Help On Circular & Linear Polarizer.
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Cheesecake said:
I see his posts every now and then, and I would feel very sorry if this community could not accept him just because he wrote some not quite correct things in the past. Despite former mistakes, he no doubt can contribute something.
Frankly, there's much more hilarious nonsense to be read on ClubSnap including from "old birds" and "gurus". I'm sure that if I searched long enough, I would also find something that you wished you never wrote.
Why pick specifically on this one person? Because he appears to have no peer support? That would be very sad indeed.
Cheesecake
Senior Member
Really amusing to read the "scientific" explanations given here of how polarizer works and the difference between linear and circular polarizers.
While Little Wolf's explanation is most technically correct I'm afraid you have not really made it easy to understand.
Sammy888, I applaud your effort in putting together such a detailed writeup in post #65 but I'm afraid your explanation is also technically incorrect. However I appreciate your good intentions. Polarizing is about the orientation of the E and H field about the axis of travel of light, and is not really related to the direction of travel of light.
Let me give it a try in explaining first about the basic principles of a polarizer, and then the difference between linear and circular polarizers.
First of all, the propagation of light can be simplistically represented by the physical vibration on a length of string. Imagine 2 person A and B each holding an end of a long and slightly elastic string and pulling the string taught. Asume the string is light enough such that there is no slack. In other words, the string is a straight line joining person A to person B.
Now lets call person A the light source and person B the object the light is falling on.
(a) If person A moves his end of the string up and down at a regular frequency he will create a wave on the string that travels from A to B. To an observer standing between A and B but to one side of the string he will see a sinusoidal wave on the string that travels from A to B. Because the vibration on the string is in the up-down direction only we call such a vibration "polarized" in the vertical direction. I guess it is obvious that the vibration can also be polarized in the horizontal direction if A moves his end of the string in a left-right manner. In fact polarization means the vibration of the string in only 1 particular direction, anywhere from 0 to 360 degrees. At any angle of vibration, the direction of travel of the wave is the same (along the length of the string), but only the orientation of the vibration is different.
(b) Imagine now that A moves his end of the string in a random fashion, including up-down and left-right motion. The disturbances on the string will still travel across the string towards B . To the observer there is no clear pattern on the string even though he can clearly see some kind of random shape on the string that travels from A to B. In this case the vibration is not polarized.
The vibration on the string in both cases represents the vibration of electrical and magnetic field in free space which is a simple explanation of what light is.
(c) Imagine the observer now holds a piece of large square board with a straight slit cut in the middle of the board. Next image that the string is passed through the slit of the board with its both ends held by A and B still. If the observer holds the board such that the slit in the board is vertical, and A moves his ends of the string in an up-down movement, the vibration on the string will travel through the slit unaffected. In this case we say the vibration is polarized and the direction of polarization is aligned with the slit on the board, so the "light" from A travels easily through the slit (polarizer) to land on B.
(d) Imagine the observer still holds the board the same way (slit is vertical) but now A moves his end of the string horizontally. Assume that the observer can hold the board firm enough, then all the vibration on the string will be blocked by the board, and no "light" reaches B. In this case we can say that the polarization of the vibration is not aligned with the slit so the "light" does not pass through to reach B.
(e) Imagine next that A moves the his end of the string at 45 degrees. He will still be sending polarized vibration down the string, as explained above. What happens when the vibration hits the board with the vertical slit? There will be only be vertically polarized vibration coming out from the other side of the board because of the restriction imposed by the vertical slit. The level of vibration coming our of the other side of the board is also smaller than the level of vibration hitting it from the A side (in theory, 1/[SqRoot 2] of the level on A side).
(f) Now imagine A moving his end of the string in a random manner, and the board is still positioned such that the slit is oriented vertically. The same thing as the previous case happens. The random vibration will hit the board and only verticle vibration comes out from the other side. No matter how A moves his end of the string, the viration that reaches B is always polarized vertically.
(g) OK, one final imagination. Now imagine A moving his end of the string in a random manner, and the observer arbitrarily changes the angle of the slit about the axis of the string. B always receives vibration that is polarized according to the angle of the slit.
I hope is is obvious noe that the board with the slit I described above represents the polarizer. The 6 scenarios described above correspond to the following optical scenarios:
(a) Polarized light from a light source hits a target.
(b) Un-polarized light from a light source hits a target.
