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[OM] Re: Protective Filters

Subject: [OM] Re: Protective Filters
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Sun, 25 Jun 2006 08:22:57 -0400
I hear you and I don't use "protective" filters myself; gave that up 
long ago for decent lens hoods.  However, something doesn't seem to 
compute.  I would assume (I know, assumptions are dangerous) that
1) multicoated surfaces pass 99.5% of the light and reflect only 0.5%
2) the low pass filter on the sensor is multicoated
3) the backside of the UV or other filter is multicoated

If all three are true then only 0.5% can be reflected back to the front 
filter, 5-15% of that will be lost getting back to the front of the 
lens, and 99.5% of that will pass out through the filter and only 0.5% 
of that will get reflected back to the sensor and 5-15% of that will be 
lost again trying to pass back through the lens.  Seems to leave only a 
teeny-tiny amount of light to cause any mischief.

So what's wrong with this analysis?  I'm sure that assumption #1 is 
pretty close to correct.  If anything it may be 0.1% reflectivity rather 
than 0.5%.  Are #2 and #3 simply bad assumptions?

Chuck Norcutt


Wayne Culberson wrote:

> 
> 
> 
>>The very best demolishing of the protective filter myth even not
>>taking into account the tests at Gary Reese's site.
>>
>>http://forums.dpreview.com/forums/read.asp?forum=1021&message=18960184
>>
>>
>>
>>Winsor
>>Long Beach, California, USA
>>
> 
> 
> Perhaps that explains why I had so many low contrast/flare pictures with the 
> C5050 on the last trip to Bolivia. It was the first time I'd used a filter 
> on the digital down there, an 81a of course. So it looks like there is no 
> winning with a digital camera at high altitudes.
> Wayne 
> 
> 
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