# Circular Polarizers - Autofocus - Loss of Sharpness



## Film4now (Sep 13, 2009)

I've taken sports photos of my sons' teams for years and didn't really need a polarizer.  However, the growing use of artificial turf fields has introduced a significant glare factor that I want to get rid of.  I had a linear one from my film days but didn't use that based on all the discussions about the impact on autofocus.  So I purchased a  Sunpak (that's what my local photo shop carried) that I use on my Nikon 18-200 VR lens.  When I compare shots with and without the filter, the ones with the filter are just not as sharp  - so much so that I don't want to use them.  This is true whether I use the D70 or D300s. Should I purchase a different brand?  Forget the filter and live with the glare?   Thoughts?


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## Derrel (Sep 13, 2009)

A polarizer has two pieces of glass, and introduces four separate air-to-glass surfaces. The chances of a poorly constructed polarizer causing easily seen sharpness loss is VERY high. A polarizing filter is one item where it truly pays to buy a top-quality model from Nikon or B+W.

In the 1980's I had a low-cost polarizer that was total garbage. I bought it and shot it,and the first rolls of slide film came back useless. It was then that I bought my first Nikon polarizer.

The choices seem clear; A) ditch the Sunpak polarizer and either B) purchase  a good quality polarizer or C) shoot without any polarization at all.


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## Plato (Sep 13, 2009)

Derrel said:


> A polarizer has two pieces of glass, and introduces four separate air-to-glass surfaces. The chances of a poorly constructed polarizer causing easily seen sharpness loss is VERY high. A polarizing filter is one item where it truly pays to buy a top-quality model from Nikon or B+W.
> 
> In the 1980's I had a low-cost polarizer that was total garbage. I bought it and shot it,and the first rolls of slide film came back useless. It was then that I bought my first Nikon polarizer.
> 
> The choices seem clear; A) ditch the Sunpak polarizer and either B) purchase  a good quality polarizer or C) shoot without any polarization at all.



Two pieces of glass?  None of my polarizers had that.

I do agree with buying high quality filters.


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## Derrel (Sep 13, 2009)

Circular polarizers are made of two pieces of glass. In a circular polarizer, there is a linear polarizing filter on the front, which allows through one polarization of light while rejecting another, followed by a quarter-wave plate, which is the second piece of glass and which turns in a rotating metal frame. The need for two,separate pieces of glass means that there are, obviously, FOUR air-to-glass surfaces.

If you want to prove this, tear apart a cheapie circular polarizer and you can easily see for yourself. Here is a nice,small on-line diagram.

http://www.anchoroptics.com/images/products/CircPolarGraphweblarge.jpg

B+W makes some of the world's best circular polarizers. Automatic, through the lens metering systems have necessitated circular polarizers for about the last 30 years,and of course, autofocusing cameras also need circular polarizers if you want the autofocus system to work.


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## Plato (Sep 14, 2009)

Derrel said:


> Circular polarizers are made of two pieces of glass. In a circular polarizer, there is a linear polarizing filter on the front, which allows through one polarization of light while rejecting another, followed by a quarter-wave plate, which is the second piece of glass and which turns in a rotating metal frame. The need for two,separate pieces of glass means that there are, obviously, FOUR air-to-glass surfaces.
> 
> If you want to prove this, tear apart a cheapie circular polarizer and you can easily see for yourself. Here is a nice,small on-line diagram.
> 
> ...



It's a "sandwich" construction and there are only two glass-to-air surfaces and, of course, the two pieces of glass rotate simultaneously.  Rotation affects the polarizing surface but the circularizer (randomizer) is oblivious to the rotation.


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## table1349 (Sep 14, 2009)

Derrel said:


> Circular polarizers are made of two pieces of glass. In a circular polarizer, there is a linear polarizing filter on the front, which allows through one polarization of light while rejecting another, followed by a quarter-wave plate, which is the second piece of glass and which turns in a rotating metal frame. The need for two,separate pieces of glass means that there are, obviously, FOUR air-to-glass surfaces.
> 
> If you want to prove this, tear apart a cheapie circular polarizer and you can easily see for yourself. Here is a nice,small on-line diagram.
> 
> ...



Except the link that you posted is not for a circular polarizing glass filter.  It is for film. Here is the link to the whole site including the film. 
Anchor Optics - Circular Polarizer Film

Polarizing filters are one piece of glass.  Here is how B&W makes their filters. 