(c) Polarized light from a light source passes through a polarizer that is aligned in the same angle.
(d) Polarized light from a light sources is completely blocked by a polarizer that is aligned perpendicularly to the orientation of electric field "vibration" in the light.
(e) Polarized light from a light source is partially blocked by a polarizer that is aligned at 45 degrees to the orientation of electric field. Only component of the energy that is aligned with the polarizer is allowed to pass through.
(f) Un-polarized light from a light source is "polarized" by the polarizer. Only component of energy that is aligned with the polarizer (vertically) is allowed to pass through.
(g) Un-polarized light from a light source is "polarized" by the polarizer. Only component of energy that is aligned with the polarizer (arbitrary angle) is allowed to pass through.
So now comes the basic operating principle of a polarizer, in terms of photography:
1. If the light from the scene is un-polarised, the polarizer behaves not much differently from a neutral density (ND) filter. By blocking energy components that are not aligned with the polarizer, the overall impact is a fixed reduction of energy passing through regardless of the angle of the polarizer.
2. If the light from the scene is polarized, the amount of light that passes through the polarizer can be attentuated at different levels. Level of attentuation is dependent on the relative angle between the angle of polarization of the light and the angle of the polarizer.
3. Light reflected off textured surfaces are not polarised. Light directly reflected off shining surfaces at particular angle (around 45 degrees) are polarized. In other words, direct reflections off glass and shining surfaces usually consist of polarized light when viewed at around 45 degrees to the surface, which can be effectively reduces by using a polarizer. Removing the direct reflection components of light from a surface brings out the true colour of the material that form the surface. If you shoot perpendicularly through a glass window you cannot remove your own reflection with a polarizer.
4. Sunlight dispersed by the atmosphere is also polarized where the rays are almost tangential to the top surface of the atmosphere. Make the thumb's up sign and point your thumb at the sun. Open and close your fingers while keeping your thumb pointed at the sun. The area of the sky where your fingers sweep across has the most polarized light. That part of the sky can be effectively darkened by a polarizer, resulting in very deep blue skies in pictures.
to be continued....
While Little Wolf's explanation is most technically correct I'm afraid you have not really made it easy to understand.
Sammy888, I applaud your effort in putting together such a detailed writeup in post #65 but I'm afraid your explanation is also technically incorrect. However I appreciate your good intentions. Polarizing is about the orientation of the E and H field about the axis of travel of light, and is not really related to the direction of travel of light.
Let me give it a try in explaining first about the basic principles of a polarizer, and then the difference between linear and circular polarizers.
First of all, the propagation of light can be simplistically represented by the physical vibration on a length of string. Imagine 2 person A and B each holding an end of a long and slightly elastic string and pulling the string taught. Asume the string is light enough such that there is no slack. In other words, the string is a straight line joining person A to person B.
Now lets call person A the light source and person B the object the light is falling on.
(a) If person A moves his end of the string up and down at a regular frequency he will create a wave on the string that travels from A to B. To an observer standing between A and B but to one side of the string he will see a sinusoidal wave on the string that travels from A to B. Because the vibration on the string is in the up-down direction only we call such a vibration "polarized" in the vertical direction. I guess it is obvious that the vibration can also be polarized in the horizontal direction if A moves his end of the string in a left-right manner. In fact polarization means the vibration of the string in only 1 particular direction, anywhere from 0 to 360 degrees. At any angle of vibration, the direction of travel of the wave is the same (along the length of the string), but only the orientation of the vibration is different.
(b) Imagine now that A moves his end of the string in a random fashion, including up-down and left-right motion. The disturbances on the string will still travel across the string towards B . To the observer there is no clear pattern on the string even though he can clearly see some kind of random shape on the string that travels from A to B. In this case the vibration is not polarized.
The vibration on the string in both cases represents the vibration of electrical and magnetic field in free space which is a simple explanation of what light is.
(c) Imagine the observer now holds a piece of large square board with a straight slit cut in the middle of the board. Next image that the string is passed through the slit of the board with its both ends held by A and B still. If the observer holds the board such that the slit in the board is vertical, and A moves his ends of the string in an up-down movement, the vibration on the string will travel through the slit unaffected. In this case we say the vibration is polarized and the direction of polarization is aligned with the slit on the board, so the "light" from A travels easily through the slit (polarizer) to land on B.
