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06/22/2003 04:03:44 PM · #1 |
(continued from another thread)
Originally posted by pitsaman:
I bought couple of filters and results are disappointing. |
I have to say that it would probably is because you don't know how or where to use them.
I have found that here in the UK, polarizers are usually worthless for skies - the skies don't really go blue enough to allow the polarizer to do its job. In Arizona, however, the blue sky went nearly black. I have noticed similar results in Norway, not sure why.
What a polarizer does
Remember what a polarizer does, though; It removes reflections from non-metallic surfaces. Water and glass panes are particularly obvious examples. Just try to turn your PL filter, and see how the scene changes.
Because of this special property, I would be inclined to claim that PL filters are one of the few types of filters that cannot be artificially added in Photoshop. (the other ones that springs to mind are close-up filters and infrared filters).
polarised light
Polarizer filters work like this: When light is emitted from a light source (i.e almost all sources of light with the exception of certain types of lasers), it is unpolarised. Think of it as waves going in all directions.
If you can imagine a light beam coming straight towards you, and imagine it flowing up and down as a wave, the different light photones would be moving like this:
| (vertical)
/ (diagonal SW/NE)
\ (diagonal SE/NW)
- (horizontal)
Remember that the light will also be polarised in all directions in between those four.
In other words, a wave of light basically has two dimensions (x,y) to vibrate back and forth in if the wave propagates along the third axis (z). The x,y vibration is our polarisation.
Reflection
When the unpolarised (-/\|) light hits a surface that is non-metallic, let us say a pane of glass, it becomes polarised. What happens is that some of the light will go through the glass, while other parts of the light will be reflected.
The reflected light will now have a new property; It does not contain all the variations of polarisation. (it is impossible to measure exactly which ones it does contain, but explaining this phenomenon takes us into quantum physics and is most easily explained in the field of quantum microparticle physics and quantum cryptography, which seems a little over-kill)
Anyway. If you will imagine, our freshly polarized light beam now only contains "|" and "/" light. the "\" and "-" light has been absorbed / dispersed by the glass.
polarizer filters
A polariser filter takes this effect to the extreme; The glass in polarizer filters is created in such a way that it only lets through a very narrow band of light (let us say that it only lets through light that has an x,y value of horizontal ("-"), give or take 10 degrees). This is also the reason for why PL filters are so dark.
Now, this filter would completely block out everything but the horizontal light waves, we get this effect:
(unpolarized light "|/\-" ) -> polarizer filter -> (polarized light "-")
In other words, our unpolarized light has been reduced to only horizontal polarized light. Everything else is absorbed by the filter.
Because PL filters rotate, you effectively have many filters in one - you can orientate the filter in any you want, choosing which lightwaves to filter out. This is why you can remove reflections from glass and water.
As for skies, this works in a similar fashion: On its way through the ozon layer and the atmosphere, the light becomes polarized along the spectrum - blue light becomes polarized slightly different than the rest of the spectrum, which is why you can effectly filter out parts of the rest of the spectrum, resulting in ridiculously blue skies.
Phew, that was quite a blotch of text. If you want more on the science of polarized light, I would suggest this article. More about polariser filters is available here. Pictures from my trip to Arizona, where I used polarisers most of the time, are available here.
Enjoy!
Haje Jan Kamps
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06/23/2003 04:33:20 AM · #2 |
In addition to shamelessly bumping this back up, I'd like to add that this is a very good resource for more information, particularly about the differences betweeen linear polarizers (PL) and circular polarizers (C-PL)
HJ
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06/23/2003 10:18:44 AM · #3 |
| something interesting I just learned about polarisers, is that they influence fur/ hair a great deal too, darkening and increasing the saturation on animals. |
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06/23/2003 10:25:17 AM · #4 |
These four shots were taken within about an hour of each other, in Las Vegas, NV. All with a polariser, but different rotations/ angles.
Two of them are probably far too dark but show the effect.
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06/23/2003 11:00:01 AM · #5 |
Page with polariser info which is pretty easy to follow (even for someone like me!)
//www.cliffshade.com/dpfwiw/polarizer.htm |
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