What's the difference between a polarizing filter and a neutral density filter? The answer is that they're both like sunglasses for your lens, but they are just as different from each other as a standard pair of sunglasses is from a pair that is polarized. A neutral density filter simply cuts back on the amount of light that reaches your camera's sensor, which allows you to shoot at slower shutter speeds in brighter conditions. A polarizing filter, on the other hand, only cuts back on a certain kind of light -
and to further complicate the matter, there are two different kinds of polarizing filters: a linear polarizing filter and a circular polarizing filter. Do you know the difference? Read on to find out the answer.
What does a polarizing filter do?
In order to really understand what a polarizing filter does as compared to what a neutral density filter does, you must first understand what “polarization" is. Light travels in waves, and those waves can oscillate in more than one direction. Linear polarization refers to light that oscillates in a single direction. Circular polarization refers to light that rotates at a constant speed—if you could view those wavelengths with your eye they would look a little bit like a corkscrew. Circularly polarized light can move either clockwise or counterclockwise (the first is called right-handed polarization and the second is called left-handed polarization).
If you own a pair of polarizing sunglasses, you already know what a polarizing filter does for your lens—oriented correctly, a polarizing filter removes glare and reflection so that you can see through the surface of shiny transparent surfaces such as glass or water. For a simple demonstration, sit in your car wearing your polarized sunglasses, and notice what happens to the windshield when you remove them. Without the sunglasses, you probably see a lot of ugly glare, some of which can even obscure your view of the road. The same thing happens when you attach a polarizing filter to your camera’s lens.
V Logs by Flickr user `James Wheeler
Besides removing reflections from shiny surfaces, polarizing filters also do helpful things like deepening or saturating the colors in an image, especially the sky. When the filter is turned in such a way so that it blocks out the polarized light in the sky, it has the effect of deepening the blue. A polarizing filter can also cut back on haze because haze is really just scattered polarized light. And polarizing filters are great for capturing rainbows, too—if you’ve ever been disappointed by the dim-looking rainbows you’ve photographed in the past, try attaching a polarizing filter the next time you see a rainbow in the sky. The filter will help bring out those brilliant colors.
How to use a polarizing filter
Polarizing filters don’t work in every situation. You’ll get the strongest effect if the lens is oriented 90 degrees from the light source. This really just means that the sun should be on the left or right side rather than in front of you or behind you, but if you want precision you can make an “L” shape with your thumb and forefinger, and then point your thumb at the sun and follow your forefinger to the part of the sky where you’ll get the best effect from the polarizing filter. And remember that you have to twist the filter while looking through your viewfinder—when you see the sky darken or the reflections disappear, you’ll know where to stop and take the photo.
Gold Rush by Flickr user ...-Wink-...
Linear polarizing filters
Again, polarizing filters for your camera come in two different varieties—linear and circular. A linear polarizer selectively allows certain orientations of polarized light to pass through. You can turn a linear polarizer so that it allows only vertically polarized light though, or you can turn it another at 90° to allow only horizontally polarized light. Most light sources will put out a mix of all different types of polarized light, but it's the reflections and glare that respond to the use of a polarizing filter.
Surfaces such as your car’s windshield or water reflect linear polarized light more strongly than other types of polarized light. When you rotate your linear polarizing filter in such a way that it blocks the reflected linear light, you effectively remove the glare and reflection from the image. This can allow you to get images you wouldn’t be able to get without the filter—for example; you can shoot through the surface of the water, past the reflections, and capture detail in the rocks and other objects beneath the surface.
A circular polarizer blocks out circularly polarized light or allows it through depending on how it is oriented. But circular polarizers also cut back on linear polarized light, which is why they're still useful for photographing those rocks through the surface of the water.
Now, a circular polarizing filter isn't really its own thing—rather, it's a combination of a linear polarizer and a second element known as a "quarter wave plate." The quarter wave plate is attached to the back of the linear polarizer and oriented in such a way that the light that passes through is circularly polarized.
Okay, this is where it gets a bit complicated, so bear with me for a moment. The mirrors in modern DSLRs are partially reflective, which means that the light that gets sent to the camera's metering system is impacted by both intensity and the polarization angle. But the camera's sensor is only sensitive to the intensity of the light, not the polarization, which means that if you use a linear polarizing filter with a modern DSLR, you could end up with metering errors. And metering errors, of course, correspond to incorrectly exposed photographs. So to avoid this problem, modern photographers use circular polarizers—because the circularly polarized light that passes through the filter is reflected by the mirror with an intensity that is not affected by the angle of the filter. So with a circular polarizer, you don't have to worry about an incorrect exposure.
So besides the differences in exposure, what are the visual differences between what a linear polarizer does to a photograph and what a circular polarizer does? The answer is: there are no differences. A circular polarizer does exactly the same thing as a linear polarizer does provided it is adjusted to the most effective angle. Both filters are effectively the same thing, except that the circular version adds a process behind the scenes that helps the camera arrive at the correct exposure decision. The visual effect is exactly the same: reduced or eliminated reflections, a reduction in haze, richer, more saturated colors and a deeper blue in the sky.
Now, what happens if you do use a linear polarizer with your modern DSLR camera? You may end up with images that are up to a stop over or underexposed, which quite honestly isn't a disaster in the golden age of post-processing unless you're shooting in less-than-ideal light. But generally speaking you want to use the right equipment with your camera, and circular polarizers aren't prohibitively expensive, so you could save yourself some time and headaches by making sure you have one in your kit. The good news is that unless you're buying used, most polarizing filters sold today are of the circular variety, so if you're buying new, you don't have to worry too much about the variety you're going to end up purchasing. If you're using an older filter, you can check to see what type it is by standing in front of a bathroom mirror and holding it about three inches from your eye (in the same orientation you would use when attaching it to your lens), then flipping it so the opposite side is facing you. If it's a linear polarizer, you will see your eye reflected back to you in the mirror regardless of which side is facing you. If it's a circular polarizer, your eye will only appear on the naturally facing side—the opposite side will appear black.
If you've never owned a polarizing filter, you may be wondering why this information is important—after all, most of the photographic filters of the past have been replaced by post-processing techniques, so photographers don't need the same giant arsenal of filters that they once did. But the fact is that a polarizing filter is one of the few filters that you really do still need to have in your kit because the effects can’t be replicated in post-processing. In other words, you can’t delete the glare on the surface of a pond and see the rocks beneath the surface—you need to do that in camera, with a polarizing filter.
297:365 - rainbow by Flickr user cavale
Again, the difference between a linear polarizer and a circular polarizer is really just in the mechanics. You don’t really have to put this information to use unless you are using a DSLR and you don’t know what sort of polarizing filter you own. It’s good to make sure that you’re using a circular polarizing filter because it can save you some disappointment down the road, but the most important takeaway from all of this should be that if you don’t own a polarizing filter of any kind, you should at least put one on your wishlist. I think you’ll find that a polarizing filter opens up a whole new world of possibilities for shooting really remarkable images, and no photographer should be without one.
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