(d) Imagine the observer still holds the board the same way (slit is vertical) but now A moves his end of the string horizontally. Assume that the observer can hold the board firm enough, then all the vibration on the string will be blocked by the board, and no "light" reaches B. In this case we can say that the polarization of the vibration is not aligned with the slit so the "light" does not pass through to reach B.
(e) Imagine next that A moves the his end of the string at 45 degrees. He will still be sending polarized vibration down the string, as explained above. What happens when the vibration hits the board with the vertical slit? There will be only be vertically polarized vibration coming out from the other side of the board because of the restriction imposed by the vertical slit. The level of vibration coming our of the other side of the board is also smaller than the level of vibration hitting it from the A side (in theory, 1/[SqRoot 2] of the level on A side).
(f) Now imagine A moving his end of the string in a random manner, and the board is still positioned such that the slit is oriented vertically. The same thing as the previous case happens. The random vibration will hit the board and only verticle vibration comes out from the other side. No matter how A moves his end of the string, the viration that reaches B is always polarized vertically.
(g) OK, one final imagination. Now imagine A moving his end of the string in a random manner, and the observer arbitrarily changes the angle of the slit about the axis of the string. B always receives vibration that is polarized according to the angle of the slit.
I hope is is obvious noe that the board with the slit I described above represents the polarizer. The 6 scenarios described above correspond to the following optical scenarios:
(a) Polarized light from a light source hits a target.
(b) Un-polarized light from a light source hits a target.
(c) Polarized light from a light source passes through a polarizer that is aligned in the same angle.
(d) Polarized light from a light sources is completely blocked by a polarizer that is aligned perpendicularly to the orientation of electric field "vibration" in the light.
(e) Polarized light from a light source is partially blocked by a polarizer that is aligned at 45 degrees to the orientation of electric field. Only component of the energy that is aligned with the polarizer is allowed to pass through.
(f) Un-polarized light from a light source is "polarized" by the polarizer. Only component of energy that is aligned with the polarizer (vertically) is allowed to pass through.
(g) Un-polarized light from a light source is "polarized" by the polarizer. Only component of energy that is aligned with the polarizer (arbitrary angle) is allowed to pass through.
So now comes the basic operating principle of a polarizer, in terms of photography:
1. If the light from the scene is un-polarised, the polarizer behaves not much differently from a neutral density (ND) filter. By blocking energy components that are not aligned with the polarizer, the overall impact is a fixed reduction of energy passing through regardless of the angle of the polarizer.
2. If the light from the scene is polarized, the amount of light that passes through the polarizer can be attentuated at different levels. Level of attentuation is dependent on the relative angle between the angle of polarization of the light and the angle of the polarizer.
3. Light reflected off textured surfaces are not polarised. Light directly reflected off shining surfaces at particular angle (around 45 degrees) are polarized. In other words, direct reflections off glass and shining surfaces usually consist of polarized light when viewed at around 45 degrees to the surface, which can be effectively reduces by using a polarizer. Removing the direct reflection components of light from a surface brings out the true colour of the material that form the surface. If you shoot perpendicularly through a glass window you cannot remove your own reflection with a polarizer.
4. Sunlight dispersed by the atmosphere is also polarized where the rays are almost tangential to the top surface of the atmosphere. Make the thumb's up sign and point your thumb at the sun. Open and close your fingers while keeping your thumb pointed at the sun. The area of the sky where your fingers sweep across has the most polarized light. That part of the sky can be effectively darkened by a polarizer, resulting in very deep blue skies in pictures.
to be continued....
Now the bit on linear versus circular polarizers.
Linear polarizers are similar to the board with a slit. Light that manage to pass through it are polarized in the same orientation as the polarizer. This works fine for most manual cameras.
Newer cameras usually employ some kind of beam splitter prism to direct part of the light comming through the lens for metering purposes. Since the beam splitter has polarizing effect also, it will not be accurate if the incomming light is already polarized.
Now remember that once you have a linear polarizer aligned correctly to give you the desired (or closest to desired) lighting effect, the light that has passed through the polarizer need not remain polarized all the way until it hits the film or sensor. If some kind of "de-polarizer" can be placed after the linear polarizer to randomize or evenly re-distribute the orientation of the electric and magnetic fields, it will still achieve the desired reflection/flare cutting or sky deepening effect, while maintianing compatibility with cameras with beam splitter metering designs.
