Polarizer

Ektagraphic · Aug 4, 2009

Hello-I just picked a circular polarizer. Was this the best type to buy or do you reccomend the roatating Polarizer?

Thanks

Q.G. · Aug 4, 2009

All polarizers rotate (or should), else they are of very limited use.

You need to be able to rotate the polarizer to make it work (or work more, or less).
If not set properly, all a polarizer is, is an expensive, 'so so' neutral density filter.

A circular polarizer is a simple linear polarizer, with an device behind it that 'scrambles' the light again.

That is only necessary if there are things behind the polarizer (like metering systems or AF sensors) that depend on the light not being polarized to function correctly (typically devices that use beam splitters or mirrors to direct light to where they need it).
Without such thingies, or with metering systems that are not particular about the polarisation of light, the cheaper linear polarizer would have done just fine. Yet a circular will always work. So no worries there.

As far as being a polarizer, both types - circular and linear - are the same.

George Collier · Aug 4, 2009

Doesn't your polarizer rotate? In other words - two glass layers, one screws into the lens filter threads and stays put, and the outside (toward the subject) layer should rotate. It's the relative position of the two "filters", in relation to the sun angle that creates the polarizing effect.

Ektagraphic · Aug 4, 2009

I'm not sure yet as I just ordered it today.

Someonenameddavid · Aug 4, 2009

The standard polarizer has a single sheet of polarizing material which can be rotated. Water molecules in the atmosphere have a polarizing and scattering effect upon sunlight which is more pronounced in the higher frequencies of visible light (which is why the sky is blue) and is why the most pronounced effect of the polarizer is at 90 degrees to the incident sunlight. For grins and chuckles (and 6th grade science fairs which inevitably win a prize) you can arrange a carousel slide projector with a circular cutout in a black slide to represent the sun: point the light source through a fish tank or other suitable water container. As long as the tank contains water only. there is not much scattering. Dropwise add milk, which forms clouds and as the concentration increases, the blue-white light is dispersed... and is polarized, and can be demonstrated so with your polarizing filter... The more milk you add the more orange the direct view of the light source becomes.

This is why the sky is blue and the setting sun is orange and polarizing filters darken the sky (and red /orange filters do so in B+W)..

Rayleigh Scattering (electrostatically dependent upon the imaginary component of the wave form)

David

randyB · Aug 4, 2009

Most polarizers have only one glass but two rings. As George says, one of the rings screws into the lens and is stationary, the outer ring which contains the glass rotates varing the polarized effect.
There are filters, usually for scientific use, that have two polarizing elements, one glass is stationary and the other one rotates. This has the effect of a variable density filter. When two polarizers are crossed they block about 90% of the light, some more, some less.

bsdunek · Aug 5, 2009

I would suggest that you use a linear polarizer if you don't have in camera metering or autofocus. I find them to have a much stronger effect than circular. Just IMHO.

Q.G. · Aug 5, 2009

Linear polarizers can only be stronger in effect if they are compared to a not-so-good circular polarizer.

Remember that the working bit is the same in both types.
So given a linear filter that has a 'strong' effect, using that same foil in a circular polarizer would produce the very same effect.
The retarder plate put behind the polarizer in circular polarizers will change nothing about what the polarizer in front of it has done. It cannot allow light through that did not make it through the polarizer in the first place.

Pumal · Aug 5, 2009

http://en.wikipedia.org/wiki/Photographic_filter#Polarizer

lxdude · Aug 5, 2009

The circular is used in place of a linear with cameras that use semisilvered mirrors for metering or focusing. If you don't have an autofocus SLR or one that uses a semisivered mirror as part of the metering system, then the cheaper linear will work fine.
So for instance, I have a Pentax LX which has a small mirror behind the main one which reflects to the photocell, so I use a circular polarizer to get correct readings. On automatic it reads off the first shutter curtain and the film, so a linear will work, but the readout before exposure may not be accurate.
My Pentax MX has its photocells near the eyepiece reading light from the focusing screen so a regular polarizer works fine-a circular is of no benefit.

Remember that circulars are considerably more expensive and are no "better"- they're just necessary for proper metering or autofocus function in some cameras.

bsdunek · Aug 5, 2009

Q.G. said:
Linear polarizers can only be stronger in effect if they are compared to a not-so-good circular polarizer.

Remember that the working bit is the same in both types.
So given a linear filter that has a 'strong' effect, using that same foil in a circular polarizer would produce the very same effect.
The retarder plate put behind the polarizer in circular polarizers will change nothing about what the polarizer in front of it has done. It cannot allow light through that did not make it through the polarizer in the first place.

Well, I am comparing different brands, however, I would think Hoya and Tiffen would both be good. In my case, the linear Tiffen seems to be better than the circular Hoya. This is just to my eye - not scientific.

Ektagraphic · Aug 5, 2009

I went with a Tiffen as I know that they are a legendary company AND the their stuff is MADE IN USA.

Q.G. · Aug 6, 2009

Where do they get their glass?

Ektagraphic · Aug 8, 2009

What is the filter factor for a Polarizer? Is there even one since it it has a variable effect? I usually do manual exposure using a light meter.

Q.G. · Aug 8, 2009

The filter factor is fixed, and usually is 2x (1 stop). It will say on the rim of the filter.

Manual metering is the best way of metering when using polarizers.

As you say, the effect is variable. Depending on the light in the scene, and the orientation of the filter, light coming from parts of the scene is held back (selectively; other parts are not affected).
That is what you want to happen (else you shouldn't use the filter ), so you do not want to compensate for that.

What you do need to compensate for is the overall, non-selective blocking that also occurs. And that is what the fixed filter factor is for.

If you would meter through the filter, the meter will also register the darkening of the bits you want to have darker. The meter of course has no notion of selective filtering, and just registers less light. So the parts that are unaffected by the filtering will be overexposed (the amount by which depending on how much the filter 'does') if you follow what a meter behind the polarizer suggests.
So you're on the right track!

Mind you: images can look a bit dark, flat, when there is a lot for the filter to block.
You could adjust by giving it a bit more exposure. But it would often be better to just reduce the filter's effect.
Exposing a bit more will produce lighter images that are still rather flat, without sparkle. It's better to not remove all sparkle in the first place.

Polarizer

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