A couple of points to add:
I don't believe that circular polarizers are necessarily of higher optical quality than their linear equivalents. Either because of market demand or more complex manufacturing needs, they do cost more, true. But they only came into existence at all because the newer TTL systems introduced in the '70's and '80's couldn't meter accurately through the traditional linear type.
Also, I don't think it's necessary to use one filter of each type. The movie industry used double-polarization for years to create fade-outs well before circular polarizers were on the market.
It used to be that all polarizers were actually sandwiches, and for all I know they still are. One sheet of extremely thin material is held between two glass plates. So you optical purists out there might want to go ahead and take your heart pills before you consider that the two-polarizer system might consist of six layers and twelve, count 'em: twelve, surfaces, all dedicated to making your expensive lenses look just a little shabbier. That said, if your pictures stay sharp with this method, there's no point counting or worrying.
A little arcania: If you have an old Polaroid camera, you'll notice that the corporate logo consists of two overlapping circles with the intersection darkened. Before it made any instant cameras, the Polaroid corporation's main product was polarizing material. Edwin Land, the company founder, and my hero, invented the stuff. Until then, polarizing relied on a very expensive mineral only useful in a research lab. In the years before World War II, Land pushed for many applications for his miraculous stuff, but few of them panned out. He wanted to make night driving safer by polarizing windshields and headlights. He tried to get house builders to install double-polarized windows for infinitely variable shading. Those commercial non-starters were among his few professional disappointments.
So, when you attach your polarizing filter, thank Edwin Land. When you watch a 3-D movie, thank Edwin Land. And as long as I'm off the topic, I ask you to consider this: When Polaroid introduced the SX-70 system in the early '70's, the company received 200 patents related to it. Land was awarded forty patents in his own name, just for that one camera. How miraculous was that camera? Miraculous film: SX-70 film had an ISO of about 100, but within a quarter second of exposure it would have completed a chemical process that would make it impervious to direct sunlight. So, the film saw one tiny unit of light to record a view, then instantly blinded itself to several million units of light, while still developing the image. Miraculous camera: the SX-70 was a folding, large format, automatic exposure, soon to be automatic focusing, close-focusing, single lens reflex camera that could fit into a jacket pocket.
And to think I was proud of myself just for getting out of bed on time this morning. If you're as interested in Land as I am, I recommend Land's Polaroid: A Company and the Man Who Invented It by Peter C. Wensberg. It's a fun, light read.
Thanks for tolerating my wandering. MB
