manual_focus
Senior Member
How does a circular polarizing filter work? A CPL works by only allowing light which is polarized in a particular plane to pass through, and filtering/absorbing/eliminating light which is vibrating in a plane different than what the filter is aligned with. Simple, huh?
There are two types of light which a CPL most interacts with (the polarizing effect): skylight and reflected sunlight. Skylight, as opposed to sunlight (light coming directly from the sun), is sunlight that has been scattered by crashing into atoms of oxygen and nitrogen high in the atmosphere. This scattered light is the reason the sky is blue. From a photographer’s perspective the most important thing to understand is that skylight, which is mostly polarized in one direction, is greatest 90 degrees to the sun. and occupies a band that is around 20-30 degrees wide, fading to white on either side.
An easy way to see where in the sky, light is most polarized, is to point your index finger at the sun and raise your thumb straight up. Now rotate your wrist from one horizon to the other and that arc generated by your thumb is where the sky will be the bluest. If you then look up in the sky you will see that darker blue band with the horizons changing to white.
Another thing to understand is that as the sun moves across the sky, so does the blue band. It changes on both a time of day and time of year basis. Be aware and don't be afraid to use your thumb.
The second type of light that a CPL effects is reflected sunlight (glare) that has been reflected (and becomes polarized) mostly from shiny surfaces like water, glass, leaves, and any non-metallic surface. This is the type of light that polarized sunglasses are made to interact with. The CPL woks by filtering out all the reflected like that is not vibrating in one particular plane. The end result is colors and shadows appear deeper, purer, with increased clarity.
These two effects together are why a CPL is unique and can be hard to reproduce in post.
A final interesting fact is that polarized light can become unpolarized by coming into contact with water vapor (clouds and/or humidity) and dust/aerosols/air pollution. The reason a CPL works with blue skylight and white clouds, is it darkens the blue sky (which is polarized) while leaving the white unpolarized light from clouds unaffected. This results in a creating a more dramatic (higher contrast) scene. Areas with a lot of particles in the atmosphere (agriculture, deserts, cities with bad air pollution, etc.) and/or high humidity will have a reduced polarizing effect from using a CPL as the light is less polarized. Light near the horizon is whitish because the skylight has interacted with the particles/water vapor in the atmosphere and become unpolarized. Also, a CPL has little effect when shooting in the direction of the sun as during a sunrise or sunset.
The sky will be darkest blue when a weather front has moved through and cleared out air pollution and/or humidity. Since the effect is highest where there is low humidity and clean air, one can find these conditions at higher elevations >4,000 ft, i.e., the mountains. The higher the better.
So, knowing some of the physics of CPLs and polarized light, photographers have a better handle on when and how to use a CPL. I have two images which I hope show the effects of a CPL on 1) skylight (scattered sunlight) and 2) reflected sunlight (glare), and a third image showing how to use a CPL with an ultra wide-lens.
1) A CPL can make the sky dark blue to the point it is near black by absorbing/filtering out all the polarized (mostly) blue skylight streaming straight down from the upper atmosphere. Remember that the darkest band of blue polarized light lies in a 20-30 degree area high in the sky. For maximum effect the camera should be pointed up into this band. The first image shows the changes in sky color as the point of view changes from horizontal to near vertical as the camera comes closer to the steeple. In all shots the CPL was rotated to maximum effect. I find the resulting light unique. It’s the light one would expect on a planet with little atmosphere. Depending on the level of polarization the effect is similar to the dark blue sky just after sundown.
2) The second image shows the results of shooting a brick façade which is illuminated with direct sunlight and rotating the CPL from full off (90o), to half way (45o), to full on polarizing effect. If one looks closely the CPL is eliminating the glare (scattered, reflected chaotic light) so the colors become purer (more monochromatic) and the shadows are deeper/darker. The light that is remaining is linear and less chaotic as it’s all vibrating in one plane. Some describe the effect as increase contrast and improved clarity. If you look at the top of each shot you will also see that the sky turns darker as the CPL is rotated, i.e., both effects can happen together.
3) The band of polarized light covers the FOV of a 50mm lens which is why some say to be aware of using a CPL with wide-angle lenses. Once you get beyond a telephoto to normal lens it’s easy to cover more of the sky than that portion which is highly polarized. This is when the image shows the dramatic band of blue polarized skylight quickly changing to lighter blue to whitish sky (less polarized) which can be unnerving.
