You don't need a CPL (circular polarizer)

[Deleted member 37895]

I keep seeing references to putting a CPL on a dashcam, but this is based on a misconception. Circular doesn't refer to the shape of the filter, it refers to the circular polarizer layer that is placed behind the linear polarizer to help SLR cameras focus.

Cameras without phase detect focusing (SLRs, DSLRs, some mirrorless) don't need the circular layer, they only need the linear polarizer layer.

But no harm done having the circular layer right? Wrong! Each polarizer cuts light by half, so you end up with at least 75% light blocked by a CPL, vs 50% blocked by a linear polarizer.

 

[lacibaci]

You are correct, however it is very hard to find linear polarized lenses in the right size.

 

[DT MI]

You are correct, however it is very hard to find linear polarized lenses in the right size.

DIY with clip-on sunglasses.

https://dashcamtalk.com/forum/threa...-sg9665xx-a118xx-and-b40xx.11698/#post-151367

 

[Deleted member 37895]

You are correct, however it is very hard to find linear polarized lenses in the right size.

You can buy a simple sheet like this https://www.amazon.com/gp/B009P8B548

 

[Rajagra]

I keep seeing references to putting a CPL on a dashcam, but this is based on a misconception. Circular doesn't refer to the shape of the filter, it refers to the circular polarizer layer that is placed behind the linear polarizer to help SLR cameras focus.

Cameras without phase detect focusing (SLRs, DSLRs, some mirrorless) don't need the circular layer, they only need the linear polarizer layer.

But no harm done having the circular layer right? Wrong! Each polarizer cuts light by half, so you end up with at least 75% light blocked by a CPL, vs 50% blocked by a linear polarizer.

You have some misconceptions yourself.
Firstly, circular polarisers were first used in photography to prevent wrong exposure, not to help focusing. This is because most SLRs measured exposure off the mirror, which had a polarising effect. Using a linear polariser would mess up the reading.
Does this mean dash cams need circular polarisers to get proper exposure? Well there is no mirror in play, so I'm guessing the answer is no, but that's just an educated guess.
Also a circular polariser is not two linear polarisers back-to-back. Maybe you are thinking of a cross polariser, which can be used as a variable neutral density filter, cutting down light by two stops upwards.
The way I think of a circular polariser is a linear polarising layer at the front, with a depolarising layer (quarter-wave plate) behind it. The depolarising layer may cut down a little light, but nowhere near 50%. Probably just a few percent.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/quarwv.html

Edit> Looks like I was wrong, Lee filters say up to 1 2/3 stops are lost! That is pretty bad for our needs.

 

[Deleted member 37895]

You have some misconceptions yourself.
Firstly, circular polarisers were first used in photography to prevent wrong exposure, not to help focusing. This is because most SLRs measured exposure off the mirror, which had a polarising effect. Using a linear polariser would mess up the reading.
Does this mean dash cams need circular polarisers to get proper exposure? Well there is no mirror in play, so I'm guessing the answer is no, but that's just an educated guess.
Also a circular polariser is not two linear polarisers back-to-back. Maybe you are thinking of a cross polariser, which can be used as a variable neutral density filter, cutting down light by two stops upwards.
The way I think of a circular polariser is a linear polarising layer at the front, with a depolarising layer (quarter-wave plate) behind it. The depolarising layer may cut down a little light, but nowhere near 50%. Probably just a few percent.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/quarwv.html

Edit> Looks like I was wrong, Lee filters say up to 1 2/3 stops are lost! That is pretty bad for our needs.

I don't have misconceptions. Phase detect focusing devices don't work properly with linearly polarized light. Exposure as I understand is also affected but that doesn't make what I said wrong.


I did not say CPL is two linear polarizers stacked. I clearly wrote that there is a circular polarizer BEHIND the linear polarizer. "...it refers to the circular polarizer layer that is placed behind the linear polarizer to help SLR cameras focus. " How can I possibly be more clear than that?

Every reference says a CPL filter cuts "2 stops" of light, that's halving light twice, hence you get 25% passed through. A simple linear polarizer cuts 1 stop by definition

 

[Nigel]

The 2nd layer is not a polariser, it simply modifies the polarisation of the light so that it is no longer linear, it does not cut out any light except through inefficiency. A good CPL with good anti-reflection coatings will cut out close to 1 stop, poorer ones will be closer to 2 stops, there is very little difference between the amount of light removed by a CPL compared to a linear polariser of similar quality, that is why it is hard to buy a linear polariser - they don't work any better.

 

[Rajagra]

I don't have misconceptions.

I'm afraid you do. I'm not being argumentative, but excuse me a moment while I be pedantic...

I clearly wrote that there is a circular polarizer BEHIND the linear polarizer. "...it refers to the circular polarizer layer that is placed behind the linear polarizer to help SLR cameras focus. " How can I possibly be more clear than that?

