How a polarizer filter works
The two best things about summer are clear blue skies and beautiful sparkling oceans as far as the eye can see. Unfortunately, these are also two of the most reflective things out there.
If you’re not careful, you can wind up with summer photos that have big ugly reflections and white, blown-out skies. How can you fix it? A polarizing filter for your camera, properly used, can help enhance the deep blue colour of the sky. It will also help you minimize harsh reflections from water, making your summer photos look gorgeous!
What is a Polarizing Filter?
A ‘polarizing filter’ or polarizer is a filter for your camera that controls how much polarized light you allow to enter the lens. There are two types of polarizing filter: linear polarizers and circular polarizers.
Originally photographers used linear polarizers, which blocked the light with a series of horizontal openings like venetian blinds. Eventually, advances in the way auto-focus mechanisms worked rendered linear polarizers useless. Circular polarizers were developed, designed to work with newer auto-focusing systems. They also allow you to adjust the effect by turning the ring around the filter.
How Polarizers Work
Light coming from a reflected surface is all the same wavelength. This allows the filter to eliminate reflected light on that specific wavelength, making the reflections fainter. This is useful if you’re taking a photo of someone wearing sunglasses or landscape photos of the ocean.
You can also apply that same effect to the sky, but it works a bit differently. Since the light is reflecting off all the moisture in the stratosphere, you make the sky appear a deeper blue. It’ll also increase your contrast between the clouds and the sky. Polarizing filters are the filters of choice for landscape photographers.
Disadvantages of Polarizing Filters
Polarizing filters can get expensive. Where your average UV filter runs from $10-20, most polarizing filters start at $60 and go up to $150. A trick to avoid buying polarizers for each of your lenses is to buy one polarizer for the diameter of your widest lens and use cheap step-up rings for smaller lenses.
Another disadvantage is that the filters are quite dark. They will force your exposure down at least one stop, making it harder for you to use a high shutter speed. This is the biggest argument against leaving them on your camera all the time.
Polarizing filters an essential tool in the landscape photographer’s toolkit. Photographers of all skill levels and fields find them useful at one time or another. In the end, it’s up to the individual photographer to decide if they want one. Just keep in mind that good use of a polarizing filter can make your summer photos really pop!
Credits
This article was written by my good friend Andrew Ferguson, who runs the Golden God blog, which is full of fabulous articles - much like Photocritic, in fact, and well worth a slot in your RSS reader. Fancy writing a guest article for Photocritic? Drop me an e-mail with an article idea!
The photo of the building is © Andrew Ferguson. The 4-up comparison of with-and-without polariser images is under creative commons, and was done by Flickr user Higashitori. All non-marked photos are © Haje Jan Kamps / Photocritic.org
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This entry is © Haje Jan Kamps / Photocritic.org, and was posted on Friday, July 6th, 2007. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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#1 - July 6th, 2007 at 21:20
“They also allow you to adjust the effect by turning the ring around the filter.”
Linear polarizers did this too, this isn’t limited to circular polarizers.
From what I understand, the “circular” refers not to the shape, or orientation of the waves, but to a little circular tweak the light gets on the way out of the filter on the way to the lens. I’ll return to this in a second, but first, the other problem with your explanation…
“Light coming from a reflected surface is all the same wavelength. This allows the filter to eliminate reflected light on that specific wavelength, making the reflections fainter. ”
No. Light which is all the same wavelength is all the same colour. What you mean is that “light coming from a reflected surface is all the same orientation.” Consider ripples of water on a pond: the water goes up and down, the distance between two “ups” is the wavelength. Tighter ripples are on blue, longer more relaxed ripples are red, intermediate length ripples are orange, yellow, or green. Now, as light waves travel through the air, they aren’t restricted to up/down movement like the surface of a pond; light waves can oscillate left to right, or at any other angle.
What a (linear) polarizing filter does is only let through light vibrating in one direction. As you turn it, you can select which direction that is. All wavelengths get through, but only if it is oriented correctly. Most light isn’t oriented correctly and is mostly filtered out, resulting in about a two-stop loss of light gettting through.
So, light (of all kinds of wavelengths) is travelling through the air. Some of it hits a surface, like a pond, and bounces back towards you. The light that bounces back will tend to be oriented in a particular direction (lets say, for example, only the left/right vibrating light gets reflected back). Light vibrating along a particular axis is called polarized light. Other light will enter the pond, get reflected around under the water and come back out in a basically random orientation.