The third image was taken with the DA* 11-18mm f2.8 with a CPL. I feel this shot works because the point of view is mostly vertical into the band of polarized skylight. If you look closely you can see the change in skylight to lighter blue on the lower right side. This is mostly obscured by the vegetation. Don’t be afraid of using a CPL even on an ultra wide-angle lens. Just be aware of what is happening. Take advantage of the fact that Pentax made the excellent 11-18mm with threads for front filters, even if they are 82mm.
4) And finally, while not as dramatic, shooting into the band of polarized blue skylight without a CPL will still give you a dark blue sky. It does not reduce any glare.
I fully enjoyed shooting this series and better understanding CPLs and polarized light. While the effects can be quite useful for photographers, my personal preference is to only use a CPL when I want its polarizing effects, i.e., I don’t leave the filter on.
Most of the shots were taken with the DA* 50-135mm f2.8 which I found tack sharp with no noticeable distortion from center to edge at f8. Impressive, and a joy to use due to its relatively light weight and overall ease of use! I bought my copy used and two weeks later the SDM motor failed. I put the lens on the shelf but finally got around to having the motor replaced at Precision Camera. Glad I did.
Feel free to leave comments and/or suggestions.
Enjoy.
1) Taken with CPL at full effect in all images. Shows the effect of shooting from near horizontal to near vertical into the band of maximum polarized skylight.
2) Taken with CPL at 90o, at 45o and at full polarizing filter effect. Viewed at original size you can better see how a CPL effects glare/sunlight that has been reflected off a nonmetallic surface. Notice how the sky darkens as the CPL is turned to full effect.
3) Taken with the DA* 11-18 f2.8 lens with a CPL.
4) Top image taken with DA Limited 20-40mm without a CPL. The bottom image was taken with the DA* 50-135mm with a CPL. Shows the effect of shooting into the polarized skylight band regardless of CPL use.
5) Full size image showing the detail that the DA* 50-135 is capable of when viewed at original size. I call this the brick test!
3 image vertical pano taken with DA* 50-135mm using Pixel Shift on the K3-ii. Slight improvement on some details when compared without using Pixel Shift.
13 image multi-row pano. Shows that you can use a CPL on a complex pano but be careful.
Full size crop from the above multi-row pano. I find the light otherworldly (very monochromatic/pure) and the detail from the DA* 50-135 impressive.
Best when viewed at "original size".
There are two types of light which a CPL most interacts with (the polarizing effect): skylight and reflected sunlight. Skylight, as opposed to sunlight (light coming directly from the sun), is sunlight that has been scattered by crashing into atoms of oxygen and nitrogen high in the atmosphere. This scattered light is the reason the sky is blue. From a photographer’s perspective the most important thing to understand is that skylight, which is mostly polarized in one direction, is greatest 90 degrees to the sun. and occupies a band that is around 20-30 degrees wide, fading to white on either side.
An easy way to see where in the sky, light is most polarized, is to point your index finger at the sun and raise your thumb straight up. Now rotate your wrist from one horizon to the other and that arc generated by your thumb is where the sky will be the bluest. If you then look up in the sky you will see that darker blue band with the horizons changing to white.
Another thing to understand is that as the sun moves across the sky, so does the blue band. It changes on both a time of day and time of year basis. Be aware and don't be afraid to use your thumb.
The second type of light that a CPL effects is reflected sunlight (glare) that has been reflected (and becomes polarized) mostly from shiny surfaces like water, glass, leaves, and any non-metallic surface. This is the type of light that polarized sunglasses are made to interact with. The CPL woks by filtering out all the reflected like that is not vibrating in one particular plane. The end result is colors and shadows appear deeper, purer, with increased clarity.
These two effects together are why a CPL is unique and can be hard to reproduce in post.
A final interesting fact is that polarized light can become unpolarized by coming into contact with water vapor (clouds and/or humidity) and dust/aerosols/air pollution. The reason a CPL works with blue skylight and white clouds, is it darkens the blue sky (which is polarized) while leaving the white unpolarized light from clouds unaffected. This results in a creating a more dramatic (higher contrast) scene. Areas with a lot of particles in the atmosphere (agriculture, deserts, cities with bad air pollution, etc.) and/or high humidity will have a reduced polarizing effect from using a CPL as the light is less polarized. Light near the horizon is whitish because the skylight has interacted with the particles/water vapor in the atmosphere and become unpolarized. Also, a CPL has little effect when shooting in the direction of the sun as during a sunrise or sunset.