You could be clearer by saying there is a quarter-wave plate behind the linear polarizer. Not a "circular polarizer layer that is placed behind the linear polarizer" Calling it a circular polarizer layer implies it is a polarizer layer (!) which would cut out 50% of light. It is not.

Every reference says a CPL filter cuts "2 stops" of light, that's halving light twice, hence you get 25% passed through.

I included a link above to Lee Filters saying "A polariser will reduce your exposure up to 1⅔ stops." But I do agree that is bad enough.

Cameras without phase detect focusing (SLRs, DSLRs, some mirrorless) don't need the circular layer, they only need the linear polarizer layer.

This is untrue. Many SLRs would not meter reliably with linear polarisers fitted. Depending on the angle of use they would cause cross polarisation when combined with the reflections off the mirror and inside the pentaprism, making the camera think it was very dark. When the mirror flipped up to take the photo, all cross-polarisation effects would disappear, and massive over exposure could happen.

Taken from the instructions for my beloved T90 manual focus camera:

 

[Deleted member 37895]

I'm afraid you do. I'm not being argumentative, but excuse me a moment while I be pedantic...

You could be clearer by saying there is a quarter-wave plate behind the linear polarizer. Not a "circular polarizer layer that is placed behind the linear polarizer" Calling it a circular polarizer layer implies it is a polarizer layer (!) which would cut out 50% of light. It is not.

I included a link above to Lee Filters saying "A polariser will reduce your exposure up to 1⅔ stops." But I do agree that is bad enough.

This is untrue. Many SLRs would not meter reliably with linear polarisers fitted. Depending on the angle of use they would cause cross polarisation when combined with the reflections off the mirror and inside the pentaprism, making the camera think it was very dark. When the mirror flipped up to take the photo, all cross-polarisation effects would disappear, and massive over exposure could happen.

Taken from the instructions for my beloved T90 manual focus camera:
View attachment 33109

You're right, it's a quarter wave filter which I didn't know-- I thought the layer at the back was the circular polarizer, but apparently it's the combination of the linear polarizer and the quarter wave polarizer that create circular polarization.

In any case, they only exist because of SLR camera quirks--- both metering and focusing. But neither of those restrictions apply for dashcams, cell phone cameras, most P&S cameras, and most mirrorless

 

[Rajagra]

I think it is worth trying a linear polariser, I'm surprised I haven't seen any tests. If the metering is done by the same sensor that takes the image, there should be no problem.
Worst case scenario is the exposure is off, but most (all?) dash cams have exposure compensation. Since the polariser should be in the same position all the time, it would be easy to correct.

 

[DT MI]

I think it is worth trying a linear polariser, I'm surprised I haven't seen any tests. If the metering is done by the same sensor that takes the image, there should be no problem.
Worst case scenario is the exposure is off, but most (all?) dash cams have exposure compensation. Since the polariser should be in the same position all the time, it would be easy to correct.

I used a linear polarizer (old clip on sunglasses) with my DIY filter - worked fine.

 

[Rajagra]

I used a linear polarizer (old clip on sunglasses) with my DIY filter - worked fine.

 

[Deleted member 37895]

I think it is worth trying a linear polariser, I'm surprised I haven't seen any tests. If the metering is done by the same sensor that takes the image, there should be no problem.
Worst case scenario is the exposure is off, but most (all?) dash cams have exposure compensation. Since the polariser should be in the same position all the time, it would be easy to correct.

Metering is done by software using the image sensor, only SLRs have separate metering

 

[Lola]

I think it is worth trying a linear polariser, I'm surprised I haven't seen any tests. If the metering is done by the same sensor that takes the image, there should be no problem.
Worst case scenario is the exposure is off, but most (all?) dash cams have exposure compensation. Since the polariser should be in the same position all the time, it would be easy to correct.

While looking for Linear polarizer I came across this, it commented on Non Metallic surfaces, I don't know what the metallic surface would have to do with it.
https://www.amazon.com/gp/B000J3L1T2
Disregard it's for a 35mm camera.

 

[Deleted member 37895]

While looking for Linear polarizer I came across this, it commented on Non Metallic surfaces, I don't know what the metallic surface would have to do with it.
https://www.amazon.com/gp/B000J3L1T2
Disregard it's for a 35mm camera.

The reflection from a polished metal surface isn't polarized-- for example chrome. You can rotate your filter and it doesn't change

 

[Nigel]

While looking for Linear polarizer I came across this, it commented on Non Metallic surfaces, I don't know what the metallic surface would have to do with it.
https://www.amazon.com/gp/B000J3L1T2
Disregard it's for a 35mm camera.

In transparent materials like glass and water, when the reflection is at around 90 degrees the horizontally polarised light gets reflected but the vertically polarised light gets refracted and goes into the water/glass. Metal reflects all the light.