_For a polarizer to work, a condition of polarized light is
necessary. Light rays diverge in a manner similar to that
of a water wave. Light from the sun or a lamp consists
of rays vibrating in many directions simultaneously. This
is called unpolarized light. Light rays vibrating in only
one direction are called linear polarized light.
*B+W polarizers have a foil, which consists of a grid
construction, cemented between two pieces of glass*.
This grid construction is invisable to the eye, and it only
allows light rays vibrating parallel to the foil grid to pass
through. Light rays which are vibrating perpendicularly to
the foil grid are totally blocked while other directions are
partially suppressed._

The rotation of a polarizing filter is to align the direction you want the vibrating light waves to enter the lens.  By changing the direction of the vibration light waves you change the effect the polarizer produces.  
B&W

This is what you get when you use two pieces of polarized glass in a filter.  http://www.singh-ray.com/varind.html.


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## Garbz (Sep 15, 2009)

Indeed quarter wave plate does not mean glass. The thickness of these "plates" is often only in the 10s of microns. Their application can for instance be vacuum backed just like an anti-glare coating. 

That said I agree with the overall premise. Cheap polarisers are nasty! Bad Nasty! In my experience cheap polarisers are worse than cheap UV filters by quite a margin. A semi decent Hoya like the SHMC Polariser or Pro1 cost more than $70


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## Plato (Sep 15, 2009)

gryphonslair99 said:


> Derrel said:
> 
> 
> > Circular polarizers are made of two pieces of glass. In a circular polarizer, there is a linear polarizing filter on the front, which allows through one polarization of light while rejecting another, followed by a quarter-wave plate, which is the second piece of glass and which turns in a rotating metal frame. The need for two,separate pieces of glass means that there are, obviously, FOUR air-to-glass surfaces.
> ...



Years ago I used dual polarizers as an infinite ND filter but it doesn't work with CPLs.


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## Dwig (Sep 15, 2009)

Plato said:


> ...
> Years ago I used dual polarizers as an infinite ND filter but it doesn't work with CPLs.



It won't if you mount both in the conventional orientation. You can either use a linear polarizer for the front filter with a circular pol behind it or invert the front circular polarizer.

One thing left out of the descriptions of polarizers is the circular polarizing element. All of the descriptions apply directly to linear polarizers only. Circular polarizers have an extra element in the sandwich.

Polarizing elements, the "grid" referred to in the B+W quote, are extremely fragile. Simply touching them will destroy them. They must be laminated with something durable to protect them. There are two methods used for photographic polarizers. One is to laminate the polarizing film between two uncoated sheets of glass and then polish to two outside surfaces flat and parallel. This leaves an uncoated filter, but it results in little optical problems otherwise. The other method is to coat, or mult-coat, one surface of each glass wafer before laminating the film between them. This results in a coated filter but keeping the two outside surfaces perfectly plano-parallel is rather difficult. Any error destroys image quality. The nature of both how coatings are applied and the polarizing film generally makes coating after laminating impossible.

Circular polarizers differ in that they have a second element behind the polarizing element for a total of 4 components. This is the "circular polarizer". It acts to de-polarize the light passing through it. The result of the completed sandwich is that the first element, the so-called linear polarizer, selects the desired portion of the light and the second element, the circular polarizer, scrambles the orientation of the light so that it doesn't create problems for the AF, metering, and viewing sysystems in the camera.

The lamination necessary is a large part of why polarizers are more expensive than simple filters made from dyed glass. Additionally, the circular polarizing element is rather difficult to make if you want it to be perfectly neutral in color. You will find that, in general, the more expensive filters are the mos neutral. Inexpensive circular polarizers very often have a distinct color tint, often yellow/amber. A third issues is sealing the outer rim of the sandwich. The cheaper filters are generally unsealed and have raw glass edges which reflect light internally (this is also true of inexpensive simple filters). The better filters have some form of edge sealing. B+W has a special sealing technique used in the Kasseman versions which is supposed to be better than other methods.

The quality of any filter, but especially a polarizer, is as important as, or possibly more important than, the quality of the lens used. All filters always degrade the image quality, period. The differences are only in the degree of the loss.


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## Plato (Sep 15, 2009)

Dwig said:


> Plato said:
> 
> 
> > ...
> ...



Minor problem with the screw threads!   



Dwig said:


> The quality of any filter, but especially a polarizer, is as important as, or possibly more important than, the quality of the lens used. All filters always degrade the image quality, period. The differences are only in the degree of the loss.



I have made that very same point many times.   :thumbup:


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