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Which Polarizing filter should I go for?
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PostPosted: Sun Oct 28, 2018 5:53 am    Post subject: Which Polarizing filter should I go for? Reply with quote

There are a lot of dirt cheap filters on ebay, but are they good? Which affordable filters will be the best to buy?


PostPosted: Sun Oct 28, 2018 7:59 am    Post subject: Reply with quote

I usually pay less than £5 for an "old-school" second-hand glass linear polariser for use with manual-focus lenses. Any one with a recognisable brand-name on it should be OK if it's not actually scratched.

With the current demand for larger circular polarise filters on autofocus zoom lenses, linear filters are readily available and often regarded as "of no further use" Wink


PostPosted: Sun Oct 28, 2018 10:18 am    Post subject: Reply with quote

that's sort of a mixed bag?
do you use film or digital?

http://forum.mflenses.com/sea-linear-polarizer-with-s-m-c-120-2-8-t16113.html


PostPosted: Sun Oct 28, 2018 11:56 am    Post subject: Reply with quote

An interesting experiment to compare filters is to let sunlight shine through onto white paper. You may see different tints and shades. Or, decide the filter degrades the image when it is not necessary, which may well be a rare occasion.


PostPosted: Sun Oct 28, 2018 1:12 pm    Post subject: Reply with quote

After having a pretty dismal experience with a no-name UV filter on one of my telephotos, I won't ever skimp on filters again. This goes for polarizers also. If folks here are like me, after having acquired a good collection of lenses, you've probably also amassed a good collection of filters, including a variety of polarizers. So I can usually just rummage through my polarizer inventory to find one that will work for what I need.

I know how linear polarizers work, but I'm not so sure about circular ones. I have a couple, but I don't recall actually using them. Back in my early days of photography I had some slides that came out weird and it was because I didn't know how they worked. Like a deep blue sky on one side of the photo fading to pale blue by the other side of the photo. Weird. Then I learned how to use them and things got better. Actually, I'd already learned about this in a physics class taken years earlier, so it was a matter of applying what I'd learned.

At any rate, if I can assume that circular ones polarize light in a similar fashion, here's a good rule of thumb (literally, as it turns out) to follow when using a polarizer. The rule is that light is polarized at a right angle from the direction of the sun and the best way to determine how this works in a photo is to use your hand with the thumb up and the fingers pointed straight out. Point your thumb at the sun and the area of the sky that your fingers will sweep as you pivot your hand on your thumb is that area of the sky that will be polarized with a filter. So, for example, if you shoot the sky at noon, with the sun directly overhead, the sky at the horizon is the area that will be polarized.

A well-known reason for using a polarizer is to eliminate glare and reflections. However, this does not occur with all materials. A second rule of polarization is that reflections off metal are not affected by a polarizer. The reason is that the light that is reflected off bright metal is already polarized, so the polarizer has no effect.

If you have more than one linear polarizer of the same size, they can be used as an ND filter and an extinction filter for fades to black. I'm assuming that most folks here probably already know this, but I'll go ahead and mention it anyway, just in case some may not. If you stack two linear polarizers together, rotating one of them will dim the scene until it goes completely black. So you have total control over exposure that way.

So what I'm getting at is this: it's not just the brand or type of polarizer -- it's knowing how they work in order to get the best use out of them.


PostPosted: Sun Oct 28, 2018 4:55 pm    Post subject: Reply with quote

cooltouch wrote:

A well-known reason for using a polarizer is to eliminate glare and reflections. However, this does not occur with all materials. A second rule of polarization is that reflections off metal are not affected by a polarizer. The reason is that the light that is reflected off bright metal is already polarized, so the polarizer has no effect.


No doubt someone will give me good reason why I'm wrong, but I remember being taught that it was polarised light that could be cancelled out by a polarising filter, such as that reflected from water or glass, but that the reflection from metal was unpolarised so there was always some reflection that'd get through the filter, whichever way you turned it Question

FWIW, I do use two linear polarisers as an adjustable neutral density filter ... a little fiddly when they both try to turn at once and especially if there's a lens hood in the way, but the results can be worth it. I've been given to understand if you try using two circularly polarised filters in this configuration the results can be a little "weird", but I've not tried.


PostPosted: Sun Oct 28, 2018 6:54 pm    Post subject: Reply with quote

I would go for a good brand, here in Germany B+W but HOYA and Nikon should have decent ones.
They are not always very neutral, so go and do some tests first!!


