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02-10-2010, 07:28 PM   #1
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Polarizers and IR's

I know I should do a search, but thought I would hit some of you experts up with a beginner question.

Polarizers...

Linear, Circular...

What am I supposed to see? I look for reflections on screen, or the water in the pond behind our home, and I eliminate them by turning the filter. Am I doing this correctly? I've seen the pictures of 'with' and 'without' but don't see the results.

When I turn the filter, I get two spots on the 360 degree circle that brighten the viewfinder, and two that darken as well as eliminate shadows. Is the latter the correct position.

When would you use a linear vs a circular?

Also, IR lens filters. I just have an 850, and have tried some high iso shots, and some long exposures. Is there any magic to these, or do you just expose to get a b&w type image?

Thanks

02-10-2010, 07:59 PM   #2
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A circular polarizer is a linear polarizer with something added that has nothing to do with polarization.

On SLR cameras use a circular polarizer.

The spot is suppose to point at the light source. Draw an imaginary line from the center of the lens and through the spot. Now turn the polarizer until that line points at your light source. Your polarizer is aligned.

The polarizer works best at 90 degrees from the light source. So at high noon, when the sun is directly overhead, you'll get the largest effect by pointing your camera anywhere along the horizon. At sunset, you'll get the greatest effect by pointing the camera north, south, at the ground, or directly overhead.

As for what you should see...that depends on what you're looking at. You should see foliage brighten up and reflections disappear. The sky should become more blue, and clouds better defined. You should be able to see through glass and water when reflections were previously blocking your view.

Here's a very good article on polarizers...
http://archive.popphoto.com/pdfs/2002/0902/Polarizer.pdf
02-10-2010, 09:13 PM   #3
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QuoteOriginally posted by Snydly Quote
...
When would you use a linear vs a circular?
I think all mine are linear. Some AF systems don't work with them. Pentax is supposed to be OK. Obviously circular costs more.

QuoteQuote:
Also, IR lens filters. I just have an 850, and have tried some high iso shots, and some long exposures. Is there any magic to these, or do you just expose to get a b&w type image?
Color doesn't really mean anything in the IR world. The image on the preview screen will be mostly red and black. I use a Hoya R72 (720nm) so it's a little different than yours. I shoot RAW so I can alter the white balance easier and get more bits to work with in processing. I try to use foliage to set a custom white balance.

Exposures are tricky because most of the data is in the red channel, which you have to be careful not to overexpose. The other channels may have so little data that you'll see tons of noise if you try to bring those levels up. Bracket exposures if you can.

Framing and focus are also tricky. Framing can be done with the filter off. Some lenses have a mark indicating how much IR focusing is off. Sometimes you have to bracket these too, or use a small aperture for lots of depth of field, so exact focus isn't critical.

Some example photos. The first is the scene without a filter.



If you screw on the Hoya R72 and forget the custom white balance trick, this is what you'll see on the preview screen:



With some processing of the best image, I got this:



If you convert to black and white, noise is much less of an issue, and you don't have to make up the colors. So processing is easier. I keep trying to do the false color style but not well.
02-10-2010, 09:16 PM   #4
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QuoteQuote:
Graystar: The polarizer works best at 90 degrees from the light source. So at high noon, when the sun is directly overhead, you'll get the largest effect by pointing your camera anywhere along the horizon.
At high noon, the sun is not directly overhead--not where I live. And not where most people live either. The Sun's path through the sky is always changing, as the Earth moves through its orbit. In fact, where I live, in New England, the Sun is no where near directly overhead.

On the equator, on the dates of the 2 equinoxes, then the Sun is directly overhead--but only if you are living on the equator. There is nowhere, in the continental US, where the Sun is ever directly overhead.

02-10-2010, 09:22 PM   #5
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QuoteOriginally posted by Jewelltrail Quote
At high noon, the sun is not directly overhead--not where I live. And not where most people live either. The Sun's path through the sky is always changing, as the Earth moves through its orbit. In fact, where I live, in New England, the Sun is no where near directly overhead.

On the equator, on the dates of the 2 equinoxes, then the Sun is directly overhead--but only if you are living on the equator. There is nowhere, in the continental US, where the Sun is ever directly overhead.
So?

..........
02-10-2010, 09:39 PM   #6
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QuoteQuote:
Graystar: So?

Sorry, I thought the conclusion to this was obvious. If the Sun is not directly overhead, you will not be 90 degrees from it anywhere along the horizon.
02-10-2010, 09:43 PM   #7
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QuoteOriginally posted by Jewelltrail Quote
...On the equator, on the dates of the 2 equinoxes, then the Sun is directly overhead--but only if you are living on the equator. There is nowhere, in the continental US, where the Sun is ever directly overhead.
If you are in Hawai'i around the beginning of June or in mid-July, the sun will be directly overhead. They've named it "Lahaina noon". Anywhere between the two Tropics has the same thing happen.
02-10-2010, 09:57 PM   #8
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QuoteOriginally posted by Jewelltrail Quote
If the Sun is not directly overhead, you will not be 90 degrees from it anywhere along the horizon.
Right. So my statement applies only "when the sun is directly overhead"...and at no other time...hence my clarification of "when the sun is directly overhead." If the sun isn't directly overhead at noon, then it doesn't apply, right? So what's the problem?

The idea behind my description is to allow a person to easily visualize what's meant by "90 degrees from the light source," which would otherwise require a more complex explanation. I really don't think there's a need to tailor my answer for each of Earth's 7 billion inhabitants.

