Hi,
I usually come here with the odd ones, so here’s another. When I use the polarizing lens on my camera I have to rotate it to get the effect. But polarizing glasses (i.e. sunglasses) don’t seem to work that way. How come?
Thanks,
…john
It works that way with polarizing glasses, too. Rotate them while you look at nonmetallic surfaces that have reflections and glare. They are designed to eliminate glare from horizontal surfaces while worn, but they work differently at all angles like the camera filters do.
The polarizing filter/coating on sunglasses is vertical. Reflection from horizontal surfaces (water, road, etc) is polarized horizontally, so a fixed vertical polarizer works well to block reflection.
A fixed vertical polarizer on a camera would work just as well for reducing reflection off water & road surfaces. But a camera filter is threaded into the lens, so the filter manufacturer has no control over what orientation the filter ends up in.
Also, another common use of polarizer filters on cameras is to darken the sky, and here, you do need to adjust the angle depending on the position of the Sun in the sky.
You can see this effect very well by trying to view a smartphone screen through polarizing glasses - rotating the phone will make it more or less readable.
Tilt your head 90 degrees (so your ear is touching your shoulder) and you’ll notice things shift. I’ve had phones and car radio displays I couldn’t see without lifting my sunglasses while others were just fine. In both cases I’d could tilt my head to get them to clear up a little, but it usually wasn’t enough and it’s a royal PITA just have a big blank (car radio) display all the time.
A lot of side windows on cars are polarized as well, usually with a checkerboard pattern.
Finally, if you really need proof, get two pairs of polarized glasses, or your glasses and your camera filter and hold them in front of each other, then rotate one of them as you look thorough the pair. As you do that, the light should go from fairly dim (as you’d expect looking through two dark filters) to nearly 100% gone then back every 180 degrees. Quite a big of light is blocked out when they’re aligned, nearly all of it is blocked when they’re perpendicular to each other.
Actually, the glass is tempered. Polarized lenses let you see the stress patterns in the glass.
Thank you all for the explanations.
… john
And more specifically, it’s the combination of a polarizing filter and a polarized light source that lets you see the stress pattern - the technique is called photoelasticity. So if you are sitting in a car and looking out through the back window with polarized sunglasses, you do not see the pattern. But when you are outside the car, looking at light that reflected off the back surface of the window (where it becomes polarized) and transmitted through the window, you see the pattern.
Is that really what it is? The effect is very regular, like a moire pattern. I’d think stress patterns would appear more random.
A polarizing filter also creates some purple hues in low-e glass depending on the angle. That can be a problem with architectural photography, especially if the glass is also tempered
I think it looks regular because you’re seeing multiple bands that indicate a continuously changing stress variable. They’re like constant altitude lines on a map. They look repetitious because the scale they represent is smaller than the total amount of change they show.
That may be part of it, but the glass is tempered by blowing cold air through an array of nozzles, which creates the regular stress patterns.
Hi.
Yes, I’ve seen that effect.
Thanks,
… john
Exactly wrong. I’ve been a working Cinematographer for over 37 years. 5 Emmy nominations, etc etc. I have never, not once, been holding a polarizing filter that was in a fixed threaded ring. By very definition it would be completely useless.
Polarizing filters are always mounted into freely spinning rings that are then attached to a threaded ring of a diameter appropriate for the taking threads at the front end of your lens. ( In the case of rectangular filter stages such as the 4x5 Panavision filters, one stage of the mattebox can rotate, allowing a filter to be rotated to kill unwanted glare, etc. Here’s the consumer B&H Photo version of that mattebox rotating device for 4x5 Polarizing filters. )
If I cannot rotate a polarizing filter, why in the world would I put it on the front of a lens??
This is a reasonable explanation of how and why polarizing filters work.
For the math wonks on The Dope, the Wikipedia page on Polarizers is pretty comprehensive.
Lastly, because I used to work for them and because I think this information is clearly presented ( with stills and videos ), I give you the Tiffen Circular, Linear and Low Light Polarizer page.
Imagine the embarrassment when I went to work one day with my hot-shot new Polarized lens sunglasses for a long day’s work in the bright sun.
Only to be reminded of the linear polarization issues inherent in most LCD monitors. I was blind to almost all of the image on my Steadicam monitor. I had to slide the shades up onto my head when I was shooting and then slide them back down when we cut.
:smack::smack::smack:
I went back to my tried and true BlueBlockers !
I’ve been a point-n-shoot photographer for 40 years (got you there:) ), and I have never, not once, been holding a polarizing filter with a free rotating ring.
Of course the main purpose was to protect the front of the lens from scratches, and the cost of the iterm was like $3, so we were in different parts of the market 
That’s not a Pola. That’s a Skylight or U.V. filter. To protect the front element.
And agreed- they never have a rotating ring.
What I meant is, this is why polarizers for cameras are mounted on freely rotating rings. Because there’s no way to make a fixed vertical polarizer. Sorry for the confusion.
Correct, Melbourne is talking about UV filters that are put on lenses to protect them (rightly or wrongly, that’s another discussion). I have a circular polarizing filter for my DSLR that would be useless for shooting if it were fixed. Almost by definition, a polarizing filter has to rotate or you could never change the orientation of the camera.
I have to admit that I never really considered the direction of polarization of polarized sunglasses, but to have them vertically polarized makes sense – it’s more likely to counter unwanted reflections from horizontal surfaces then. But polarization in any direction will almost certainly reduce some glare, because it isn’t all coming from reflections from horizontal surfaces.
I did naively think at one time that polarized glasses for 3D movies were polarized one vertical and one horizontal, but if you play with a pair, you quickly find that this isn’t so – their directions of polarization are each at 45 degrees to the horizontal, although perpendicular to each other. Don’t try this with glasses made for a “Real3D” system, or anything like it – those use circularly polarized light. I notice that the 3D glasses used by Imax theaters still use linear polarizers, though.
Your original statement was entirely clear to me — unlike the Wikipedia explanation of circular polarizers. I’ll have to give that one another go, after I’ve been awake a few more hours.