square polariser

I was recently left some photography equipment. One item was a square polarizing filter. This has a circular component with a flat square dark sheet. Although I am familiar with the circular polarizer I have no idea how this works. Do you have to turn the whole thing like a circular polarizer. or is it fixed?

I'm not sure what you mean.

There are polarizers in square mount for Cokin-style filter holders - the polarizing element rotates...

 

A "circular" polarizer can also refer to a special kind of polarizer needed for accurate AF on certain cameras (LINK)

 

A polarizer in a Cokin mount may be either linear or circular in this latter sense.

"Do you have to turn the whole thing like a circular polarizer. or is it fixed?"

 

- You have to turn any kind of polariser, regardless of shape or type. The filter's polarising plane has to be oriented at right angles to the polarised light that you're trying to block. That's how polarisers work.

 

Incidentally, the polarising membrane used is the same in both Linear and 'Circular' polarising filters. The difference being that a Circular polariser also has a following quarter-wave plate element that de-polarises (or scrambles) the light coming from the rear of the filter. Therefore Circular polarisers are unidirectional and must be used with the front facing the subject. A Linear polariser will work either way round and their effect on the subject is otherwise identical.

 

'Explanatory' diagrams showing Circular polarising filters removing a spiral-shaped wave of light, and Linear polarisers removing transverse waves are just nonsensical. It's what happens to the light after passing through the polarising membrane that differs between the two types of filter.

You have the "80's version of "Cokin filters - I own multiple lenses, but don't want to buy an expensive polarize / filter for each lens. For goodness sakes, they still make them and other companies have their variations ! So this big company came out with filter adapters. A big square filter and threaded lens adapters that fit on the big square holder. You can still find them on E-bay or Amazon. So there are adapters for each lens thread size that fits on the one square filter holder. I will assume there is a polarizer model that will rotate. Now some bad news. The old polarisers work on film. Newer digital camera need circular polarisers in order for the AF to function right. There are thousands of used glass models available. Back in the day, there were few zoom lenses. So your 5 lenses all had different diameters, this system was perfect. With modern zoom lenses you now only have 1 or 2 lenses. Even two new polarisers would only cost some $20 each size....unless you are talking about B&W makes. And the Cokin were plastic, don't shoot through plastic. On the other hand, don't buy a $9 China filter.

It isn't only the AF.

 

The optical low-pass filter, at least the ones I know, are made with birefringent materials.

 

You might remember these from school days, specifically calcite, where you can look through a crystal and see two images.

 

The refractive index is different for different polarizations.

Six months past its sell-by date for the OP, but this circular/linear polariser thing seems to have been one of the biggest marketing cons in history. Because I'm darned if I can see any difference in function or result between fitting a linear or circular polariser to an AF film camera or DSLR.

 

I can hear the marketing meeting now:

'Here's the plan; we tell the suckers that they all absolutely need a different and more expensive type of filter to go with their new camera. They'll be too scared to actually test if that's the case. Heh, heh, heh, heh, heh!'

Six months past its sell-by date for the OP, but this circular/linear polariser thing seems to have been one of the biggest marketing cons in history. Because I'm darned if I can see any difference in function or result between fitting a linear or circular polariser to an AF film camera or DSLR.

 

I can hear the marketing meeting now:

'Here's the plan; we tell the suckers that they all absolutely need a different and more expensive type of filter to go with their new camera. They'll be too scared to actually test if that's the case. Heh, heh, heh, heh, heh!'

I have also used older linear polarizers with newer DSLRs and exposure metering and AF seems to work as well as with circular polarizers--I'm with you about marketing hype on this one.

Interesting. I'll try to remember to take a photo through my sunglasses (aka "whoops I forgot the polariser") and see what happens. I'm not quite keen enough to buy a linear polariser just for experimentation.

 

It looks like Lee still have circular (as in "round") polarisers intended to fit into their square filter system, via an adaptor. They seem to be fairly expensive, but then they're 105mm polarisers, so they would be. Otherwise, my first rummage for linear polarisers turned up some that are about the same cost as circular (output) polarisers, which are obviously more commonly useful these days. The old "buy linear to save money" adage seems to be missing twenty years of economy of scale.

The old "buy linear to save money" adage seems to be missing twenty years of economy of scale.

 

- Yes, true that there's almost no difference in cost these days, but there must be literally tons of perfectly good linear polarisers lying around unused; for fear that they won't work on, or actually cause damage to, a digital camera.

 

The only redeeming thing about the situation, is that polarisers are now routinely multi-coated. A nicety that was omitted in the days of plain-vanilla linear pols.

Interesting. I'll try to remember to take a photo through my sunglasses (aka "whoops I forgot the polariser") and see what happens. I'm not quite keen enough to buy a linear polariser just for experimentation.

 

It looks like Lee still have circular (as in "round") polarisers intended to fit into their square filter system, via an adaptor. They seem to be fairly expensive, but then they're 105mm polarisers, so they would be. Otherwise, my first rummage for linear polarisers turned up some that are about the same cost as circular (output) polarisers, which are obviously more commonly useful these days. The old "buy linear to save money" adage seems to be missing twenty years of economy of scale.

 

The Lee holder will take any brand of 105mm polarizer...the Lee tends to vignette quite a bit at wider focal lengths...best option would be a slim 105, but they can get quite pricey.

 

Lee also has a 4x4 square glass circular polarizer that slides into the holder...you can simply rotate the holder to get the desired amount of polarization. One bit of caution...the square polarizer will only "work" as long as you keep the engraved markings on the filter facing the subject...if they're facing you, there's no polarization effect. One other bit of caution...since the filter slides into the holder, you can use it in conjunction with graduated filters. However, in this case, to get proper polarization you'd need to remove the polarizer from the holder, rotate it 90 degrees, and slide back in, which could limit the amount of polarization you'd get.

 

I use this polarizer occasionally, however I do keep multiple round CPL's in various filter sizes...not a fan of step up rings.