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[OM] Re: Protective Filters

Subject: [OM] Re: Protective Filters
From: "Walters, Martin" <mwalters@xxxxxxxxxxx>
Date: Tue, 27 Jun 2006 12:04:22 -0400
The article refers to using filters on digital. Rightly or wrongly, I have 
always used either UV or Skylight filters on my lenses for film. When I was 
"retiring" my 28/3.5 I was surprised to see all the small scratches on the 
filter, while the lens element itself was pristine. Now, I pretty much always 
used the metal hood with this lens. The hood is largely unmarked also, so a 
hood may not be everything.

Also on the filter issue, there was an interesting thread on the 4/3 forum 
concerning a test of a Tamron 300/2.8 and the (beneficial) effect on the lens' 
performance wide open with the big Tamron filter mounted. The thread discusses 
why this may happen, and why it appears to most noticeable with large 
apertures. Now, I'm no expert, but the thread seemed reasonable to me. Any 
other views? The thread is: 
http://www.fourthirdsphoto.com/vbb/showthread.php?t=643


Martin 

-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx]
Sent: June 25, 2006 8:23
To: olympus@xxxxxxxxxx
Subject: [OM] Re: Protective Filters


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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