Archived posting to the Leica Users Group, 1997/06/11

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Subject: Re: Noncoated filter
From: Jim Brick <jim@brick.org>
Date: Wed, 11 Jun 1997 23:49:36 -0700

At 10:42 PM 6/11/97 -0700, you wrote:
>> >uncoated. Is it less important for a filter to
>> >be coated than for a lens? 
>> 
>> Take it back. It will be the first surface to greet the incoming light
>> rays. If it's uncoated, a lot of scattering, reflecting, and other ugly
>> phenomenon will take place. 
>> 
>It seems to me that any light reflected from the first surface of the
>filter will simply be reflected back to the subject and therefore not
>degrade the picture. I can understand why the surface of the filter closest
>to the lens benefits from coating. Am I missing something? 
>

Yes, you are indeed missing something. The first surface is indeed the most
important. I quote from Leica and B+W... "lens glass reflects about 4% of
the incident light on each surface. It mirrors! This reduces light
transmission, increases stray light flare, which superimposes itself on the
true image, reducing contrast. In unfavorable situations (high contrast
subject) this may cause double images of the bright areas. Surface coatings
significantly reduce these damaging effects. This involves application of a
layer of a special coating, such as magnesium fluoride, in a thickness of
about one ten-thousandth of a millimeter on every glass-to-air surface. A
single coating reduces the reflection on glass-to-air surfaces to 1.5%.
Multicoating can reduce the glass-to-air reflections to 0.2%." Modern Leica
lenses have a transmission of 99.8% A single uncoated filter will totally
destroy this. If you use a filter, use the very best you can get.
Multicoated if possible. There is no reason to purchase exquisite Leica
lenses if they are going to be crippled by a layer of window glass.

Jim