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12-27-2013, 08:01 PM   #1
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Is a CPL supposed to produce an effect like this at 17mm?



I'm referring to the darker blue sky in the center. I noticed on my recent death valley trip that I couldn't get an even shade of blue for the sky. It was always blue on one end or middle when I rotated the CPL.

12-27-2013, 08:08 PM   #2
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This reminds me of some images I saw which showed the polarisation orientation in the sky. The polarisation is not, by nature, constant across the sky. I was shown the image as part of a presentation about the sight and navigation methods of some kind of insect. The effect can be used as a compass.

Therefore I would say that it is not surprising. The fact that it moved around when you rotated the CPL is consistent with the navigation matter observation.

What does it do with an old style linear polariser?
12-27-2013, 08:16 PM   #3
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What's the shape of the lens hood, perhaps the light getting past the lens hood - if its a tulip design, is interacting with the polarizer?
12-27-2013, 08:23 PM   #4
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QuoteOriginally posted by tim60 Quote
This reminds me of some images I saw which showed the polarisation orientation in the sky. The polarisation is not, by nature, constant across the sky. I was shown the image as part of a presentation about the sight and navigation methods of some kind of insect. The effect can be used as a compass.

Therefore I would say that it is not surprising. The fact that it moved around when you rotated the CPL is consistent with the navigation matter observation.

What does it do with an old style linear polariser?
I don't have a LPL to test it out

QuoteOriginally posted by philbaum Quote
What's the shape of the lens hood, perhaps the light getting past the lens hood - if its a tulip design, is interacting with the polarizer?
yes it is tulip

12-27-2013, 08:59 PM   #5
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Linear polarizers don't work on digital sensors anyway, but the effect you're seeing is consistent with what I've seen in other places. Very wide lenses don't get along too well with polarizers, but I must admit I've never seen such an extreme example at 17mm. I'd guess it's the filter. What's the brand?
12-27-2013, 09:02 PM   #6
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You got the effect one would expect. The effect of a polarizer depends on the angle to the sun. With a super wide lens the angle to the sun varies enough over the frame that you get the effect you got!
12-27-2013, 09:02 PM   #7
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It's not supposed to produce that effect at any FL. If it was the lens hood you'd think the bottom would be affected as well. Since the light is from the side and low on the horizon I would not think it is spilling over from above.
12-27-2013, 09:06 PM   #8
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It is a common effect for such a wide lens. Light rays from the sun hit the frame at different angles put the polarizer only works at a certain angle. The magnitude depends on your angle with the sun. It is strongest with the sun at 90 degrees as in the photos you bosted (just guessing). To avoid it, use a lens longer than 24mm (35mm equiv)

12-27-2013, 09:49 PM - 1 Like   #9
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QuoteOriginally posted by Ole Quote
You got the effect one would expect. The effect of a polarizer depends on the angle to the sun. With a super wide lens the angle to the sun varies enough over the frame that you get the effect you got!
For darkening of the sky with a super-wide angle lens, Ole is 100% correct. It's all the angle between the polarizer orientation and the sun.

For pictures that do not have the sky in them, such as a wet surface, or leaves with indirect bluish sky glare, a CPL can be used without this overriding dark band effect. There may be some differential effect across the frame, but not the extreme effect of the direct sky.
12-27-2013, 10:29 PM   #10
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Yep, agreed. Too much polarization at too wide a setting. A CPL works much more consistently at medium focal lengths. Weird things happen to the sky at very wide settings.
12-27-2013, 10:36 PM   #11
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QuoteOriginally posted by Kharan Quote
Linear polarizers don't work on digital sensors anyway, but the effect you're seeing is consistent with what I've seen in other places. Very wide lenses don't get along too well with polarizers, but I must admit I've never seen such an extreme example at 17mm. I'd guess it's the filter. What's the brand?
Sure they do. They just screw with the light metering. And maybe autofocus.

To the OP, there's a good explanation of the problem, including photos taken in a similar location.
12-27-2013, 11:18 PM   #12
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QuoteOriginally posted by Kharan Quote
Linear polarizers don't work on digital sensors anyway, but the effect you're seeing is consistent with what I've seen in other places. Very wide lenses don't get along too well with polarizers, but I must admit I've never seen such an extreme example at 17mm. I'd guess it's the filter. What's the brand?
Linear polarizers work fine with digital sensors and the metering system. The compatibility issues are with the AF system.

As for the OP's photo. I am with Ole. The image is well in line with what you might see for some sun angles with a wide angle lens where a polarizer filter (either linear and circular) has been used.


Steve

Last edited by stevebrot; 12-27-2013 at 11:42 PM.
12-27-2013, 11:30 PM   #13
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I go with the CPL wide view of sky causing the effect. It is expected. It is not restricted to using a polarizing filter -- it can occur with any wide lens, even without a filter. I was once asked about a wide sky shot taken with the M 28/3.5 on Velvia 50, a very "punchy" slide film many of us liked a lot in the dark ages, particularly for scenics and the great greens.

The slide was not taken with a polarizer, but the sky was much darker on one side than the other. Polarizing filters just make it even more prominent. 90 degrees from the sun is the darkest point. Velvia 50 just made it stand out even without the filter.

PS - the comments about the effect not being different between the linear and polarizing versions is 100% correct. The problem, specifically, is the half silvered area on the mirror that has a polarizing effect of its own. This can -- but most of the time will not -- affect auto focus and also can affect metering, particularly when using a spot setting that sees relatively more of that area of the image reflected to the meter.
12-27-2013, 11:50 PM   #14
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QuoteOriginally posted by Canada_Rockies Quote
I go with the CPL wide view of sky causing the effect. It is expected. It is not restricted to using a polarizing filter -- it can occur with any wide lens, even without a filter. I was once asked about a wide sky shot taken with the M 28/3.5 on Velvia 50, a very "punchy" slide film many of us liked a lot in the dark ages, particularly for scenics and the great greens.

The slide was not taken with a polarizer, but the sky was much darker on one side than the other. Polarizing filters just make it even more prominent. 90 degrees from the sun is the darkest point. Velvia 50 just made it stand out even without the filter.

PS - the comments about the effect not being different between the linear and polarizing versions is 100% correct. The problem, specifically, is the half silvered area on the mirror that has a polarizing effect of its own. This can -- but most of the time will not -- affect auto focus and also can affect metering, particularly when using a spot setting that sees relatively more of that area of the image reflected to the meter.
This is absolutely correct and immediately obvious when shooting with a fisheye or any other wide angle lens where a wide slice of sky is present. The issue with spot metering is also quite true, though it is related to the angle of view more than the how or whether the light is polarized.


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12-28-2013, 12:17 AM   #15
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QuoteOriginally posted by abmj Quote
Yep, agreed. Too much polarization at too wide a setting. A CPL works much more consistently at medium focal lengths. Weird things happen to the sky at very wide settings.
+1 That about sums it up
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