[TomK]

Just some side-by-side comparisons of shots done with a Marumi linear polarizer on the K20D through a Super Takumar 85/1.9. The shots on the left/top are done with the filter rotated to kill the filter's effect and on the right/bottom with the filter rotated to maximize the filter's effect.

First of all photo #3 is a great example of what polarizers do and don't do in general.
Should be in every photo textbook!

Now, I have a question about photos #1 and #2.
Note that the other day I took some shots in a quickie test of my new K200D. About a dozen shots w/out filter and as an afterthought 2 shots with the (circular) pola on, at maximum.
The pola shots were of the same picture that I just took without the filter. In both shots the pola ones look about 1 or 1.5 stops underexposed. I am not talking about the polarized elements - the whole shot. Just like your #1 and #2; in these there is a definite darker photo and a lighter one. Note that in your car photo the distant white fence is dark and in the shed one the white siding is also dark - but only in one photo.
Using PhotoMe, the Exif indicates for my photos that the pola ones were indeed exposed at a different EV so the pola filter factor should have been negated.
Anyone have any answers?
I have used pola filters on film cameras both linear and circular but have never seen this result before.

Thanks, TomK

[Canada_Rockies]

First of all photo #3 is a great example of what polarizers do and don't do in general.
Should be in every photo textbook!

Now, I have a question about photos #1 and #2.
Note that the other day I took some shots in a quickie test of my new K200D. About a dozen shots w/out filter and as an afterthought 2 shots with the (circular) pola on, at maximum.
The pola shots were of the same picture that I just took without the filter. In both shots the pola ones look about 1 or 1.5 stops underexposed. I am not talking about the polarized elements - the whole shot. Just like your #1 and #2; in these there is a definite darker photo and a lighter one. Note that in your car photo the distant white fence is dark and in the shed one the white siding is also dark - but only in one photo.
Using PhotoMe, the Exif indicates for my photos that the pola ones were indeed exposed at a different EV so the pola filter factor should have been negated.
Anyone have any answers?
I have used pola filters on film cameras both linear and circular but have never seen this result before.

Thanks, TomK

You just hit the angle that causes erroneous metering. The problem only occurs when the polarizer and the beam splitter interfere with each other. You could take thousands of pictures with little or no problem, and then the killer one is no good at all - and it was worth all the others. I'm afraid that I (expensively) subscribe to the "if it can possibly happen in 1 in 1,000 shots" that is too many for me.

[TomK]

You just hit the angle that causes erroneous metering.

Ah. That makes sense. The darkening effect is immediately apparent when pixel peeping so knowing to look for it in the future I can correct it by changing my position or whatever.
As I said these 2 pix were just an afterthought.
Thanks Canada Rockies for the quick reply.
-TomK-

[res3567]

Boy, I was way off-base!

Thanks for the corrections mattdm and bdery!

I have to admit to being almost clueless regarding circular polarizers, but I do know how the linear version works. That is why I put the comment in regarding accuracy. My ignorance regarding the circular polarizer is based on being out of the photo world for about five years during which these items came to market. Having made my confession, I can say that my understanding is similar to the explanation given by mattdm. Having never actually worked with a circular polarizer, I just assumed that the need to rotate the filter was gone. After some consideration, that assumption was pretty silly.

So let me get this straight.

Linear polarizer: Bad for AF systems, works for MF

Circular polarizer: Works for both AF and MF systems

Steve

Thanks for trying. You gave it all what you had and even said you may stand corrected. Bravo for sharing your info. DON'T FELL BAD AT AT ALL! That is what this forum is about.

[SOldBear]

hi Mike, very nice test. what is the difference between the linear and circular polarizer

Now that you know the financial and the technical differences between linear and circular polarizers. Maybe you want to know the answer to the practical question: given a polarizer, how can one tell if it is a linear or a circular polarizer?

1. Read the label. If it says "circular polarizer," chances are it is. If it says "polarizer," it can be either.

2. Stand in front of a mirror, hold the polarizer so it covers one of your eyes. Look at the eye behind the polarizer in the mirror. If the eye is NOT visible, the polarizer is circular. If the eye is visible, the polarizer can be either. Now, flip the polarizer so that the side previously facing the eye now faces the mirror. If the eye is NOT visible, the polarizer is circular. If the eye is visible, the polarizer is linear.

(#2 is based on the fact that circular polarizer is not "symmetrical").

[DAZ]

You just hit the angle that causes erroneous metering. The problem only occurs when the polarizer and the beam splitter interfere with each other. You could take thousands of pictures with little or no problem, and then the killer one is no good at all - and it was worth all the others. I'm afraid that I (expensively) subscribe to the "if it can possibly happen in 1 in 1,000 shots" that is too many for me.

Now that you know the financial and the technical differences between linear and circular polarizers. Maybe you want to know the answer to the practical question: given a polarizer, how can one tell if it is a linear or a circular polarizer?

1. Read the label. If it says "circular polarizer," chances are it is. If it says "polarizer," it can be either.

2. Stand in front of a mirror, hold the polarizer so it covers one of your eyes. Look at the eye behind the polarizer in the mirror. If the eye is NOT visible, the polarizer is circular. If the eye is visible, the polarizer can be either. Now, flip the polarizer so that the side previously facing the eye now faces the mirror. If the eye is NOT visible, the polarizer is circular. If the eye is visible, the polarizer is linear.

(#2 is based on the fact that circular polarizer is not "symmetrical").

At the risky of way oversimplifying this I will take a crack at this.