There is no effective way to randomize the electric and magnetic field, but there is a workaround.
To help visualize that I will have to bring you back to the string representaion. Imagine now that there is a string held between A and B with no polarizing boards in between. Now imagine A moves his end of the string in a clockwise circular motion about the axis of the string (like kids playing skipping robe). He will be sending a cockscrew kind of wave pattern towards B. Such a kind of vibration is representative of circularly polarized light.
At this point I am stucked because I cannot come up with a board + slit design that would turn un-polarised or linearly polarised vibrations on the string into circularly polarized vibration.
For that we will have to get a bit more technical at look at it purely as light.
A circular polarizer consists of a layer of linear polarizer followed by a layer of "wave retarder" (I hope I got the term right, even if the term is wrong the principle should be correct). The wave retarder lets 50% of the light through untouched, but slows the other 50% by quarter wavelength as well as rotating the electric and magnetic field vibration by 90 degrees. Imagine a single frequency of light coming thorugh this 2-layer setup. After passing through the linear polarizer layer the light will be polarised in a particular orientation. 50% will pass through the "wave retarder" while the other 50% gets delayed by 1/4 wavelength and rotated 90 degrees before being mixed with the 50% that passed through. The resulting electric field at a point in space after the wave retarder will be a vector of constant magnitude but rotating about the axis of travel of the light ray.
I think this might be confusing but if you now look at the diagrams in the link supplied by LittleWolf (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html) it should be clearer.
Because a circular polarizer produces circularly polarized (duh) light that has even distribution of the electric and magnetic field orientation it works well with beam splitter type of metering systems.
Phew...:sweat:
Thanks for your attention and I sincerely welcome corrections to my explanation above.
- Roy
Linear polarizers are similar to the board with a slit. Light that manage to pass through it are polarized in the same orientation as the polarizer. This works fine for most manual cameras.
Newer cameras usually employ some kind of beam splitter prism to direct part of the light comming through the lens for metering purposes. Since the beam splitter has polarizing effect also, it will not be accurate if the incomming light is already polarized.
Now remember that once you have a linear polarizer aligned correctly to give you the desired (or closest to desired) lighting effect, the light that has passed through the polarizer need not remain polarized all the way until it hits the film or sensor. If some kind of "de-polarizer" can be placed after the linear polarizer to randomize or evenly re-distribute the orientation of the electric and magnetic fields, it will still achieve the desired reflection/flare cutting or sky deepening effect, while maintianing compatibility with cameras with beam splitter metering designs.
There is no effective way to randomize the electric and magnetic field, but there is a workaround.
To help visualize that I will have to bring you back to the string representaion. Imagine now that there is a string held between A and B with no polarizing boards in between. Now imagine A moves his end of the string in a clockwise circular motion about the axis of the string (like kids playing skipping robe). He will be sending a cockscrew kind of wave pattern towards B. Such a kind of vibration is representative of circularly polarized light.
At this point I am stucked because I cannot come up with a board + slit design that would turn un-polarised or linearly polarised vibrations on the string into circularly polarized vibration.
For that we will have to get a bit more technical at look at it purely as light.
A circular polarizer consists of a layer of linear polarizer followed by a layer of "wave retarder" (I hope I got the term right, even if the term is wrong the principle should be correct). The wave retarder lets 50% of the light through untouched, but slows the other 50% by quarter wavelength as well as rotating the electric and magnetic field vibration by 90 degrees. Imagine a single frequency of light coming thorugh this 2-layer setup. After passing through the linear polarizer layer the light will be polarised in a particular orientation. 50% will pass through the "wave retarder" while the other 50% gets delayed by 1/4 wavelength and rotated 90 degrees before being mixed with the 50% that passed through. The resulting electric field at a point in space after the wave retarder will be a vector of constant magnitude but rotating about the axis of travel of the light ray.
I think this might be confusing but if you now look at the diagrams in the link supplied by LittleWolf (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html) it should be clearer.
Because a circular polarizer produces circularly polarized (duh) light that has even distribution of the electric and magnetic field orientation it works well with beam splitter type of metering systems.
Phew...:sweat:
Thanks for your attention and I sincerely welcome corrections to my explanation above.
