[Wheatfield]

As I mentioned in my other thread, I am still using linear polarizing filters, but for the new 50/1.4 I decided to buy a good quality circular polarizer. I ended up with a
Zeiss T*, which seems quite good. Going through some boxes of shelf litter yesterday, I came across a 77mm B+W circular polarizer and a 77-72 step down ring that i don't recall why I purchased.
Anyway, one of the things I used to do with linear polarizers was stack one on the other to use as a variable ND filter, so for no particular reason, I put the 2 filters together to see what would happen.
With 2 linear filters darkening is virtually complete, but with the circular ones, there doesn't seem to be any effect that way.

So, I've discovered a great big gaping hole in what i know, and I'm curious as to why this is. Knowing that there are many people here far smarter than I, this seems like a good place to come for an explanation.

As an aside, I also picked up a couple of Breakthrough Photography ND filters, so the question is purely academic.

thanks

bill

[pentaxus]

A circular polarizer polarizes the light just like your old polarizer but then it depolarizes what is left before it comes out of the filter. Polarizing filters are polarizing material sandwiched between two pieces of glass. The inner piece of glass is a quarter wave plate that does the depolarization of what is remaining.

[photoptimist]

Circular polarizers contain two filter layers. The first is a linear polarizer and the second is a quarter-wave retardation plate that shifts part of the electromagnetic wave to create a circular-polarized light.

You can stack a linear polarizer in front of a circular polarizer to create a variable ND but the reverse stack won't work. You can also reverse a circular polarizer and stack it on the front of another linear or circular polarizer to create a variable ND.

[Mikesul]

Circular polarizers contain two filter layers. The first is a linear polarizer and the second is a quarter-wave retardation plate that shifts part of the electromagnetic wave to create a circular-polarized light.

You can stack a linear polarizer in front of a circular polarizer to create a variable ND but the reverse stack won't work. You can also reverse a circular polarizer and stack it on the front of another linear or circular polarizer to create a variable ND.

Nice tip! Thanks.

[Wheatfield]

I love this place. Thanks for the explanations.

[Bob 256]

Circular polarized light and linearly polarized light are apples and oranges so a linear filter has no effect on circular polarized light and visa versa. A retarder is used to convert one to the other so if linear polarized light passes through a retarder (the correct one since there are different types) it becomes circularly polarized. It can be switched back to linear by passing it through another retarder.

Circular polarized light has two "senses", clockwise, and counter-clockwise (right hand and left hand are sometimes used terms), and there are those two types of circular filters. If the light is cw polarized, it is blocked by a ccw filter (and visa versa). If polarized light is reflected, its sense changes so looking through a circular filter into a mirror will show an opaque filter. Circular filters are used in 3D movies since they are less affected by head angle (tilting your head to the left or right has little effect on them as opposed to a lot with linear which used to be the 3D norm).

Using polarizing (linear) filters for ND filters is possible but you need to be aware, they can impart color casts and they transmit infrared so don't ever use one to look at the sun or a very bright source because the IR can cause eye damage even though they are blocking visible light.

Another bit of trivia - place a third linear polarizer between two crossed linear polarizers (meaning they are adjusted for minimum light transmission). You can "unpolarize" the light and get light to come through if the middle filter is rotated. Figure that one out (has to do with vectors adding to produce a new vector). Both linear and circular polarizers are cool!!