What comes to mind when you think of a polarizing filter? It’s probably how it can darken blue skies. This is just one of the many things this versatile polarizer can do. Many photographers swear by them and some go so far as to keep them on their lenses all the time. But as far as darkening blue skies are concerned, polarizers can create more problems than they solve if you’re not careful.
But before getting into all that, just exactly what does a polarizing filter do? How does it darken blue skies?
It all starts with the fact that light is a wave. We speak of the color of light in terms of the frequency of the wave, just as we speak of the pitch of a sound in terms of its frequency. Red light has a lower frequency and blue light, a higher frequency. It’s as if light vibrates – up and down. And most light vibrates in all directions. But some forms of light vibrate in a single direction. This is called polarized light. For example, glaring light bouncing off the highway can be polarized in a horizontal direction. That’s why Polaroid sunglasses work. They block horizontally polarized light while allowing light polarized in the other directions to pass.
The same is true of a blue sky, or at least some of it. Depending on where the sun is, blue sky light is polarized to a greater or lesser extent. If you stand facing the sun and look through your polarizer, you will notice that it has no effect – the sky is not darkened. But if you continue to look through the polarizer and slowly turn away from the sun you will notice the sky gets darker and darker until the sun is directly over your shoulder. Continuing your turn, the sky will get lighter and lighter until the sun is directly behind you.
…as far as darkening blue skies are concerned, polarizers can create more problems than they solve if you’re not careful.
You can tell in which direction the effect is the greatest with this simple trick.
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Figure 1 – Finding the polarized sky |
Make an L with your thumb and forefinger. Point your finger at the sun. Rotate your hand, keeping the finger pointed at the sun. Your thumb will be pointing in the direction where the polarizing filter has its greatest effect.
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Figure 2 – Polarizer Filter |
The filter itself has two parts, an inner ring that screws into your lens and an outer ring that rotates. The effect is adjusted by rotating the outer ring.
Here is an example of an image taken with and without a polarizing filter.
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Figure 3 – 105 mm with polarizer |
Figure 4 – 105 mm without polarizer |
Both photographs were taken with a 105 mm lens. Figure 3 is polarized and Figure 4 is not. The difference is dramatic. The first thing to jump out is the sky. The polarized image shows the clouds very clearly while they are barely visible in the unpolarized image. This is due to the effect described above; the image is shot at such an angle to the sun that the sky is polarized and the filter darkens it but not the clouds.
But there’s another beneficial effect in Figure 3. When these images were shot, the sky was very bright but the land was still in relatively dark shadow. In other words, the image had a high dynamic range. In the unpolarized image, the sky dominates the exposure, rendering the land very dark. But in the polarized image, the sky is not as bright resulting in a dynamic range that is not as great. Therefore, the earth is not as dark. So two good things happen.
Now let’s see what happens when we take a wider shot.
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Figure 5 – 70 mm with polarization |
Figure 6 – 70 mm without polarization |
Both of these images were shot with a 70 mm lens. Again, Figure 5 is polarized and Figure 6 is not. Very much the same thing is going on here as was with the 105 mm lens shots. The clouds are enhanced and the earth is brighter in Figure 5. But the left edge of the this image is a little bit lighter than the right edge. We’re starting to see that part of the sky is not being darkened quite as much by the filter.
What happens when we go even wider?
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Figure 7 – 50 mm with polarization |
Figure 8 – 50 mm without polarization |
These images are shot at 50 mm. Something new is starting to show up now. The left edge is even brighter and the right edge is a little brighter than the center of the image. We have reached a focal length that is wide enough that the field of view actually extends beyond the part of the sky that is affected by the polarizer.
Is there any way to get the effect of a polarizer on the sky with a wide angle lens without the problems that the filter causes?
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Figure 9 – 35 mm with polarization |
Figure 10 – 35 mm without polarization |
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Figure 11 – 24 mm with polarization |
Figure 12 – 24 mm without polarization |
At 35 mm and 24 mm, the angle of view is so wide that a substantial part of the image is outside the range where the polarizer is effective. Now the limitations of polarizing filters become obvious. The effect first noticed in the 50 mm shot is much more prominent to the point where the polarizer actually causes a problem. There is a dark area in the middle of the frame and this can be very difficult to recover from in Lightroom.
And these shots were taken at right angles to the sun. If you are shooting with a wide angle lens at, say, 45 degrees from the sun then one side of the image will be darkened, the side closest to 90 degrees from the sun, and the other side will not. This is generally not what we are looking for in the sky.
Is there any way to get the effect of a polarizer on the sky with a wide angle lens without the problems that the filter causes? Yes. Lightroom can be used to bring the unpolarized RAW image to life. The Exposure, Highlights, Shadows, Whites and Blacks adjustments all play a role in realizing the potential stored in the RAW file. These are pretty standard adjustments that affect the entire image.
But one adjustment is worth singling out, an adjustment that is often not fully appreciated. It is the Luminance adjustment in the HSL group. With this adjustment it is possible to darken just the blue sky. This has very much the same effect as the polarizing filter without the problem that it has with wide angle lenses. Here’s the result.
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Figure 13 – 24 mm without polarization with Lightroom enhancements |
There is one more thing to keep in mind when using any filters on a wide-angle lens. You will notice some vignetting in the corners of the image above. It’s really apparent in the top corners. When using any kind of filter on a wide-angle lens it’s possible the rim of the filter will be in your photo. Filter manufacturers make thin filters that reduce this problem. But fortunately, this too can be corrected in Lightroom.
It’s important to not forget, however, that polarizing filters are much more versatile than simply darkening blue skies. And these other uses don’t run into the problems we’ve been exploring here. These include cutting glare on shiny objects, especially still water. With a polarizer you can look into the water and see the lake or stream bed.
Polarizers are also magical in bringing out the colors of foliage, especially on an overcast or rainy day. In fact, it’s worth checking a scene by holding up your polarizer in front of your eye and rotating it to see what it will do for you.
But in those situations where a polarizing filter creates more problems than it solves, there are ways to achieve comparable results in Lightroom.
I hope you find this information helpful. Join us in one of our photography workshops and learn about these and other techniques while photographing some of the most beautiful places in the West. Click here for more information.
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