- Roy
roygoh said:
Hey Roy. thanks for the lengthy but highly informative bit on how a polariser filter works but my attempt was not to get technical, too indepth and overwhelm the newbies or readers. I just wanted to get the basic understanding across in as simple a way as possible and then let the reader's imagination spur them on to try it out for themselve. Some self discovery will help them learn and remember how to effectively use the filter on the long run. I just wanted to also demystified this filter and clear some of the misunderstood claims people here and outside of clubsnap have had with this filter. Yes I agree with your technical explanation but to the lay person...it is too much to take in. At the end of the day they want to be confidence enough to go buy one to try..most likely a cheap one to start with to see what is the big deal about it. The techincal stuff if they ever want to know will only come in handy if they get serious into know more about how the polarising effect takes place.
So now..they know they need to do more then plug and play, they know you need to rotate the outer rim to adjust the effect, they know it has something to do with the fact that it distort or control how various kind of light passes through the lens to the film or CMOS/CCD and that they should get a circular polariser as it is geared more to meet the modern day Auto FOCUS camera and exposure metering system and they also know that the filter is not always the best choice to get deep blue sky..etc. With some of those basic concept, they only need to go out and with some confidence to go buy maybe a reasonable one and go try it out. No theory, technical facts from you and me will give these newbies the best lesson then something they will now go out to try for themselve. Yes there are more stuff I have left out but that is just it...leave it to them to find it out lah. That was how I learn to use and appreciate this filter for example. Too much details just spoil the fun of discovery in my opinion. hehe..
But I did enjoy your info and link...since I am abit of a techno weenie.
reachme2003 said:
You can't make it right, I guess. If the target audience has some basic science background, one can explain it very quickly (but some people will complain it's not understandable). If one explains it from the ground up, people will complain it's too long. Sometimes I suspect that the acceptable size limit to the cellphone generation is given by the maximum length of an SMS.
I would recommend to anyone interested in photography to read up on elementary optics in a science textbook (secondary/JC level should get one quite far). This will not only answer a lot of questions, but also clear up common misconceptions. A better understanding of light (and more) will also help to appreciate and develop the creative/artistic side. It's not only "know your tools", but also "know your medium".
sammy888 said:Hey Roy. thanks for the lengthy but highly informative bit on how a polariser filter works but my attempt was not to get technical, too indepth and overwhelm the newbies or readers. I just wanted to get the basic understanding across in as simple a way as possible and then let the reader's imagination spur them on to try it out for themselve. Some self discovery will help them learn and remember how to effectively use the filter on the long run. I just wanted to also demystified this filter and clear some of the misunderstood claims people here and outside of clubsnap have had with this filter. Yes I agree with your technical explanation but to the lay person...it is too much to take in. At the end of the day they want to be confidence enough to go buy one to try..most likely a cheap one to start with to see what is the big deal about it. The techincal stuff if they ever want to know will only come in handy if they get serious into know more about how the polarising effect takes place.
So now..they know they need to do more then plug and play, they know you need to rotate the outer rim to adjust the effect, they know it has something to do with the fact that it distort or control how various kind of light passes through the lens to the film or CMOS/CCD and that they should get a circular polariser as it is geared more to meet the modern day Auto FOCUS camera and exposure metering system and they also know that the filter is not always the best choice to get deep blue sky..etc. With some of those basic concept, they only need to go out and with some confidence to go buy maybe a reasonable one and go try it out. No theory, technical facts from you and me will give these newbies the best lesson then something they will now go out to try for themselve. Yes there are more stuff I have left out but that is just it...leave it to them to find it out lah. That was how I learn to use and appreciate this filter for example. Too much details just spoil the fun of discovery in my opinion. hehe..
But I did enjoy your info and link...since I am abit of a techno weenie.
Hi Sammy,
The basics on how to use a polarizer and how linear and circular polarizer can be compatible with different camera systems has already been explained by many including yourself (which by the way you have elegantly summarized in your post here), so my intention was to go a little deeper for the more technically inclined, plus satisfy my own desire to practice my writing skills.
I understand your point about getting the basics across but I have to stress that your explanation given earlier about how a polarizer works is simply incorrect. Please don't take this personally. I am not attacking you in anyway, but just want to clarify the technical misconception presented here so far (not only by you). We are all here to learn from each other, and if in my haste to put down what I know about polarizers in just a few paragraphs I might have unintentionally created a perception of arrogance on my part then I apologize for that. I just feel strongly that though it is OK and even desired to simplify the explanations to appeal to the less technically inclined, it is not OK to present the wrong concept in trying to do so.
