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 Author
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Topic: Circular polorizing filter questions (Read 20996 times)
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2_Rastas
Newbie
Posts: 11
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Hello all, I have a two part question. First off, I'm using a Canon 20D witha Canon 28-135mm lens. I want to get a circular polorizing filter to cut down on glare from water etc and make my colors more vivid as I'm going to be using in on a vacation in St. Lucia. I'm told that by using this lens I can shoot at slower shutter speeds to enhance shots of waterfalls. How does this work?
Also, I'm told that the filter works best at 90 degrees from the sun. So then as I understand it the sun should always be shining at me from either hard left or right? Shooting across it so to speak?
Thanks in advance for any help you may be able to furnish
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KeithB
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"I'm told that by using this lens I can shoot at slower shutter speeds to enhance shots of waterfalls. How does this work?"
Simple, by cutting down on the light that gets into the camera. A polarizer only transmits about half the light, but in a very neutral way, it affects all colors equally. In other words, it acts like a neutral density filter. By cutting down on the light, you need to decrease the shutter speed in order to keep the same exposure and aperture.
It is not that great of a neutral density filter. As I said, you tend to lose only about 1 - 2 stops of light. Kodachrome 25 would probably work better, 8^).
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2_Rastas
Newbie
Posts: 11
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How do you know how much slower to go then?
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Bob Atkins
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The camera metering will take care of exposure. You'll typically lose about 1.3 stops of shutter speed with the polarizer attached, which means, for example, that if you get a shutter speed of 1/60s without the polarizer, you'll get a shutter speed somewhere around 1/20s - 1/25s with the polarizer attached. You don't have to do the math, the camera will do it all for you! The "90 degree from the sun" rule really applies to the degree of polarization of the sky. It doesn't apply to reflections from water, leaves etc. You should look up "Brewser's Angle" for a complete explanation (see for example http://en.wikipedia.org/wiki/Brewster_angle) |
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« Last Edit: November 04, 2008, 03:37:01 PM by Bob Atkins »
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2_Rastas
Newbie
Posts: 11
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OK, I get that the camera will automaticaly compensate for the correct speed. I'm curious tho, I read about that wikipedia explanation of polorizing filters, the filter I would be using is a circular type so I wouldn't be able to rotate for different types of relections.
Am I still doing the right thing here? Is this still the right way to go I mean?
Also, the filter I'm considering is a B&W Kaesemann circular polorizing filter. Is this a right way to go or is the quality not all that noticable?
The Kaesemann is 125.00 and a tiffen circular plorizing filter is around 55.00 is the Kaesemann worth the extra cost? Would it be noticable?
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Bob Atkins
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Kaesemann polarizers are fully environmentally sealed, which means they will last longer under conditions of high temperature and humidity. Strictly from an optical point of view thet are just the same as any other quality polarizing filter. As far as you and the image is concerned, it makes no difference whether a polarizer is linear or circular. You get exactly the same effect. Only the internal camera optics care. See http://www.bobatkins.com/photography/technical/polarizers.html |
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2_Rastas
Newbie
Posts: 11
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Thanks guys.....youve been most helpfull and patient. It would appear the less costly filter is the way to go for me then.
One last question...would you recomend using it as a lens cover, that is to say leaving it on all the time instead of say a UV filter?
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KeithB
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No, that extra stop of lost light might be needed for night shots! The polarizer has too much of an effect to leave it on the lens all the time.
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2_Rastas
Newbie
Posts: 11
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Well, kinda.
I want to get some pics of the waterfalls, but not too much slower like Bob's pics. Maybe just a little faster shutter speed but I'm really a beginer. The waterfalls are rather tall with small collecting pools at the bottom. Maybe just slow enough to get a fuzzy look to the pool but still be able to see the cascading effect of the fall. Does that seem reasonable?
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KeithB
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The nice thing about digital is that you can put the camera in Tv mode, set a shutter speed and experiment. You will probably need a tripod, though.
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2_Rastas
Newbie
Posts: 11
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Yes, I plan on taking a tripod even though the lens is image stabilized.
Is it a good idea to keep a UV filter on for protection, and when I want to use the circular polorizing filter snap it on OVER the UV filter. So that you NEVER take the UV filter off?
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Bob Atkins
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Using multiple filters is generally a bad idea. It can cause vignetting (dark corers) with many wideangle lenses and you run a greater chance of flare.
I'd only use one filter at once unless there's some real need to use more.
Some people keep a UV filter on their lens all the time to "protect" it, some don't. I'm one of the ones who doesn't!
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2_Rastas
Newbie
Posts: 11
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Thanks so much for helping me out with this.
I feel like I've learned so much on here.
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