the polarizing effect is quite marked when the scene is viewed from the male threaded side of the filter, but almost non existent when viewed from the other side.

Thank you UHH!

Blake Loc: Alfred NY

MT,
Great to the point answer. My wife has her PhD in physics, she teaches engineering students how to make light work for them and the like. She taught me the difference but it took about 10 minutes. But the again the student may not have been as stellar :thumbup:
Blake

dpullum Loc: Tampa Florida

The test for a fake/real circular pol filter is similar to that for vampires. If you look thru you can see your reflection in the mirror; flip it over and it will be black. If you are in the army, vampirism still falls under the old a don't ask/don't tell rule.
http://www.youtube.com/watch?v=6lVIo9C0NDA

Get educated on polarized filters (list at bottom of 1st page) Note, summary shows a graph... several of the "expensive" filters tested poorly ... so don't tell me you get what you pay for!!!
http://www.lenstip.com/115.1-article-Polarizing_filters_test.html

Oh! Why circular polarizing name... liner filters allow only a linear .. one... orientation. If that sorted uniform light goes thru a 1/4 wave filter it is disrupted and becomes random or non linear again... light rays are oriented in every direction ... the axis of the light rays drawn will form a circle... thus circular.

Do an UHH search and you will find we have danced this dance before. Some good references there.

BboH Loc: s of 2/21, Ellicott City, MD

The neat thing is that every time the music plays there are new steps to learn

twowindsbear

Are you wondering what the initials stand for? Here's my understanding: Circular PoLarizr.

Nikonian72 Loc: Chico CA

CPL stands for Circular Polarizing Lens, and is a misnomer. The proper term is CPF for Circular Polarizing Filter.
Lenses alter the path of light, while filters alter only the quality of light.
FAQ: Photographic Filters: http://www.uglyhedgehog.com/t-26502-1.html

dpullum Loc: Tampa Florida

Surly my Idle of wild life photography, Nikonian 72, .. is now in my eyes... man who suddenly is reduced to bickering about lens/filter distinction when it comes to the single most complex lens attachment to a camera lens. Nikonian, is a 2x lens attached to a 50mm lens not a lens and now you claim it is a filter? Humph! NO, it is a lens. Likewise that linear polarizing attachment becomes a circular one with the addition of a 1/4 retarder (disrupter as in from Star Trek)

I ask you the UHH readers, Jury of peers, Has he done this to mask the fact that he lacks knowledge of the mystical, inscrutable, 1/4 wave light modifier that work similar to pyramids and can, when used properly, sharpen single edge razor blades? He has either no knowledge or is hiding from us that Man (or lady in a lab) did not invent the circular (de)polarizing lens/filter; it was stolen from nature. May I quote from Scientific American: "The compound eye of the peacock mantis, the new study's authors found, harbors a natural quarter-wave retarder, ... that converts circularly polarized light to linearly polarized light, which then activates receptors below."

Nikonian... this is the LENS of the shrimp's eye.. not the FILTER of the shrimp's eye!!! My case rests..

http://www.theawl.com/2009/10/achromatic-quarter-wave-retarder-having-shrimp-are-terrifying
...................................
Additionally:
A waveplate is a transparent slab that can alter the polarization of light because it is birefringent -- exhibits double refraction. Note the word REFRACTION.
http://live.psu.edu/story/53917
A material that exhibits refraction IS A LENS by definition. Nikonian72, have you no shame in you statement that CPL is simply a CPF! I am astonished. Examples of refraction devices below ... are they filters or lenses?:
http://www.google.com/search?q=refraction+lenses&hl=en&qscrl=1&nord=1&rlz=1T4GGLL_enUS397US452&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=HX4vUM65EYK89gS5woHICA&sqi=2&ved=0CFUQsAQ&biw=1095&bih=709

REALITY CHECK: The deeper I dig into the physics of CPF or CPL, the more complex these become and it is no wonder that they confuse our brains as well as the focusing mechanism of some of the more "expensive" cameras such as Canon and Nikon. (which I suspect have chips in the screw rings that require you to use their brand of filter! :roll: )

dpullum Loc: Tampa Florida

UNDERSTANDING THE PHYSICS OF POLARIZATION
(General Physics course 341)

It is important that Photographers understand the Camera, and Composition, but also the very nature of Light.

For those who really want to know about Polarization of light you can read these lab experiments, section 4 discusses polerization, section 4.6 and 4.7 discuss circular polarization using 1/4 wave filters such as scotch tape.
http://instructor.physics.lsa.umich.edu/int-labs/Chapter4.pdf
(Many thanks to the University of Michigan, me ol' alma-mater)
Please ignore the complex mathematical equations or you will personally become a section 8.
(to understand that play on sections see ... http://health.vic.gov.au/mentalhealth/archive/pmc/amend_invol.htm)