Where is the mark on the polarizer supposed to be?

Started 2 months ago | Questions thread
ThrillaMozilla Veteran Member • Posts: 4,959
Re: Some technical info on polarization.

petrochemist wrote:

ThrillaMozilla wrote:

Gerry Winterbourne wrote:

No filter removes all polarised light.

In the right orientation they do remove virtually all polarized light. Photographic polarizing filters are almost perfect in that respect. (Footnote: someone once tested a horrible, cheap filter that transmitted a few percent of polarized light, but that was an exception.)

However, most light that you see in nature is not 100% polarized.

Virtually all is IMO a rather unhelpful term.

Note that this is the Beginners Forum, and the OP seems like a very basic question. Hence the basic answer. But since no good deed goes unpunished, check out some extinction measurements:

Polarizing fillter transmission and extinction tests

The small amounts of transmission occur at the extreme ends of the visible spectrum, where the sensitivity of the sensors is at a minimum.

For those who want a simple statement, all you really need are Roger Cicala's words:

If you are buying a circular polarizing filter because you want some circular polarizing, it doesn’t seem to matter much which one you choose; they all polarize like gangbusters.

In some cases 0.002% transmission could be significant, in many the few percent transmitted by that cheap filter will not matter.

Considering that the sky is what, maybe 30-50% polarized at most, and very few surfaces are conveniently arranged at the Brewster's angle, are you really worried about 0.002% transmission? No filter at all will give you 100.000% transmission, so I'd consider 0.002% pretty good. That's 15.6 stops. Keep in mind, this is the Beginners' Forum.

Polarizers will indeed remove the bulk of visual light that is polarized in the opposite orientation, but very few are effective with Near infrared which many digital cameras have some sensitivity to.

The question is not about how to implement IR photography. Photographic polarizing filters are intended for visible light use. I don't know what kind of cameras you are using, but as far as I know, most consumer cameras have IR blocking filters. Older video cameras are an exception.

Check it out here.

You can buy polarizers that are effective over the entire wavelength range silicon sensor can record, sadly they are quite expensive so I haven't got any.

I don't know of any natural source where light is 100% polarized & I think lasers may be the only man made sources that actually achieve 100% (even the very best polarizers leak a few ppm of the polarization they are designed to block). Most natural sources have photons of random polarization but fortunately this can be treated as vectors the proportion in the diracton of the polarizer being blocked & the remainder passed. Refection etc will only polarize a portion of the reflected light with the maximum portion when the light is incident at the materials brewster angle (63 degrees for water IIRC).

I'm trying not to be sarcastic here, but keep in mind, this is the Beginners Forum.

Using two crossed normal polarisers gives a fairly good indication of how much light of each polarization is removed it not hard to get down to around 0.5% transmission in the visual with this approach. With all of my polarisers this raises to around 50% transmission by about 750nm - easily seen with several of my unmodified cameras and not dissimilar to a R72 filter on my modified bodies.

That sounds high for a modern photographic filter, and even higher for a camera. Check the references above. Can you make a case for inadequate extinction for one of those reviewed filters in ordinary photographic use?

 ThrillaMozilla's gear list:ThrillaMozilla's gear list
Canon EOS Rebel SL1
Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow