CPL filter is not bi-directional

[erkme73]

No real good way to explain that in the title.

When I verified my CPL filters to see if the white tick mark was standardized - that is, at 12 o'clock, they all had the same level of polarization - I became discouraged to find that no matter how I turned the lens, I could not get it to black out my LCD monitor. The hue of the monitor would go from a bluish tint to a brownish tint.

I was about to fire off a mean email to the seller, saying these were junk, when I discovered something significant...

Unlike my polarized sunglasses which are bi-directional, the polarization on the CPL only works when you look through from the camera-side:

Glad I figured that out before I ate crow with the seller...

 

[RavenManiac]

Good info. Thanks for sharing.

 

[niko]

There is an easier way I discovered for myself many years ago without looking though camera and can be seen with naked eyes.
- when you look trough CPL filter on common LCD clock display and turning it, - you will also be able to determine which side is polarized

 

[Sunny]

Most of threaded cpl filters are one directional including the ones used in DSLR's.

 

[RavenManiac]

Is the Mini 0806 CPL marked to indicate which side is out?

 

[niko]

Is the Mini 0806 CPL marked to indicate which side is out?

The side which has glass close to outer edge of glass holder ring is facing outside towards the road.

 

[niko]

Most of threaded cpl filters are one directional including the ones used in DSLR's.

I had to use many time "My LCD clock Technique" in those DIY project cases where disassembled CPL filter from it's original casing / holder and forgot which side is which.

 

[RavenManiac]

Gotcha. Thanks!

I do have one more question concerning CPL's. If I n=intend to mount a second camera in the back of my CR-V, which has blacked out windows and relatively flat glass, do I need a CPL?

 

[Sunny]

Gotcha. Thanks!

I do have one more question concerning CPL's. If I n=intend to mount a second camera in the back of my CR-V, which has blacked out windows and relatively flat glass, do I need a CPL?

Blacked out is not CPL but adding CPL on blacked out windows will make it more dark.

 

[niko]

Gotcha. Thanks!

I do have one more question concerning CPL's. If I n=intend to mount a second camera in the back of my CR-V, which has blacked out windows and relatively flat glass, do I need a CPL?

Most problematic glare comes from horizontal surface vs close to 30-45 degree front windshield combination. Your CR-V rear windows has around 65-70 degree angle, so there should be no similar glare like from windshield.

 

[erkme73]

That's interesting that camera lenses have polarity. My only experience with polarized glass has been with sunglasses. I wonder why sunglasses are polarized in both directions, and why photography equipment isn't.

As for the blackout of any LCD material (clocks, TVs, etc) that I knew - I just thought it'd be more dramatic to show the first post on my screen being manipulated by the lens...

 

[RavenManiac]

erkme73, if I'm not mistaken doesn't your SUV have blacked out windows? Are you using a CPL with your side and rear cameras?

 

[erkme73]

There is a dark tint on the side and rear, yes. And, with the angle of those windows being nearly vertical, there is no need for the CPL. The front is different - especially in the Highlander since it has a very light interior/dash.

 

[RavenManiac]

Okay, good to know. I suspect my Honda will not require one either.

 

[Nigel]

That's interesting that camera lenses have polarity. My only experience with polarized glass has been with sunglasses. I wonder why sunglasses are polarized in both directions, and why photography equipment isn't.

As for the blackout of any LCD material (clocks, TVs, etc) that I knew - I just thought it'd be more dramatic to show the first post on my screen being manipulated by the lens...

Sunglasses are just simple linear polarisers, the CPL is a simple linear polariser with an added quarter wave plate on the camera side which is there so that SLR cameras focus and exposure mechanisms aren't affected by the polarisation of the light and it is that which causes the directionality. Try looking through the CPL at a mirror, one way around you can see your eye, the other you cant, with sunglasses it makes no difference.

For a dashcam there is no need for the quarter wave plate, in fact it cuts out a tiny bit of light so it would be better not to have it, we shouldn't be using CPLs, just PLs.

Note that LCD clocks etc are often polarised at 45 degrees so that you can read them with sunglasses either vertical or horizontal, the same is true of a lot of car LCD displays so you cant use them for finding the correct way up of your polarizer, TVs are designed to be viewed only one way up so they are always vertically polarised so they can be clearly viewed through polarized glasses (assuming you have your head vertical!). Not always true with monitors if they are designed to be used either landscape or portrait but most are vertical like TVs.

P.S. Just looking at post #1 - the TV/Monitor should be mounted vertical to do the test!

I do have one more question concerning CPL's. If I n=intend to mount a second camera in the back of my CR-V, which has blacked out windows and relatively flat glass, do I need a CPL?

The CPL will not work effectively on nearly vertical glass anyway, they work best at around a 50 degrees glass angle, I suspect the rear window on the CR-V is too steep for the effect of the polariser to be worth the penalty of extra motion blur. I suggest you try the one from the front camera to see if it is useful before bothering to purchase one for the back.