December 2008: HelpLine
Defining Termsfont style="font-weight: bold">A Polarizing Topic
Q) I read your article in the September issue of PCPhoto, which referred to “polarizer” filters. Since I got back into picture-taking about seven years ago, I’ve been confused about the terms ”polarizer” and ”polarizing” being used to describe filters. To give you a bit of background, I’ve been playing around with cameras since my first, which was a 35mm Argus, circa 1938. I’m also a retired aerospace engineer. Now, it’s my understanding that ”polarizing” is done by nature. An example is reflections from water, which are ”polarized” light and can be removed by use of the filter. In other words, nature does the polarizing and the filter blocks or filters out the polarized light. As you can see, the term ”polarizer” confuses me when used to describe filters.
A) I should start off by saying this isn’t rocket science, but I’m sure you’ve heard that one before. Your question brings up a difficult issue that I and many others deal with on a regular basis: terminology.
There are times when being “technically accurate” often doesn’t help people when they’re in the real world talking with other photographers. There are other times when it’s useful to bend the rules to explain a concept.
I only have to start talking about what happens when you mount a lens on a digital SLR with a sensor that’s smaller than a 35mm film frame to get my mailbox burning with e-mail. Some are justified and some are unprintable. But it can be a delicate balance between trying to be exact in technology terms and yet understandable.
Cropping factor, magnification factor, focal-length multiplier, field-of-view factor have all made their appearances in this column at one time or another. Most times I’ve qualified the use of the term to assuage my entire audience, but not always. I suppose I should bring up “RAW” versus “raw,” but I’ll leave that for another day.
But it took a rocket scientist (okay, I just like writing ”rocket scientist”)—I mean an aerospace engineer—to bring digital photography back to my high school physics class. My teacher Mr. Gearan (who once asked on a test, ”How fast would you have to go through a red light to make it turn green?”) would probably excuse me a little bit for my use of the term ”polarizer.”
There are different uses for the term. I recall that, in the lab, when we used a ”filter” that passed only light that was polarized in one direction and blocked the rest, we used a polaroid filter. But since we ended up with only the light that was polarized in one direction (as opposed to mixed polarized light), we said we were polarizing the light. Edwin Land, who developed filters for polarizing light, created a company called Polaroid and produced some polaroid filters with that trademark. As a result, I think it’s possible that the “Polaroid” term through the years morphed into a more active ”polarizer.”
(As I wrote this response to your question, my word processor wanted to capitalize the ”p” in polaroid filter, so even my word processor wants me to use polarizer!)
Circular Vs. Linear
Q) In the September issue of PCPhoto, you talked about ”circular and linear” polarizing filters. I have an old Nikon 52mm polarizer that I’ve had for about 10 years. It works on my “digital SLR” Nikon, but is it circular or linear?
Via the Internet
Well, first of all, as a rocket scientist once told me, you should be asking about a 52mm polaroid filter, but I still can give you some guidance. The easiest way to determine if you have a circular or linear polarizer is to use a mirror. Stand in front of a mirror and hold the polarizer up to your eye with the threads facing you (as if your eye were a camera body and you were going to screw the filter into your eye; that is, the male side of the filter). Look through the filter and note its reflection in the mirror. Unlike when you use the filter on a camera, in this test it doesn’t matter how the filter is rotated.
Now flip the polarizer over (so the other side of the glass faces the mirror—the threads are facing out) and see if its reflection, as you look through it, looks darker than when you were looking at the other side. If it does, then you have a circular polarizer.
To understand what’s happening, we need to go back to what comprises a circular polarizer (and this is going to get a bit complicated). The filter has a normal linear polarizer on the front. Behind it is a special optical filter (called a quarter-wave plate) that causes the light to become circularly polarized.
In the mirror test, the mirror reverses the circular polarization of the light coming from the filter. It then gets converted back into linearly polarized light that’s polarized in the opposite plane of the original light and gets blocked by the linear polarizer.
One note I want to mention is that you say it “works” on your camera. Remember that both a linear and a circular polaroid filter (there, I said it!) will give you the same polarizing results in the photograph. It’s the effect on the camera’s autofocus system that may be noticed.