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Author Topic: Do Fisheye lenses use polarizing coatings  (Read 3205 times)
fike
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« on: October 11, 2012, 08:42:16 AM »
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I have two fisheye lenses (sigma 15mm f/2.8 and Rokinon 7.5mm MFT) and both seem to be more contrasty (particularly bluer skies) than more normal or ultra-wide lenses. It occurs to me that the coatings on these lenses may be more like those on a linear polarizer.  It kind of makes sense to do something like this when you are seeing such a wide-angle view that will almost always include bright sky highlights. Of course circular polarizers cause issues when you use them on lenses that are too wide...they tend to make "sky bands" (whatever you call them) where there are regions of deep blue (well polarized at a right angle to the sun) and lighter bands where the polarizing effect is smaller.  With the rounded front element of the fisheyes, this effect could be reduced. 

Am I imagining this, or do they put polarizer-like coatings on fisheyes?
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Slobodan Blagojevic
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« Reply #1 on: October 11, 2012, 08:57:54 AM »
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Pass that bong, dude! Grin
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Rob C
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« Reply #2 on: October 11, 2012, 10:02:00 AM »
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AFAIK linear Polas are not compatible with digital cameras, leading me to believe that even were it possible to coat like that, lens manufacturers would shy off such ideas.

Rob C
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JeffKohn
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« Reply #3 on: October 11, 2012, 10:17:58 AM »
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AFAIK linear Polas are not compatible with digital cameras
It's not so much a digital thing, it's the phase-detect AF and matrix metering that linear polarizers interfere with.

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JeffKohn
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« Reply #4 on: October 11, 2012, 10:21:10 AM »
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Am I imagining this, or do they put polarizer-like coatings on fisheyes?
You're imagining it. Wink

As far as bluer skies, I think what you're seeing is that the fisheye will capture a greater contrast range (from ligher near the sun to substantially darker opposite) due to the extremely wide FOV.

I suppose it could also be that some of the corrections to prevent aberrations in a rectilinear lens are not needed in a fisheye design, and therefore the fisheyes tend to have very good contrast.
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ErikKaffehr
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« Reply #5 on: October 11, 2012, 03:28:13 PM »
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Hi,

I don't think so. Polarization on sky is very angle dependent so it would result in a very uneven sky.

Best regards
Erik
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fike
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« Reply #6 on: October 12, 2012, 12:39:51 PM »
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I might be inclined to accept the simplistic 'no' answers I am getting based on the idea that you would have banding due to the wide angle....but....

The front element of a fisheye lens is very curved.  The front of a polarizer is flat.  It is easy to see how the angle of light would change across a wide angle view through a flat filter, but if you consider the angle of light around a curved lens element, you could probably mitigate that problem a bit.  The also could be putting the coatings inside the lens after the sharp angled light has been bent a bit. 

I know that you guys think this is hair-brained, but I need to do some experiments between wide angle lenses and fisheyes to confirm or deny my hypothesis.  I am just not convinced yet.
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« Reply #7 on: October 12, 2012, 12:47:14 PM »
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Wouldn't a polarizer need to rotate in order to achieve the maximum effect? Something lens front elements do not do?
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fike
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« Reply #8 on: October 12, 2012, 12:58:56 PM »
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Wouldn't a polarizer need to rotate in order to achieve the maximum effect? Something lens front elements do not do?

Not a linear polarizer.
http://en.wikipedia.org/wiki/Linear_polarizer#Linear_polarizers

As someone above pointed out, linear polarizers do interfere with phase detect focus systems.  I have two comments about this: 1) polarization isn't a binary thing. There are degrees of polarizations and 2) fisheye lenses are notorious for having poor and slow auto focus, if they have auto focus at all. 

The criticality of focus precision in fisheye photography is diminished due to the extremely small subject sizes that make precise focus difficult.  Anyway, you are generally shooting with so much depth of field that it is less critical. 
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« Reply #9 on: October 12, 2012, 01:14:03 PM »
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Not a linear polarizer...

I am not a physicist, so I will restrict my comments to what I know about photographic polarizers. Any polarizing filter I've ever come across, linear or circular, square or round, had to rotate.
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ErikKaffehr
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« Reply #10 on: October 12, 2012, 03:01:52 PM »
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Hi,

The reason the sky gets dark is that some of the light reflected by air is polarized. The amount of polarized light depends on the incident angle of the light coming from the sun. With a wide angle that angle varies a lot, with a fish eye 180 degrees. So some light will be polarized and some not.

Simple physics, better to believe it...

Best regards
Erik

I might be inclined to accept the simplistic 'no' answers I am getting based on the idea that you would have banding due to the wide angle....but....

The front element of a fisheye lens is very curved.  The front of a polarizer is flat.  It is easy to see how the angle of light would change across a wide angle view through a flat filter, but if you consider the angle of light around a curved lens element, you could probably mitigate that problem a bit.  The also could be putting the coatings inside the lens after the sharp angled light has been bent a bit. 

