CPL filter - best alignment

[Ken]

Anyone know the best alignment for the CPL filter? It rotates (has a slip-ring) and notches, so you can adjust the polarization from horizontal, vertical, 45 degrees etc.

Sunglasses also are polarized to reduce glare. It sounds like vertical polarization is best to reduce glare, which I think is the slip-ring notches at 12 and 6 o'clock.

Look through the CPL filter at any LCD screen (which is a polarized source) and rotate the filter, and you can see clearly to black then back again.

 

[jokiin]

there's no best setting, it's different for every application, trial and error is the way to go

 

[panthar]

Stick a sheet of paper on your dash and rotate the filter until the paper is the least visible.

 

[Swordfish]

Optimal setting is for the filter to be facing 90 degrees from the sun. You would need a monitor to see the result as you turn the ring.

Sent from my Galaxy Note

 

[jokiin]

Optimal setting is for the filter to be facing 90 degrees from the sun. You would need a monitor to see the result as you turn the ring.

Sent from my Galaxy Note

Lucky the sun doesn't move and we never drive in different directions

You raise a good point that these filters are really designed for photography and we're not constantly in a position to be able to take advantage of the correct function of the product, yes they can help in some situations but it really is a trial and error process to find a happy medium

 

[Quick]

Ummm. The main purpose of a polarizing filter (at least in this application) is to remove reflection and glare (from reflection). Like glare off of snow, or the glare off the surface of water so you can see through it, or ... the reflections off the inside of your windshield. Since your windshield does not change position relative to your camera there IS a setting that will be best for you that will not change. Your windshield is curved to some extent so the predominant reflection off your dash may change a bit due to the angle of light coming through the windshield but not much. Setting the polarization in the horizontal plane will work best for most everybody. Will also cut the glare off the road surface and other car glass and chrome which is mostly in the horizontal plane also.

 

[Swordfish]

No, the sun doesn't move.

Sent from my Galaxy Note

 

[jokiin]

No, the sun doesn't move.

Sent from my Galaxy Note

Correct, but our position to it when driving varies a whole lot, what I meant but not explained clearly

No doubt a filter can offer some better results under certain situations but its not a cure all unfortunately

 

[Quick]

Correct, but our position to it when driving varies a whole lot, what I meant but not explained clearly

No doubt a filter can offer some better results under certain situations but its not a cure all unfortunately

No. It is not the ambient or direct light you're trying to filter. You're trying to filter *reflected* light. Most reflected light is reflected off a flat surfaces. Like your dashboard or your windshield and is mostly in one plane. A polarizing filter blocks light in one plane. If you block the light in one plane the entire picture is darkened slightly but the entire reflected image is removed. I'm sure there are some diagrams showing the basics on the net.
Here are the first hits when googling "how does a polarizing filter work"

Scroll down to the picture of the car mirror and windows. That is what we're using it for.
http://en.wikipedia.org/wiki/Polarizing_filter_(photography)

http://www.cambridgeincolour.com/tutorials/polarizing-filters.htm

Go to about 2:05 into this video. Imagine the water being the inside of your windshield
http://fstoppers.com/a-great-example-of-what-a-polarizing-filter-does

We're using a polarizing filter to remove (or at least greatly reduce) reflected images from the inside of the windshield we're pointing the dash cam through.

 

[Ken]

With my eyes I looked through the CPL filter (while driving of course) and V-polarization eliminated glare from the road and car hoods/roofs. H-polarization got rid of some glare from sides of cars. It can make a difference. I think worst glare is a wet road at night.

I found the best compromise was 30° or notches at 2&8 o'clock.