If you set the filter so that it allows through the left/right polarized light is allowed through, the surface of the pond appears very reflective. This is because (most) of what the filter allows through is what has bounced off the surface. On the other hand, you could turn the filter ninety degrees, and none of the polarized light would get through. This would eliminate surface reflections on the pond; all that would be getting through would be (a little of) the randomly oriented light from under the water. Turning the filter to an intermediate position gives an intermediate result.
The same kind of idea applies to reflective buildings, tree leaves, and clouds — all of which can be enhanced using a polarizing filter. Note for digital photographers: unlike a skylight or tungsten balancing filter or any filter that changes the colour of the image, a polarizing filter is one effect that can’t be duplicated in an image editing program.
Now, back to the circular polarizing filter. With a linear polarizing filter, the light coming out of the filter passing to the lens is all oriented in a particular direction. Unfortunately, the autofocus systems of your SLR uses light in different orientations to detect whether the subject is in focus or not. This means that you can’t autofocus with a linear polarizing filter on you camera. To get around this, another layer is added to the filter (between the camera and the polarizing layer) which gives the light a little twist as it exits. Light leaving the circular polarizer will then have an assortment of orientations, and the autofocus system works. (Note that this doesn’t defeat the purpose of the filter because it has already eliminated light with an incorrect orientation, and the orientation of light hitting the film/sensor doesn’t matter.)
Wow. This is longer than I intended. It’s also longer than your article. Oh well, enjoy.
E.
#2 - July 6th, 2007 at 21:48
great read!
#3 - July 6th, 2007 at 23:10
“Light coming from a reflected surface is all the same wavelength.”
No, same wavelength would mean it all had the same color, which it doesn’t. Reflected light does have roughly the same *polarization*, though. (See: http://en.wikipedia.org/wiki/Polarization )
#4 - July 7th, 2007 at 07:44
Thanks for the article, but you got the physics wrong!
Polarization of light has nothing to do with wavelength. If light reflected off a surface was all the same wavelength it would appear as a single rainbow color, not white or any other color.
Polarized light is light where the electromagnetic waves that make it up are oscillating (mostly) along a single plane. A light ray which undergoes reflection on a non-metallic surface becomes typically polarized along a plane parallel to the reflecting surface.
A polarizing filter is coated with a substance which filters out most waves that are not along a single plane of oscillation. By rotating the plane of the filter so that it is perpendicular to the plane of polarization of reflected light, you effectively block most of the latter.
#5 - July 7th, 2007 at 08:20
“How Polarizers Work
Light coming from a reflected surface is all the same wavelength. This allows the filter to eliminate reflected light on that specific wavelength, making the reflections fainter.”
This is not true. Polarization has nothing to do with wavelength. Please read this article and correct your own text:
http://en.wikipedia.org/wiki/Polarizer
#6 - July 7th, 2007 at 15:38
“Light coming from a reflected surface is all the same wavelength.”
This is not in fact correct, when light is reflected off a surface, it is actually all polarised in the same direction. Wavelength determines the colour of the light, polarisation is the direction in which light waves are “vibrating”. The concept is rather hard to explain without diagrams but a quick search on wikipedia should explain polarisation.
#7 - July 7th, 2007 at 16:23
This article makes my decision of buying a polarizing filter really easy! Great
#8 - July 7th, 2007 at 18:37
Great article!
Article like this are sooooooooooooooooooooooooo tasty.
#9 - July 9th, 2007 at 00:24
I love polarizers, not only for their ability to mute reflections but the saturation they can add. The loss of light is indeed a huge detraction in low light situations unfortunately. I still will leave it on almost all of the time, I just remember to carry a filter case in my bags.
#10 - July 9th, 2007 at 07:48
Very useful article!!!
Can I translate it to the russian for readers of the my blog?
#11 - July 11th, 2007 at 18:35
@Silverhalide, tcliu, Ugo Cei, observer, James Thomas:
Yikes, how’d I let that get through? I’m really sorry! I intended the article to be less-technically focused and more of a focus on why they’re useful, but not to the point that I was disseminating incorrect information. I screwed up.
Thank you all very much for giving everyone the correct information!
@Dumitru, Dennis: Thank you, I’m glad you liked it!
@Alexander: Sure! You have my permission to reproduce it, in Russian, for a non-commercial blog. You have to maintain the links back to my site (goldengod.net) and author/photo credits though. :)
#12 - November 14th, 2007 at 00:52
I have had some experience with polarized filters.I still expect to have more skills with this technique of photo.
Yours sincerely
Nguyen Anh Dung
#13 - December 26th, 2007 at 22:43
is there a way to fake polarization in Photoshop?
#14 - July 27th, 2008 at 01:43
This is NOT an article on “how they work”, it’s an article on what effect they have.