If you think of light as a wave and that these waves go back and forth. (This is the oversimplifying part as light is a wave and a particle and they go around and around. Skip the last part, as it is confusing.) Most of the light goes in all directions. Some of the light when it bounces off an item will be more one way then any other. Lets say it is going up and down. Think of a polarizes as a lot of bars really really close to each other and parallel to each other. If the light is going up and down and the bars are up and down then the light is blocked by the bars and can’t get through. So when you use a polarizer you turn is to block the light (usually some reflection you don’t want) that is mostly of one pole.

Now in most cameras there are splitters for the auto focusing and light metering. These can either have or act like polarizes. If you have a linear polarizer and turn it to the same as the splitters in the camera the focusing and/or light metering can be off. If you meter, focus and control the camera manually then this is not important.

As you may not know when this will happen this is why “they” say to you should use a circular polarizer on modern cameras. A circular polarizer that are used on cameras (a true circular polarizer filters left/right rotating, but that is back to the confusing stuff) are a linear polarizer in the side to the subject and on the camera side a layer that makes the light close to the random poles before the liner part. One pole is still not getting through but the output looks random so the splitters will work (this is why the mirror trick works for circular polarizer). This means you still need to turn the circular polarizer to filter out the part you don’t want.

Hope this helps and not confuses.

DAZ

[Wheatfield]

Pentax cameras, with only one exception, meter off the focusing screen, if I am not mistaken.
With this in mind, a linear polarizer isn't going to affect metering at all, since there is no beam splitter involved.
If you are like me and use mostly manual focus lenses, there is not much reason to spend the bucks on a circular polarizer, and you are better off buying a good quality coated linear than an uncoated circular.

[Ben_Edict]

Now in most cameras there are splitters for the auto focusing and light metering. These can either have or act like polarizes. If you have a linear polarizer and turn it to the same as the splitters in the camera the focusing and/or light metering can be off. If you meter, focus and control the camera manually then this is not important.

As you may not know when this will happen this is why “they” say to you should use a circular polarizer on modern cameras. A circular polarizer that are used on cameras (a true circular polarizer filters left/right rotating, but that is back to the confusing stuff) are a linear polarizer in the side to the subject and on the camera side a layer that makes the light close to the random poles before the liner part. One pole is still not getting through but the output looks random so the splitters will work (this is why the mirror trick works for circular polarizer). This means you still need to turn the circular polarizer to filter out the part you don’t want.

In most cameras today there are NO beamsplitters in front of the lightmeter. All Pentax DSLRs and most others have the lightmeter residing in the prism housing and take their reading (as Wheatfield noted) directly off the matte screen.

Only the AF sensors usually get their share of light via a semi-transparent mirror, which also polarizes the light. What is the consequence? The AF sensor may receive a bit less light when using a linear versus a circular polarizer. So, under dim light condition AF may work less reliably or not at all with a linear polarizer, wheras it might still be useable with a circular polarizer. But for most polarizer shots (not all, of course) this is of no consequence, as most are taken under sunny conditions to increase the sky's saturation. So, at the end of the day, the influence of the LPol versus CPol is minimal and may for many photogs not justify the price difference.

The only Pentax, that comes to my mind, that was really not useable (officialy) with a LPol, was the LX, as its lightmeter cell received the light via the polarizing semi-transparent mirror. But to be honest: even then I did not notice an ill effect...

Ben

[newarts]

I tested my K100D with a Hoya Linear polarizer and a source of uniform unpolarized light (white paper).

As the polarizer was rotated with respect to the camera, the metered speed changed from 1/45 sec to 1/30 sec - about 1/2 stop.

This effect did not depend on how the camera was rotated with respect to the light source, demonstrating the effect was soley due to the orientation of the linear polarizer with respect to the camera.

The effect was much less pronounced with a circular polarizer.

Iowa Dave

PS there's no inherent reason a linear and circular polarizer should vary in their effectiveness in screening polarized light; this is because the circular polarizer is a linear polarizer followed by a de-polarizer ("quarter wave plate".)

[jjdgti]

Interesting info. I have some older Polirizer that I use with my manual (Film) cameras. I was afraid to use them in my DSLR. I will try to do atest and see if ther is a difference.

[a.severin]

Hallo, I am brand new to the forum as well as to my K20D. I have been shooting some photos which would have been helped by a polirizer filter. So now the question is linear or circular? I understand that your experience is that on manual focus a less expensive filter is ok, but what about autofocus?

And another question, it is only a matter of spending more or less money or the circular filter doesn't do everything the linear filter does?

Thanks everybody.

[tr13]

Circular, find any on ebay and buy.
They are from $6 to $100.
Depends on quality and brand.

[Tuner571]

Wow, I didn't realize the difference. I do recall when I got my first circular polarizer I was told that I want to stay away from linear polarizers as they do mess up the autofocus and the metering on todays newer cameras. However, through this test it clearly looks like it doesn't matter which on you use.

[Ben_Edict]

Wow, I didn't realize the difference. I do recall when I got my first circular polarizer I was told that I want to stay away from linear polarizers as they do mess up the autofocus and the metering on todays newer cameras. However, through this test it clearly looks like it doesn't matter which on you use.

There are only a handfull of cameras, that are actually affected by the linear polarizer and there is no Pentax among those few (except for the old film Pentax LX.)

Ben

[Ben_Edict]

Hallo, I am brand new to the forum as well as to my K20D. I have been shooting some photos which would have been helped by a polirizer filter. So now the question is linear or circular? I understand that your experience is that on manual focus a less expensive filter is ok, but what about autofocus?

And another question, it is only a matter of spending more or less money or the circular filter doesn't do everything the linear filter does?

Thanks everybody.

No, that experience is valid for AF lenses too. Linear polarizers are perfectly suited for the current Pentax DSLRs. Better a good linear pol, than a poor circular pol. (see my Post No. 23 on this page)

Ben