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Old Jun 7, 2011, 10:45 PM   #1
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Default Polarizers - One, Two, Three (warning lots of imgs)

As I understand polarizers, they are designed to work much like a venetian blind does - it sort-of works like slats that allow light from only one direction to come in. That's why it's effective in cutting out reflections.

Here's a photo of a circular polarizer. It's darker than a clear piece of glass, so you can also use it as a (relatively) weak neutral density filter, to cut the total amount of light reaching the camera.



Another picture of the same filter. But wait - now it's black!



How can that be? The answer is simple when you look at these next two pictures:



Two polarizers (the small one I would assume is a linear polarizer, it doesn't say one way or the other, but was bought in the early 1980's, before the days of auto focus). Change the orientation of one of them so that it lets in only light that the first one blocks gives you no light at all:



The first set of pictures were taken with the linear polarizer on the lens, first with it rotated so that it was aligned to allow light through in the same direction as the circular polarizer, the second one to block the light the CPL allows through.

Then, because I have a curious mind, I decided to add a third polarizer to the mix. These pictures were taken with a linear on the camera.





All are letting light through at the same angle.

This shot has the two that are on top of each other stopping the light, while the one on the camera is aligned the same as the bottom one.



This shot has the polarizer on the lens stopping the light from the bottom filter. Even though the one on the camera is now aligned with the filter on the top, the one on the bottom still blocks the light.



You'll get essentially the same picture if you have the two filters on the ground aligned like in the first shot of the two filters on top of each other, but with the lens filter adjusted to block that light. You can see it here: http://mtngal.zenfolio.com/img/v21/p9837439.jpg - take a look at how the filters are aligned to see that it's not the same picture as the one just above.

For the last challenge I posted a backlit picture where I used two polarizers and a clear piece of plastic. I did something similar today, only the scene isn't backlit. I have a polarizer on the lens, with the two other polarizers on the ground. It's not as effective as the light-table one I posted for that challenge, but shows you what is going on:



Notice the difference between the two - the plastic case on top of the circular polarizer shows the bending of the light when you put a piece of clear plastic between two polarizers.

Time to put things away, the linear polarizer in it's case with the polarizer still on the lens.



And finally, the second polarizer is put away, a picture without a polarizer on it at all.



I know, there are far more pictures in this thread than the rules allow, but I thought they should go together as the series is related. If you think it would be better, I'll either split this up or delete some parts and make it less all-inclusive.

Perhaps I'm the only one that finds this whole topic fascinating.
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Old Jun 7, 2011, 11:13 PM   #2
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I was going to comment on the number of photos in your posting but I see that you already aware. However, since this is a very interesting and informative series, I will allow it.

Referring back to your first photo in the series, typically, a polarizing filter is roughly equivalent to a ND2 neutral density filter. That means you lose about one f-stop of light with the filter rotated for minimum effect.

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Old Jun 7, 2011, 11:14 PM   #3
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This has to be one of the more detailed expositions I've seen. Like Calr, I think bending the rules here allows us to see the various angles to this theme.
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Old Jun 8, 2011, 8:34 AM   #4
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Thank you very much for allowing this one thread to go on so much - I couldn't figure out a graceful way of excluding things (except for perhaps the last two, but I thought they added a slightly different dimension to the topic from the picture before - showing how only the part of the plastic above the polarizer shows the bending of light). Breaking it up (thought that would be possible) would have required 4 threads, would have been more disjointed, and would have broken the rules, too, and thought perhaps doing it this way was better overall. I appreciate you temporarily suspending the rules to allow it to stand as-is.

Cal - thank you also for equating the polarizer with its corresponding neutral density filter number. I'd like to eventually get a ND filter but had no idea what the numbers meant in terms of what I have. So if I want less light I'll need to go with a higher number than ND2, that's great to know!
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Old Jun 8, 2011, 9:36 AM   #5
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That was a great lesson. Thanks!
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Old Jun 8, 2011, 10:11 AM   #6
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This is a very interesting and informative series! Thanks
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Old Jun 8, 2011, 6:04 PM   #7
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I have a circular polariser, but don't yet know how to use it (have trouble trying to find the max & min spots in the first place!). I'd like to see this thread and some of the others from this challenge combined into a sticky as reference material. Everyone has been quite informative with their shots and explanations.

Thank you all.

Bernice
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Old Jun 8, 2011, 6:38 PM   #8
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I've been carefully watching responses to this thread. I do have a polarizer but it has seen limited use, mostly because when I am out taking pictures I seldom think of using it and how it might enhance my pictures.

I think I need to just put it on a lens, go out and shoot a variety of subjects in different lighting conditions and carefully look at the results to see what worked, what didn't and make a conscious effort to utilize it more often.

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Old Jun 8, 2011, 9:42 PM   #9
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I tend to forget about my polarizer much of the time also. But when I do think about it and actually think about using it effectively, I sometimes get unexpected results.

It surprised me how little trouble my camera had focusing through the linear polarizer I used for this series - I think I'll pull one of them out more often now. I find that the effect I get with it is greater than I do with the circular polarizer.

The best way to figure one out is to go out and play. Keep in mind that the polarizer is sort-of like a blind, so it will block reflected light from windows, shiny leaves and water. It will also block some of the reflected light in the atmosphere (reason for darker skies) and can "see" a bit through haze (though my experience is that it's somewhat limited, it won't help when it's foggy). If it's morning or afternoon, put the sun on your shoulder, point straight ahead (right angle to the sun) and twist the polarizer, watching the effect change. It can get quite absorbing, facing different directions and seeing the differences you get (some things will get lighter, some darker, some colors will change/be more saturated). I should have tried one when I was shooting water drops on flowers Sunday - oh well, next time...
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Old Jun 11, 2011, 7:38 AM   #10
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Great job. Very informative! Thanks.
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