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Old Jul 31, 2004, 3:43 PM   #1
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I see a lot of people use UV filters to protect their lenses from dust, but what exactly is their function?

I also would like to know what type of polarizer is the best to use, and does it knock your f-stop down?

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Roy
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Old Jul 31, 2004, 4:53 PM   #2
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Polarizing filters usually have a filter factor between 2.3 to 2.8 (approx 1.3+ stops). This varies depending upon how much light is blocked as you turn the filter. With TTL metering, you will not have to compensate for this reduced light affecting your exposure. Linear polarizing filters work fine on older cameras. However, with most modern cameras, you will need a circular polarizing filter. This circular polarizer has a built in wave retarder, thus making it compatible to many modern built in TTL exposure systems. If you use an older linear polarizer with a new camera, then you will run the risk of either having your exposure off, and/or a difficulty in using auto-focus depending upon how the camera infrastructure works. You see, polarizers are designed to block wave lengths of light in one direction. Thus, a linear polarizer is more effective then a circular polarizer due to the 1/4 wavelength retarder included with the circular polarizer. However, most modern cameras use a semi porous mirror that directs a stream of light to the exposure light sensor and theauto-focus sensor(s). A linear polarizer is too effective, decreasing or eliminating the light to the exposure meter.

UV filters are designed to remove the blue cast seen in photographs at high altitude, as UV increases the higher you go. Thus if your at sea level, UV filters won't do very much for you.

However, with digital cameras, there is already a built in UV filter, as Photo-sensors tend to be more susceptible to UV radiation. Thus, many camera makers already have a built in UV filter to eliminate this problem to UV sensitivity. This makes digital cameras appear to be less susceptible to UV radiation, thus making a screw on UV filter almost redundant for digital cameras.

With this said, UV filters are still popular because many UV filters out there do not block all the UV radiation, they only reduce it. Furthermore, because there is no filter factor, they make useful lens protectors as they do not affect the exposure. The flip side of this is filter build quality. It is possible to put a cheap filter onto a lens, and this will degrade the quality of your work. Also, an increase in light flares may occur with an external UV filter, causing the digital camera to blow out some highlights and lose information in those areas.
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Old Aug 3, 2004, 6:29 PM   #3
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There is more talking on Polarizers on

Steves Forums > Digicam Help > Misc Accessories > Circular or Linear Polarising filter, or more simply:
http://www.stevesforums.com/forums/v...mp;forum_id=67
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Old Aug 4, 2004, 3:13 AM   #4
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It's not just to protect the lens, but from damage as well (like scratches)...it's a lot cheaper to replace a filter than to replace a lens. Also on cameras with retracting lenses the filter and filter-tube protects the lens barrel from being bumped out of alignment.

A linear polarizer will work fine on most non-dSLR cameras (if you mentioned which camera, we can say for sure). Now if you plan to upgrade to a dSLR in future you may want to get a cicular polarizer now, but then again your dSLR may not have the same diameter filter thread. Here's a site on polarizers:
http://www.cs.mtu.edu/~shene/DigiCam...polarizer.html
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Old Aug 4, 2004, 8:53 PM   #5
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Quote:
Also on cameras with retracting lenses the filter and filter-tube protects the lens barrel from being bumped out of alignment.
...and if you have an Oly Cx series (C3020 like me), check into this site:

http://dpfwiw.com/c-2000z/lens_cap/index.htm
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Old Aug 5, 2004, 3:25 AM   #6
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What Circular Polariser and UV filter do you suggest for use on a Canon Pro 1? For someone living in a sunshine state/country like Spain or Portugal?
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Old Aug 5, 2004, 6:38 AM   #7
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Most of them are good quality. However, many local pros in my area seem to be willing to pay a little extra for B+W filters. I have heard some bad things about Hoya filters as well.
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