02-10-2010, 09:58 PM   #9
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QuoteQuote:
Just1moreDave: If you are in Hawai'i around the beginning of June or in mid-July, the sun will be directly overhead. They've named it "Lahaina noon". Anywhere between the two Tropics has the same thing happen.
Precisely, which is why I used the words no where in the continental US is the Sun ever directly overhead. In fact, these days, in New England, the Sun is relatively low in the southern sky.
02-10-2010, 10:37 PM   #10
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QuoteOriginally posted by Jewelltrail Quote
Precisely, which is why I used the words no where in the continental US is the Sun ever directly overhead. In fact, these days, in New England, the Sun is relatively low in the southern sky.
My parents live near Hartford, so I am sadly familiar with this. We always go home for Christmas, and I can never believe how dark it is there, compared to Colorado. And then it's cloudy most of the time. Who can use a polarizer in those conditions?
02-10-2010, 10:55 PM   #11
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QuoteQuote:
Graystar: Right. So my statement applies only "when the sun is directly overhead"...and at no other time...hence my clarification of "when the sun is directly overhead." If the sun isn't directly overhead at noon, then it doesn't apply, right? So what's the problem?

The idea behind my description is to allow a person to easily visualize what's meant by "90 degrees from the light source," which would otherwise require a more complex explanation. I really don't think there's a need to tailor my answer for each of Earth's 7 billion inhabitants.
This is the Beginner's section of our forum. I'm not sure about you, but I remember what it was like to be a beginner. Correct, you are not here to tailor your advice for all of the world's people; rather, you, as well as the rest of us, are here to tailor our advice for the OP. BTW, the OP, you and I, all live in the continental US.

Moreover, your statement above is not equivalent to the original one which you presented to the OP. Let me put that back up here, just in case:

QuoteQuote:
Graystar: So at high noon, when the sun is directly overhead, you'll get the largest effect by pointing your camera anywhere along the horizon.
02-10-2010, 10:59 PM   #12
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QuoteQuote:
Just1moredave: My parents live near Hartford, so I am sadly familiar with this. We always go home for Christmas, and I can never believe how dark it is there, compared to Colorado. And then it's cloudy most of the time. Who can use a polarizer in those conditions?
So true! But we have snow, at least tonight, and sometimes at Christmas. This allows us to have genuine empathy when we sing, "I'm dreaming of a White Christmas."
02-10-2010, 11:40 PM   #13
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QuoteOriginally posted by Jewelltrail Quote
This is the Beginner's section of our forum. I'm not sure about you, but I remember what it was like to be a beginner. Correct, you are not here to tailor your advice for all of the world's people; rather, you, as well as the rest of us, are here to tailor our advice for the OP. BTW, the OP, you and I, all live in the continental US.
I apologize to Snydly for not having provided a description of the correct shooting angle with the crystal clear clarity, pinpoint precision, wondrous wit, and salient succinctness of the description that Jewelltrail will be providing in his very next post.

I can hardly wait to bask in glow of his enlightened wisdom…
02-11-2010, 12:42 AM   #14
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QuoteQuote:
Graystar: I apologize to Snydly for not having provided a description of the correct shooting angle with the crystal clear clarity, pinpoint precision, wondrous wit, and salient succinctness of the description that Jewelltrail will be providing in his very next post.

I can hardly wait to bask in glow of his enlightened wisdom
Well, since you are so nice, we'll forgive you.
02-11-2010, 02:53 AM   #15
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Back to the InfraRed query

I'm going to expand this a bit, shift over to the other end of the visible spectrum, to one of my crazes, ACTINIC light. Basic photo emulsions, the chemistry of films and plates, are ONLY sensitive to UV-violet-blue light, called ACTINIC. Film etc can only 'see' blue, green, yellow, red, and IR, by adding dyes to the emulsion. Such dyes were gradually developed after about 1880.

I try to reproduce that pre-1880 spectrum-slice with violet or blue-violet optical filters (*no* UV-blockers allowed!) on my K20D. Just setting JPG or RAW controls to blue-channel isn't enough, although I do adjust the channels when processing actinic images. The output is B&W, and it DOESN'T look like sepia filters applied by pose-in-old-duds portrait studios. Try an 80c filter and see!

Back to IR: one can't capture near-IR spectra by selecting IR in the JPG menu or RAW lab. An optical filter is mandatory. A R72 (720nm) passes a lot of visible light (VL); I use 780nm (still some VL) on my K20D, and 900-930-1000nm (zero VL) on a couple digicams with IR hot-filters removed. Virtually all dSLRs (Fuji and Sigma forensic cams excepted) have such IR hot-filters deep inside; serious IR work is much easier if the hot-filter is gone, but that's non-reversible surgery,and it ain't cheap nor easy. With a Sony NightShot cam, it's trivial -- hot a switch and the hot-filter flips away. I shoot handheld at 1000nm.

As Dave said above, shoot RAW and adjust white balance in processing. WB *can* be set for JPGs prior to shooting, but that's tricky and (to me) frustrating. The processed output can be B&W or colored. But it won't look like IR color film, which uses spectrum-shifting to yield false colors, unless one is willing to tediously apply a lot of color substitutions.

Playing around at the edges of the visible spectrum (and beyond) is fun. Or maybe I'm just crazed. Whatever.
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