Again, I appreciate your effort in sharing your knowledge here, and I thank you also for actually reading through my explanation.
As for what I have written, I would say it is up to each member whether they want to read it or not. Read only if you are interested. It is definitely not a difficult read, despite the length. I can understand if someone is put off simply by the length of my post, and it is totally OK with me as I too am sometimes impatient to read through lengthy posts if I am not really interested in the subject.
- Roy
LittleWolf said:
Haha, this statement takes the cake!
However, sad to say it is actually most representative of the nature of the forum. Lengthy technical replies most often go unappreciated. This is because those who are technically inclined and with the attention span to follow would usually have grasped the concepts well enough to understand the functions without one having to resort to explanations at the fundamental level.
Those who are unable to understand basic theoretical explanations, will certainly be overwhelmed by the sheer amount of background knowledge that is thrown at them when such a detailed one is attempted.
Sort of a "catch 22" situation here.
From what I observed, a long explanation is more likely to have the enquirer come back later asking you the same thing that you've already stated somewhere in your reply.;p
Zerstorer said:Haha, this statement takes the cake!
However, sad to say it is actually most representative of the nature of the forum. Lengthy technical replies most often go unappreciated. This is because those who are technically inclined and with the attention span to follow would usually have grasped the concepts well enough to understand the functions without one having to resort to explanations at the fundamental level.
Those who are unable to understand basic theoretical explanations, will certainly be overwhelmed by the sheer amount of background knowledge that is thrown at them when such a detailed one is attempted.
Sort of a "catch 22" situation here.
From what I observed, a long explanation is more likely to have the enquirer come back later asking you the same thing that you've already stated somewhere in your reply.;p
Haha...so true!
I was hoping that my explanation will benefit those who are interested enough in the subject but do not wish to read through at least a few chapters on optics from one or more optics/photography text books to get a more complete picture of the theory behind how polarizers are used in photography. In other words, a group (perhaps small) of people who are somewhat between the 2 groups you have described.
Another thing that prompted me to write what I have written was that I want to point out that some of the explanations provided are technically incorrect but don't feel comfortable in just saying "your explanation is incorrect" without giving my own version of an explanation.
Finally I just enjoyed writing all that stuff as it helps me organise my thoughts and is a good practise for me in terms of organizing material for technical presentations, which is a signficant part of my job.
- Roy
Hi guys,
First of all thanks for the long but informative information (especially by Roy and Sammy) on how polarisers work. It helped alot for a newbie like me!
Question: So in a way CIR-POL is differentiated from a linear-POL by the fact that, in simplistic talk, it doesn't mess up the metering. So a CIR-POL is so called "designed" for an AF body? What if a manual focus body uses a CIR-POL? Will there be any difference from using a linear polariser instead?
Oh and another question: Roy, with regards to the quoted text above, how does one know when light, before it reaches the polariser, is polarised or not? What constitutes to the difference between a textured and a shining surface for the fact that light becomes polarised after reflecting off a textured surface and not one that shines? Does this mean a textured surface has some kind of property that polarises light?
Pardon me if this has been discussed before.
Thanks in advance.
First of all thanks for the long but informative information (especially by Roy and Sammy) on how polarisers work. It helped alot for a newbie like me!
Question: So in a way CIR-POL is differentiated from a linear-POL by the fact that, in simplistic talk, it doesn't mess up the metering. So a CIR-POL is so called "designed" for an AF body? What if a manual focus body uses a CIR-POL? Will there be any difference from using a linear polariser instead?
RoyGoh said:
Oh and another question: Roy, with regards to the quoted text above, how does one know when light, before it reaches the polariser, is polarised or not? What constitutes to the difference between a textured and a shining surface for the fact that light becomes polarised after reflecting off a textured surface and not one that shines? Does this mean a textured surface has some kind of property that polarises light?
Pardon me if this has been discussed before.
Thanks in advance.
LordAeRo said:
heheh funny you should mention that. I happen to have both and my Hoya linear one is about 20 yrs old now while my B+W circular polariser is about 10yrs. I have tested it on my antiqued Nikon FG which is the only non AF body I have still and the Circular polariser works fine like my linear version.
sammy888 said:
Haha, okay got it. Thanks!
