A Circular Polarizer Doesn't Doesn't Have to be Round, and Vice Versa
By cutting haze and cutting down on reflections, a polarizer is one of the most useful filters for outdoor photography. But not all polarizers are created equal. Most modern cameras require what is known as a "circular polarizer." And I can hear at least some of you saying to yourself "aren't all polarizers round?" Ah, but in the world of polarizers "circular" doesn't mean "round."
Light travels as an electromagnetic wave, essentially a squiggly line that doesn't inherently have any more preference for zigging and zagging up and down than it does for left and right. Or any other angle either for that matter. The waves are all randomly oriented. But when light reflects off a surface, it often becomes polarized at the same angle as that surface. The light reflecting off a storefront window that prevents you from seeing what's on the other side is vertically polarized since the surface of the window is itself vertical. Light reflecting off of the surface of a lake that makes large portions appear white is horizontally polarized since the water's surface is horizontal.
To understand what this whole "circular polarizer" thing is all about, we need to start where polarizers first started, with something that is now known as a "linear polarizer." Typical construction for a linear polarizer consists of a sheet of polarizing film sandwiched between two pieces of plate glass. The film does all the work with the glass used solely to protect the fragile film inside. Polarizing film will allow only light with a given polarization to pass through with the rest being blocked. Since all the light that makes it through to the other side therefore must be polarized at the same angle, the light is said to be "linearly" polarized.
But the beam splitters, focus sensors and meters inside modern cameras can become confused by linearly polarized light. As you change the orientation of the camera, the amount of light that reaches the meter or sensor will change based on how close or far away it is from angle of light polarization. It won't break anything but it will affect the accuracy. In order to solve this problem, photographers made the shift to circular polarizers.
The term "circular polarizer" though is a bit of a misnomer since in reality the actual polarizing effect produced is no different from that of a linear polarizer. Indeed, a circular polarizer is a linear polarizer with one critical addition. Glued to the backside of the filter is something called a "quarter wave retarder" or "quarter wave plate" that will take some explaining.
But before I do so, I need to first explain birefringence. Growing up, I collected rocks and minerals, although they haven't been taken out of the back of the closet in years except to move from one apartment or house to another. Until now that is. For you, dear readers, I have dug out a calcite crystal. Calcite exhibits an odd property known as birefringence. What this means is that light passing through it is split into two components at right angles to each other, with one (known as the "extraordinary axis") getting through the crystal to the other side more quickly than the other (known as the "ordinary axis"). You can see my calcite crystal and the somewhat "double vision" effect of birefringence in the image here. As a kid, without being able to give a name to it, I thought it was cool. Now that I know what it is called, I still do.
It's time to get back to where we left our quarter wave retarder. A wave retarder is made out of a birefringent material of appropriate thickness that is similar to, but much more optically perfect than (and obviously much thinner than), my calcite crystal. As such, the phase of one component of light passing through it is delayed ever so slightly relative to the opposite component at right angles. As you may have guessed by this point, in the case of a quarter wave retarder, the amount of this slight delay is a quarter wavelength, far less than in my calcite crystal. But by retarding only one axis of the wave, the light becomes basically "twisted," dragged down along one axis but not on the other. This leaves the light in a state known as "circular polarization" where the light effectively corkscrews its way along as it leaves the filter.
So, a circular polarizer takes the linearly polarized light produced by an ordinary linear polarizer and re-scrambles it via the birefringent properties of a quarter wave retarder so that it won't interfere with your camera's metering and focus.
OK, now that we've completed our science lesson for the day by learning just what a circular polarizer actually is, what about the "circular" versus "round" question? The vast majority of polarizers, both circular and linear, are round so that they can be screwed onto the front of a lens. They can also be rotated to allow for control of the angle of polarization. But there are square polarizers, both linear and circular in function. Even well known filter manufacturers such as Tiffen make square and rectangular, yet circular, polarizers. You slide them into holders much the way you would Cokin or Singh Ray filters.
By the way, if they had used a half wave retarder to modify a linear polarizer instead of a quarter wave, the light coming out the back wouldn't be circularly polarized at all. Instead, it would still be linearly polarized, but now at right angles to the original orientation. Just thought I'd throw that in for you science trivia buffs out there.
While the effects of many other filters can be reproduced digitally, an actual polarizer that you place in front of your lens while shooting is the only thing that can truly do what a polarizer does. And what a strange and marvelous thing that is indeed.