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  1. #1
    Join Date
    Dec 2006
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    God's Country - Australia
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    How does a CPL work ?

    i know how it works in practice, but WHY does it work like that ? ie: how does rotating the cpl make difference etc ?
    D800e l V3 l AW1 l 16-35 l 35 l 50 l 85 l 105 l EM1 l 7.5 l 12-40 l 14 l 17 l 25 l 45 l 60 l 75
    flickr

  2. #2
    Join Date
    Jul 2005
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    Hmmm ok...

    Light "particles" behave like waves. They go up and down, or from side to side, while they move ahead, just like other waves.

    So, the wave characteristic makes the light waves wider in the direction they "wave" in.
    Light that gets reflected, in the air, or on water, or on foliage, changes wave direction. This is called polarized light.

    A polarized light filter has very narrow bars in one direction. light waves going up and down for instance can pass when the bars are vertical. But when you twist the pol. filter 90 degrees, and the bars are horizontal, the up and down light waves can not pass through. This is how a pol. filter blocks polarized light.

    This is of course simplified, I do not know the exact science behind it.

    Circular pol. filters work exactly the same as linear pol. filters with the filtering, what makes them "circular" is that after the filtering the light wave "directions" are scattered again, so not cause trouble for AF sensors.
    Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30

  3. #3
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    ok. so by rotating it, it disperses the light waves in a differnt way ?
    D800e l V3 l AW1 l 16-35 l 35 l 50 l 85 l 105 l EM1 l 7.5 l 12-40 l 14 l 17 l 25 l 45 l 60 l 75
    flickr

  4. #4
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    No, with rotating it, you block different wave movement directions.
    That is what the rotating is about, trying to block certain directions fo waves to pass through the gitter.

    The circular part of the came has nothing to do with that the pol. filter can rotate. It is just that it twists the waves in different directions, so the AF sensors will not be blind to the light if they happen to have the wrong wave direction.

    I hope this crappy quick sketch makes things clearer...
    Attached Images Attached Images  
    Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30

  5. #5
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    thats brilliant, made things very clear. thank you for taking the time to do that, i appreciate it.

    now...next question. some people say they can see the differences thru the viewfinder. i cant see it at all, i need to take a test shot and review. am i turning it too fast or am i going blind ? lol
    D800e l V3 l AW1 l 16-35 l 35 l 50 l 85 l 105 l EM1 l 7.5 l 12-40 l 14 l 17 l 25 l 45 l 60 l 75
    flickr

  6. #6
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    Jul 2005
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    I quite clearly see the differences through the view finder, but not as strong as in the caprtured image. I see the sky getting darker, and notice when it is at its darkest while turning to and fro. But not as strong as in the actual photo.

    Why that is I do not know, probably the same reason why the depth of field and bokeh also looks different on the focus screen than on the photo (and I do not know the reason for that either).
    Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30

  7. #7
    Join Date
    Mar 2007
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    I thought the reason depth of field looks different in the viewfinder from the actual photo is because the lens keeps the aperture at its max opening during pre-shutter-actuation, in order to let the most light through for AF and such. Then adjusts to the actual aperture used as the photo is taken.
    Nikon D40 + kit lens

    Nikkor 50mm f/1.8 D AF(...or not)

  8. #8
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    Jul 2005
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    Quote Originally Posted by fionndruinne View Post
    I thought the reason depth of field looks different in the viewfinder from the actual photo is because the lens keeps the aperture at its max opening during pre-shutter-actuation, in order to let the most light through for AF and such. Then adjusts to the actual aperture used as the photo is taken.
    No, most cameras have a depth of field button that can close the aperture so you can judge the depth of field. Your D40 just does not.
    So, next time you try out an XTi/400D, try it out... you will notice that the actual image looks a bit different from what you see through the view finder. And that is the case with every SLR. I do not know why.
    Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30

  9. #9
    Join Date
    Feb 2007
    Location
    Illinois, USA
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    Also keep in mind that many lenses have a rotating front element, which will effectively turn the CPL while you're focusing. Meaning it won't always stay where you rotated it to.

    And I'm guessing the mirrors that images go through to get what you see in the viewfinder have something to do with why it's hard to notice in the viewfinder. If you take the CPL off your lens, and actually hold it up directly in front of your eyes, and pick a few subjects like a lamp, a car window, a mirror, water, etc. you can get a better understanding of how different rotations effect different things.

  10. #10
    Join Date
    Jan 2006
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    Delfgauw, The Netherlands
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    Quote Originally Posted by Rooz View Post
    now...next question. some people say they can see the differences thru the viewfinder. i cant see it at all, i need to take a test shot and review. am i turning it too fast or am i going blind ? lol
    I can see mostly see the difference in the viewfinder, though sometimes the effect is minimal. Sometimes I only see a very slight change in colour cast in some areas of the picture.
    Then again, sometimes the effect is hardly visible in the result either.

    By the way, when I was in Italy I found that my polarizer worked a lot better than in does at the same time of the day with the same type of sky as in the Netherlands. In fact, the effect I saw in the viewfinder was a lot more than I had ever seen before. Does anybody know what could be the reason for that? Perhaps all the moisture in the sky due to the heat?
    Nikon D-50
    // Nikkor 70-300 f/4-5.6 VR // Nikkor 50 mm f/1.8
    // Sigma 17-70 f/2.8-4.5 ...// Nikon SB-600
    // Sigma 10-20 f/4-5.6......// Nikon Series E 135 mm f/2.8
    // Kiron 105 f/2.8 Macro....// Manfrotto 190XPROB + 488RC4
    // Nikkor 35 f/1.8..........// Sigma 500 mm f/8

    My website: http://www.dennisdolkens.nl

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