PL filter: circular or linear?

[Akira]

So far as I'm aware, the circular PL filter replaced the liniar PL when AF camera became popular.  The main reason was to avoid the interference with the half-mirror commonly used in AF cameras (and also in some newer TTL metering systems in the MF cameras even before the AF had become the norm).  The linear PL filters then became obsolete when virtually all SLRs and DSLRs started to employ half-mirrors as their reflex mirrors.

But now, facing with the increasing popularity of mirrorless cameras which obviously don't use any half-mirrors, would there be any scenario for the revival of the linear PL?  If there would be any potential advantage of the linear PL over the circular PL (something like the less degradation of image quality or less amount of light loss, for example), there could be need for the linear PL.

What would you folks think?

[David H. Hartman]

All modern cameras require circular.

[I posted a bit too fast then could not return until now. I'd check the manual for mirrorless cameras. I don't know if they would require a circular polarizer or not.]

Dave

[Bjørn Rørslett]

The older linear polarisers tend to introduce a "dull" appearance of the colours, which is not equally apparent in the circular types. I noticed this effect very clearly in my Nikon polarisers of different kind and age.

However, the "dullness" could also be a spurious side effect from differences in coatings.

A scrap of inside information is the peculiar colour phenomena one can obtain combining a linear and a circular polariser. Best seen with a wide spectrum camera.

[David H. Hartman]

A quick search of the Sony A9 user manual didn't turn up a reference to a polarizing filter. I might have missed a reference. I searched for polarizer, polarizing, circular, linear and filter. Filter came up a few times but I didn't notice it in reference to a polarizing filter.

My hasty post was probably dead wrong. As far as I know all modern dSLR(s) require a circular polarizing filter for many features to work correctly. One might use manual focus and exposure and use a linear polarizing filter in a pinch. I tried one on my F5 many years ago and everything didn't fail. I don't remember what worked correctly and what was a bit weird.

Dave

[Akira]

Dave, As I mentioned in the initial post, the reason for the necessity of the circular PL is well known fact: the linear PL will interfere with the half-mirror of modern (D)SLRs.

Bjørn, the reason for my question also comes from the realization that the linear PL could have been isolated from all the developments of the coating, optical glasses, PL films, etc, because of the uselessness (or meaninglessness) with the modern (D)SLRs.  As you say, the inferior performance of linear PL may be owing to that.

[benveniste]

But now, facing with the increasing popularity of mirrorless cameras which obviously don't use any half-mirrors, would there be any scenario for the revival of the linear PL?  If there would be any potential advantage of the linear PL over the circular PL (something like the less degradation of image quality or less amount of light loss, for example), there could be need for the linear PL.

A circular polarizer is a linear polarizer with a quarter-wave "plate" stuck on the back.  Any time you can take something out of the light path you don't need there's a potential advantage.  That potential advantage is pretty small both in terms of cost and optical performance, and in my opinion, there are a couple of real-world considerations which make it prudent to stick with circular polarizers.  The first is that mirrorless technology is still evolving, especially when it comes to focus mechanisms.  I've read Nikon's patent for "split-pupil" phase detection and I'll be damned if I can figure out if a linear polarizer would interfere or not.  But the patent does not rule out the use of polarization to perform the split.  The Nikon V1 manual is silent on polarizer use as well.  So even if a linear polarizer works with today's cameras, it might interfere with the next generation of sensors.

The second reason is purely market-driven.  There simply aren't a lot of good quality linear polarizers available.  I think B+W still makes some, but their mainline products are all circular.  The other linear polarizers I've see  tend to be older products with less modern coatings or no coatings at all.  While that may change in the future, I haven't seen any sign of it with newer products.  Yet.

[Akira]

Benveniste, thanks for sharing your observation.  That is convincing.  Some products employing the future technology may indeed interfere with the linear PL again.

[Roland Vink]

It would be interesting to know if on-chip phase-detection pixels, or Canon's dual pixel sensors are affected by linear polarizers. My guess is they would be since they employ similar concepts as phase-detection used by SLR cameras, but on a much smaller scale.

[Akira]

Roland, I'm also curious if there would be something other technology than the half-mirror that may not like the linear PL.

[benveniste]

Roland, I'm also curious if there would be something other technology than the half-mirror that may not like the linear PL.

Here's the Nikon patent for on-chip phase detection.  Perhaps someone with more of a physics background can figure it out?
https://www.google.com/patents/US20090167927

[Les Olson]

The issue with linear polarising filters and the beam-splitters needed for AF systems is that some beam-splitters polarise the light, with the transmitted and reflected beams being polarised at 90 degrees to one another.  Prism beam-splitters are generally polarising, and half-silvered mirror beam-splitters are non-polarising. 

If there is a linear polariser in front of the lens a polarising beam-splitter will, at best, not split the light in the expected proportions.  If the linear polariser is oriented so that the light reaching the beam splitter is at just the wrong angle, the beam-splitter will not divert any light to the AF sensor. 

So the problem with linear polarisers and AF tends to be intermittent, depending on the orientation of the polarising filter, and it only happens at all when the camera has a polarising beam-splitter. 

Which Nikons have a polarising beam-splitter?  I don't know, but I have read that prism beam-splitters were used in older models - the D60 was maybe the last? - and recent dSLRs use half-silvered mirror beam-splitters. 

[arthurking83]

Strangely, I've never had any issues with my linear polariser on either my Sigma 10-20 and or Nikon 80-200/2.8(AF-D) nor Tamron 70-200/2.8 with respect to AF performance.
This was mainly on a D70s and D300, and very little on the D800E.

Linear Pol was a Hoya brand from about 25 years ago(new).

I also have a high end C-Pol to compare with directly, and try as I may .. could never find any AF issues with the linear type that didn't occur with the much more modern circular type.

They do render differently, but that's more to do with coatings than the polarzation type of the filter.

[Akira]

Les, thanks for the detailed concern about the possible interference between the linear PL and certain types of AF systems.  That makes lots of sense.

Arthur, thanks for sharing your real-world experiences of comparing linear and circular PLs.  Apparently the occurrence of interference depends on the combination of the involving equipments, just like the hotspot in IR photography.

[PeterN]

Interesting read. Thanks for starting and replying to it.

[Akira]

Interesting read. Thanks for starting and replying to it.

Peter, thanks for commenting.  Hope this thread to offer some food for thought.