[Digitalis]

This post seems to indicate otherwise.

There was a situation where there was said to be an effect from the LP, but the poster did not test it to confirm this. As Ben Edict stated Pentax DSLR architecture isn't the sort that suffers from failure with Linear polarizers. Error perhaps, No camera system is 100% accurate under every circumstance, but no complete AE/AF failure has been reported. The problem with testing for equipment faults with things like this is that the chance of an error occurring is so small it is statistically improbable for such a failure to occur in everyday situations.

because we are dealing here with 2 glass plates and 4 glass-to-air surfaces.

It's actually two glass to air surfaces, typically they cement the two plates together,and fit them in the rotating mount. Because if they weren't cemented together the quarter wave plate would shift out of sync with the polariser foil and believe me, you don't want that to happen.

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Last edited by Digitalis; 10-29-2010 at 05:59 AM.

[Eruditass]

The circular polarizers are only necessary for cameras, which have a semi-transparent mirror or the like between the lens and the exposure meter. That is for example what the Pentax LX had. No Pentax DSLR has such a construction, as all of them take the meter reading inside the prism housing and get the light reflected directly from the main mirror. So, no polarizer will affect the metering, whether it be a linear or circualr type. And that is true for almost all current DSLRs.

AF is different, as in front of the AF sensors, there is a polarizing element (aka a semi-transparent mirror). BUT this will not lead to AF errors or malfunction. In the worst case it will readuce the amount of useable light for the sensor, which may lead to the AF stop working in very dim conditions. But in those conditions, you are highly unlikely to use the polarizer, anyway.

wouldn't a polarizing semi-transparent mirror split half the polarizations up to metering and the other half behind to the AF sensor, thus making both affected by the polarity of light?

So, in theory using a linear polarizer instead of the circular type, may affect AF function. In practice this effect is irrelevant.




I personally prefer high quality multicoated polarizers, because we are dealing here with 2 glass plates and 4 glass-to-air surfaces. That increases the probablity for flareing ang ghosting very much. I don't care much about the reduced loss of light, as the polarizer will reduce the light much more.

Ben

P.S.: we have had a fair amount of in-dpth discussions of polarizers a couple of months ago and you'll find a lot of info in those threads.

I also prefer high quality multicoated polarizers but can't find any information on the linear ones.

edit: B+W seems to be the only one I've found with a coating on their linear polarizers, although they are more expensive than the Marumi DHG CPL's which received higher marks!

[lmd91343]

As a practical note, I only buy circular polarizers because:
1) My old film manual focus SLRs have beam splitters in the condenser for metering (Canon F1N AE)
2) My future cameras MAY require them even though my K10D & K20D do not

I bought linear polarizers 40 years ago when circular ones were not available. I had to estimate the effect on the metering system. I subsequently had to replace the five LPL I had with CPL at a cost of over $400.

I see it as silly to purchase a 67mm LPL for my new lens (pentax 50-135). I would rather take the hit on the small additional cost now than later having to buy a 67mm CPL for what ever my next new camera may require.

Hmmm, What do I do with the 49mm, 52mm, 2x 55mm, 58mm, 62mm, & 2x 72mm LPLs that I have?

[Eruditass]

Well, I am only buying two 77mm filters (to act as a variable ND), and have step-up adapter rings for everything below

[Ben_Edict]

This post seems to indicate otherwise.

That's a very old thread, search for the more current ones and you
ll find lots of first-hand comparissons and experience, which support the theory. Also, I cannot see any contradiction between something I wrote and the posts in this thread - but that may be just me.

Ben

[Ben_Edict]

It's actually two glass to air surfaces, typically they cement the two plates together,and fit them in the rotating mount. Because if they weren't cemented together the quarter wave plate would shift out of sync with the polariser foil and believe me, you don't want that to happen.

You are right. I repeated the 4-air-to-glass surfaces, without getting up-to-date with filter production. In the past, cememting the polarizing foil to the glass wasn't feasable and many old Pols can degrade severly (shrinking and crumpling foil). But today they are cemented together, which reduces the number of glass-air transitions, as you wrote. Thanks!

Ben

[Ben_Edict]

wouldn't a polarizing semi-transparent mirror split half the polarizations up to metering and the other half behind to the AF sensor, thus making both affected by the polarity of light?

The problem with semi-transparent mirrors is, that the light, that goes through the mirror gets polarized by that mirror. The light that is just reflected on its surface isn't affected/polarized. Thus, light going through the semi-transparent mirror will be double-polarized, when you use a linear pol in front of the lens, which leads to gross errors during metering.