The sky will be darkest blue when a weather front has moved through and cleared out air pollution and/or humidity. Since the effect is highest where there is low humidity and clean air, one can find these conditions at higher elevations >4,000 ft, i.e., the mountains. The higher the better.
So, knowing some of the physics of CPLs and polarized light, photographers have a better handle on when and how to use a CPL. I have two images which I hope show the effects of a CPL on 1) skylight (scattered sunlight) and 2) reflected sunlight (glare), and a third image showing how to use a CPL with an ultra wide-lens.
1) A CPL can make the sky dark blue to the point it is near black by absorbing/filtering out all the polarized (mostly) blue skylight streaming straight down from the upper atmosphere. Remember that the darkest band of blue polarized light lies in a 20-30 degree area high in the sky. For maximum effect the camera should be pointed up into this band. The first image shows the changes in sky color as the point of view changes from horizontal to near vertical as the camera comes closer to the steeple. In all shots the CPL was rotated to maximum effect. I find the resulting light unique. It’s the light one would expect on a planet with little atmosphere. Depending on the level of polarization the effect is similar to the dark blue sky just after sundown.
2) The second image shows the results of shooting a brick façade which is illuminated with direct sunlight and rotating the CPL from full off (90o), to half way (45o), to full on polarizing effect. If one looks closely the CPL is eliminating the glare (scattered, reflected chaotic light) so the colors become purer (more monochromatic) and the shadows are deeper/darker. The light that is remaining is linear and less chaotic as it’s all vibrating in one plane. Some describe the effect as increase contrast and improved clarity. If you look at the top of each shot you will also see that the sky turns darker as the CPL is rotated, i.e., both effects can happen together.
3) The band of polarized light covers the FOV of a 50mm lens which is why some say to be aware of using a CPL with wide-angle lenses. Once you get beyond a telephoto to normal lens it’s easy to cover more of the sky than that portion which is highly polarized. This is when the image shows the dramatic band of blue polarized skylight quickly changing to lighter blue to whitish sky (less polarized) which can be unnerving.
The third image was taken with the DA* 11-18mm f2.8 with a CPL. I feel this shot works because the point of view is mostly vertical into the band of polarized skylight. If you look closely you can see the change in skylight to lighter blue on the lower right side. This is mostly obscured by the vegetation. Don’t be afraid of using a CPL even on an ultra wide-angle lens. Just be aware of what is happening. Take advantage of the fact that Pentax made the excellent 11-18mm with threads for front filters, even if they are 82mm.
4) And finally, while not as dramatic, shooting into the band of polarized blue skylight without a CPL will still give you a dark blue sky. It does not reduce any glare.
I fully enjoyed shooting this series and better understanding CPLs and polarized light. While the effects can be quite useful for photographers, my personal preference is to only use a CPL when I want its polarizing effects, i.e., I don’t leave the filter on.
Most of the shots were taken with the DA* 50-135mm f2.8 which I found tack sharp with no noticeable distortion from center to edge at f8. Impressive, and a joy to use due to its relatively light weight and overall ease of use! I bought my copy used and two weeks later the SDM motor failed. I put the lens on the shelf but finally got around to having the motor replaced at Precision Camera. Glad I did.
Feel free to leave comments and/or suggestions.
Enjoy.
1) Taken with CPL at full effect in all images. Shows the effect of shooting from near horizontal to near vertical into the band of maximum polarized skylight.
2) Taken with CPL at 90o, at 45o and at full polarizing filter effect. Viewed at original size you can better see how a CPL effects glare/sunlight that has been reflected off a nonmetallic surface. Notice how the sky darkens as the CPL is turned to full effect.
3) Taken with the DA* 11-18 f2.8 lens with a CPL.
4) Top image taken with DA Limited 20-40mm without a CPL. The bottom image was taken with the DA* 50-135mm with a CPL. Shows the effect of shooting into the polarized skylight band regardless of CPL use.
5) Full size image showing the detail that the DA* 50-135 is capable of when viewed at original size. I call this the brick test!
3 image vertical pano taken with DA* 50-135mm using Pixel Shift on the K3-ii. Slight improvement on some details when compared without using Pixel Shift.
13 image multi-row pano. Shows that you can use a CPL on a complex pano but be careful.
Full size crop from the above multi-row pano. I find the light otherworldly (very monochromatic/pure) and the detail from the DA* 50-135 impressive.
Best when viewed at "original size".