PostPosted: Sun Oct 28, 2018 7:39 pm    Post subject: Reply with quote

kypfer wrote:
cooltouch wrote:

A well-known reason for using a polarizer is to eliminate glare and reflections. However, this does not occur with all materials. A second rule of polarization is that reflections off metal are not affected by a polarizer. The reason is that the light that is reflected off bright metal is already polarized, so the polarizer has no effect.


No doubt someone will give me good reason why I'm wrong, but I remember being taught that it was polarised light that could be cancelled out by a polarising filter, such as that reflected from water or glass, but that the reflection from metal was unpolarised so there was always some reflection that'd get through the filter, whichever way you turned it Question


You could very well be right. I'm having to dredge up memories from a physics class I took some 38 years ago. Maybe somebody with a stronger physics background than mine can weigh in on this.

Klaus, I agree with your filter choices, especially B+W. In addition to Nikon and Hoya, Tiffen also made (or makes?) good filters, as did or does Canon, and even Cokin if you want to put up with its filter holders.


Last edited by cooltouch on Sun Oct 28, 2018 9:57 pm; edited 1 time in total


PostPosted: Sun Oct 28, 2018 9:38 pm    Post subject: Reply with quote

Even name brand filters are graded from economy to best. Selecting good name brand filter is not much easier than selecting from entire marketplace. Lol.

What are some good tests or experiments to differentiate quality of filters?


PostPosted: Sun Oct 28, 2018 10:03 pm    Post subject: Reply with quote

visualopsins wrote:
What are some good tests or experiments to differentiate quality of filters?


My quick "is it a polariser or not" test is to hold the filter in front of a flat-screen TV or computer monitor and rotate it. A "good" filter will cause the image from the screen to completely "black out" at a specific point in it's rotation. This is the point where the filter is exactly at 90 degrees to the built-in filter of the screen.

For whatever reason, some filters don't completely "black out" the screen image, these appear to have slightly less effect on the sky as well, so, in my book, they're less desirable, but maybe that's just my eyes playing tricks. I've never bothered to put a camera on a tripod and take a series of pictures in rapid succession with different filters to demonstrate or otherwise that there is actually a difference photographically ... mostly because I don't have more than one or two filters in any given size Wink


PostPosted: Sun Oct 28, 2018 10:05 pm    Post subject: Reply with quote

visualopsins wrote:
What are some good tests or experiments to differentiate quality of filters?


One test I discovered accidentally was due to some very poor performance I was getting from a 400mm f/6.3 telephoto. I was puzzled by this until I noticed it had an off brand UV filter hanging off the front. When I removed the filter, the lens's performance improved dramatically. So, if I were to take this "test" to the next level, the way to proceed would be to try various filters on the front of this same lens and then observe the results. Doesn't really have to be any more complicated than that unless one wants to analyze exactly how each filter affects performance.


PostPosted: Mon Oct 29, 2018 2:34 pm    Post subject: Reply with quote

kypfer wrote:
cooltouch wrote:

A well-known reason for using a polarizer is to eliminate glare and reflections. However, this does not occur with all materials. A second rule of polarization is that reflections off metal are not affected by a polarizer. The reason is that the light that is reflected off bright metal is already polarized, so the polarizer has no effect.


No doubt someone will give me good reason why I'm wrong, but I remember being taught that it was polarised light that could be cancelled out by a polarising filter, such as that reflected from water or glass, but that the reflection from metal was unpolarised so there was always some reflection that'd get through the filter, whichever way you turned it Question

FWIW, I do use two linear polarisers as an adjustable neutral density filter ... a little fiddly when they both try to turn at once and especially if there's a lens hood in the way, but the results can be worth it. I've been given to understand if you try using two circularly polarised filters in this configuration the results can be a little "weird", but I've not tried.


I doubt anyone can give a good reason why you're wrong - because you're not!

I have played with multiple CPLs - You can use 2 CPLs to make a variable ND, just use them front to front. You get really weird effects if you use them back to back. Both in the normal orientation should give similar results to a single CPL but with the brightness reduced by around 50%. Combining filters front to front & back to back is considerably more fiddly than just stacking normal filters

Variable NDs are usually made from a linear polarizer followed by a CPL. This gives the same effect as 2 linear but then outputs non polarised light so AF & metering are still reliable.