I know that you guys think this is hair-brained, but I need to do some experiments between wide angle lenses and fisheyes to confirm or deny my hypothesis.  I am just not convinced yet.
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JeffKohn
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« Reply #11 on: October 12, 2012, 03:28:03 PM »
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Linear polarizing filters still need to be rotated to dial in the effect.

A few other points:

* If a manufacturer were to put a polarizing filter in the front element, this would add to the cost. I think most manufacturers would not do this when it's not a universally-wanted option.
* If a manufacturer did this, they would mention it in the marketing materials and product specs as a feature
* If a lens had built-in polarizing filter, it wouldn't just increase contrast, it would also affect reflections and glare on many surfaces
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Glenn NK
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« Reply #12 on: October 12, 2012, 07:38:12 PM »
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Linear polarizing filters still need to be rotated to dial in the effect.

A few other points:

* If a manufacturer were to put a polarizing filter in the front element, this would add to the cost. I think most manufacturers would not do this when it's not a universally-wanted option.
* If a manufacturer did this, they would mention it in the marketing materials and product specs as a feature
* If a lens had built-in polarizing filter, it wouldn't just increase contrast, it would also affect reflections and glare on many surfaces


I had a L POL on my Canon A-1.  It rotated as I recall.

However, since my landscapes are virtually shot fully manual (focusing because of the TSE, and metering because I use the RGB histo) I've seriously considered acquiring a L POL to avoid the maddening angle effect.

Any want to comment on this?

Glenn
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Slobodan Blagojevic
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« Reply #13 on: October 12, 2012, 09:15:47 PM »
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... considered acquiring a L POL to avoid the maddening angle effect...

What would be that angle effect?
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Slobodan

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JeffKohn
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« Reply #14 on: October 12, 2012, 09:24:35 PM »
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I had a L POL on my Canon A-1.  It rotated as I recall.

However, since my landscapes are virtually shot fully manual (focusing because of the TSE, and metering because I use the RGB histo) I've seriously considered acquiring a L POL to avoid the maddening angle effect.

Any want to comment on this?
The visual effects of linear and circular polarizers are the same. The "uneven sky" effect has nothing to do with circular versus linear. A circular polarizer is just a linear polarizer with an additional filter on the back to "re-scatter" the light so that it doesn't confuse metering/AF systems.

If you don't care about AF/metering and want to save some money you can buy a linear polarizer instead of circular. But don't think you'll get different results visually in your photos.
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Glenn NK
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« Reply #15 on: October 13, 2012, 12:05:25 AM »
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The visual effects of linear and circular polarizers are the same. The "uneven sky" effect has nothing to do with circular versus linear. A circular polarizer is just a linear polarizer with an additional filter on the back to "re-scatter" the light so that it doesn't confuse metering/AF systems.

If you don't care about AF/metering and want to save some money you can buy a linear polarizer instead of circular. But don't think you'll get different results visually in your photos.

I think you are right.

As for the "angle" I referred to, I think we know what I mean.  The only thing I use a CPOL for is with my 200 mm (effective) macro lens where the angle of view is so small that the amount of polarization is effectively (or visually) constant.

G
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Petrus
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« Reply #16 on: October 13, 2012, 12:13:16 AM »
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Just check the exposure; a polarizer would cut 1/2 to 2/3 of the light also making a f:4 into a t:6.3...

They have no polarizers.
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Slobodan Blagojevic
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« Reply #17 on: October 13, 2012, 01:28:48 AM »
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... As for the "angle" I referred to, I think we know what I mean...

If I knew what you meant, I wouldn't have asked Wink

I asked especially because you thought that a linear polarizer would take care of that "angle" effect, whatever it might be. I was curious what you think a linear could do that a circular couldn't, given that both produce identical visual results.
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Glenn NK
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« Reply #18 on: October 13, 2012, 12:29:32 PM »
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The "angle effect":  It's my understanding that the amount of polarization is a function of the angle between the lens axis and the sun's axis resulting in an uneven polarization effect across the width of an image - the variation being more noticeable with wider angle lenses.

If anyone could devise a method to polarize in PP, the problem would be solved - a bit anyway.  I don't expect this to happen any time soon.  Grin

Glenn
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« Reply #19 on: October 13, 2012, 01:03:50 PM »
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A linear polariser won't solve the 'angle' problem either.  It has nothing to do with the type of filter, as has already been stated. 

Fike, since all polarisers have to be rotated for maximum effect based on polarisation of reflected light coming into the lens, it would do no good to put a polarising type of coating on thefront element because it ould not always have optimal effect.

It is possible that your two fisheye lenses have different types of anti-glare or similar coatings due to the extremely wide angle the lens is taking in and these types of lens coatings can impact contrast and colour.
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