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[OM] Re: Protective Filters

Subject: [OM] Re: Protective Filters
From: Winsor Crosby <wincros@xxxxxxxxxxx>
Date: Sun, 25 Jun 2006 13:01:41 -0700
This list went though a period of deciding whether a lens for single  
or multicoated by holding a lens up to a lamp and looking at the  
reflections from the "non-reflective" surfaces.  If you can see it a  
sensor can capture it.



Winsor
Long Beach, California, USA




On Jun 25, 2006, at 5:22 AM, Chuck Norcutt wrote:

> 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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