I had a brain wave after posting the 2nd question and here is my take on it:
Remembering my primary school science, I recalled a diagram whereby reflective surfaces are so-called "straight lined surfaces" and textured surfaces (i presume rough) have "jagged lined surfaces". So two things can happen when light hits a surface. When light hits a...
Reflective surface
Since it is a reflective surface aka. straight lined surface, light will bounce off at the same angle (think angle of incidence = angle of reflection), so in other words, it is non-polarised, as they still travel at the same angle as per before it was reflected by the surface.
Textured surface
Since this time it's a textured surface aka. jagged lined surface, light will NOT bounce off at the same angle (angle of incidence != angle of reflection), simply because the jagged surfaces will not allow the light to bounce off at the same direction at which it travelled in. Yes, for each "ray" of light, the angle of incidence will be the angle of reflection because each jagged surface is made up of a flat surface, but when compared to the other rays, the light ray will not be at the same angle as the others because the jagged surfaces are not aligned the same way. So based on the fact that light will presumably be darker when viewed at an angle (aka. your eye is directed at the surface), light can be said as "polarised". In practice however, it may not be as said because light may bounce off other surfaces and end up hitting the eye.
This 5minute diagram should explain things well, if you understand what I've been saying.
http://lordaero.multiply.com/photos/photo/3/1.gif
Feel free to rebutt if my concepts are wrong, it seems logical from my point of view. :sweatsm:
http://images.lordaero.multiply.com...Z.Z7my5dG6UkbqLXJiOSbVPo30lfg.YUNHjlgAUVuZQ,Y
Feel free to rebutt if my concepts are wrong, it seemed logical from my point of view.
Not really to rebutt...but I think your LINK has been "polarised" heheh it is missing in action from my screen when I clicked it. hahahahahah
LordAeRo said:
Ya it is fixed now heheh
Well in a way you are right with your theory. That is why I mentioned that this filter is nota cure all it really depend on how the right wave of light hits the filter At the right frequency or angle it will cut the glare, deepens teh colour by elimiate the shine on it and oh..almost foget..it can also warm some colours up. That is why Roy's explaination is more technical as the elimination is not just somethign about light travelling at acertain angle but of a certain colour or frequency range. So with you rough surface as you draw it, some of those reflection or bright spots will be cut off but some others will not.
In fact you can do this by place say 5 shiny surface objects with many sharp or blunt corners and multi dimensional. (like a small status for example) Have then all spread out on a desk and you aim your camera with the polariser mounted on your lens at the entire scene. Rotating the rim slowly as you looked through your viewfinder. You can see the filter doing its job of eliminating shine or glares from surfaces and deepening colours from all objects. But it is not eliminating them from all the same places of each object. This is because each object is not place in the exact same position to catch the same illumination angle and relative position you have placed the camera. That is basically how I see the use of the polariser from way back then. It is as basic a way for a lay person to understand how to use the filter but of course it does not teach much in terms of technical knowledge but it will do for me as that is all I want to know. I like my PDA, Mountain Bike and guitar for examle but I am not going to go learn how to built one hehe I just want to know how to use one. period. hehe
LordAeRo said:
On a camera that does not use beam splitter optics for metering or AF, there is no difference between using linear or circular polarizer. Not all manual focus cameras are OK with linear polarizers. Read some where that some Canon MF SLRs use beam splitter for metering (http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00496f).
LordAeRo said:
Light that is reflected off a non metallic shiny surface at roughly 45 degrees is polarized. This has to do with the electric field component that is perpendicular to the reflecting surface cancelling eah other at the point of reflection, so the reflected light consist mainly of electric field component parallel to the reflecting surface, ie, polarized. If the reflecting surface is metallic then the reflection properties are different because metal is conductive - reflected light is not polarized.
A textured sureface, if I understand what I read correctly, can be said to be made up of countless tiny shining surfaces facing different directions. As such from any viewing angle only a small portion of light that reaches the observer is reflected at the right angles to be polarized.
Sunlight can also be polarized when scattered by the atmosphere at close to tangential angles. That's why some poarts of the sky can be easily darkened by using a polarizer (more than the normal attenuation effect that the polarizer has on all sources of light).
I came across this website also that does a good job of explaiing about polarization of light:
http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l1e.html
Hope this is helpful.
- Roy
- Status
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