Ben

[Eruditass]

The problem with semi-transparent mirrors is, that the light, that goes through the mirror gets polarized by that mirror. The light that is just reflected on its surface isn't affected/polarized. Thus, light going through the semi-transparent mirror will be double-polarized, when you use a linear pol in front of the lens, which leads to gross errors during metering.

Ben

This doesn't make sense to me. It can't make light out of nothing, so it reflects what it doesn't pass through (or absorb), which means both are affected by the polarization.

If the mirror doesn't split it based on polarizations, then neither are affected. Either both or neither should be affected.

[abacus07]

I don't have a problem with linear filters since all of my lenses are MF (and I will probably only have one DA lens (a 15 or a 21) in the next 3-5 years). But I'm kind of wondering how big of a price difference there actually is between a linear and circular (in the same product line). by the way that everyone talks it should be substantial but I've only found 10-20 dollars. And there are a lot more options with circular like the Marumi coated which looks like a terrific value (cheaper than the only coated linear that I've found).

I'm kind of interested in knowing how important the coating is or is it overrated and you should go with a top brand, non-coated circular (or comparable linear).

Unrelated to this but related to my filter purchase is the dreaded step-down ring. All of my lenses except one is a 49mm (unless I find a bargain on a tak 24mm/f3.5). The vivitar 28mm has a 52mm thread. I'm not even sure how much I'll use this lens because it's close to the tak 35/f3.5. BUT if my understanding of the issue with step down rings is correct this shouldn't be an issue (or reduced) with the 1.5x crop factor. And if you still had a problem then you could stop down. I'm just not interested in having an extra unnecessary ring that I need to use on all of my lenses.

[Eruditass]

So the company I bought from sent me Marumi CPL's instead of LPL's. They were out of stock, but sent me SunPak LPL's temporarily. I took some test images:

CPL vs LPL

Reference:



LPL:



CPL:



And 2 stacked LPL's (the reason I got them... to simulate a Var ND):


Quite frankly, I was shocked at how bad the CPL was. I had taken shots with it before but hadn't noticed anything this bad. Perhaps it was just the worst case for this one. I re-shot it several times, no change.

Setup: Indoors, mix of incandescent and daylight. Manual focused. Close-ish object distance. Tamron 70-200mm on a tripod with 3 second remote trigger. NOT cropped, only resized and horribly compressed.

Reshot CPL with hood, adjusted manual focus, turning filter, all yielded similar results.

Maybe I got a bad copy or something. I have the option to switch my Sunpaks with Marumi LPL's early next month, anyone have any opinions on these two companies?

[Manel Brand]

I took some test images:

Hi Eruditass.

Please excuse my basic question (I was reading the thread and ended on this test photos you did). Question is: why are using/testing a polarizer with this kind of photo? Polarizers are not design for product shots, are they? Correct me if I'm wrong. Thanks.

[Eruditass]

Hi Eruditass.

Please excuse my basic question (I was reading the thread and ended on this test photos you did). Question is: why are using/testing a polarizer with this kind of photo? Polarizers are not design for product shots, are they? Correct me if I'm wrong. Thanks.

Image quality degredation. All polarizers will all filter out reflections and turn skies blue just the same. Other factors not tested here are flare control, transmission functions, vignetting, and homogeneity.

[Manel Brand]

All polarizers will all filter out reflections and turn skies blue just the same

Not in every conditions to my knowledge (the use of a polarizer is limited to certain circumstances, I believe). For instance, they are effective if you stand at a 90 degree angle to the sun. Change this radius value, and you will begin to suffer from degradation of image quality. Also, you shouldn't stand with the sun in front or directly behind you as degradation will certainly occur. Please excuse me if you knew this already, but I've got the feeling that you could be missing that info.

[amoringello]

My guess is the CPL polarizer was not directly at fault.
Was SR on but perhaps not yet active?
Perhaps the CPL caused more stops of light to be cut out (likely possibility) and your shutter speed dropped? Be nice to see the shutter, f-stop and ISO for each, as well as status of SR.

No piece of glass should cause that pattern in your image. It looks like motion blur.
Certainly not a factor of circular v.s Linear polarizers in general -- not that a bad one couldn't exist but this doesn't look like a glass issue.

[Digitalis]

No piece of glass should cause that pattern in your image. It looks like motion blur.

I have seen that before, coming from a very very cheap CPL filter.