Re: Noncoated filter

[aglang@sprynet.com]

Interesting.
I have an old Summitar (SM 5cm F2) which is coated on each surface EXCEPT the 
front. It is sufficiently late to have been factory coated. I guess the front 
coating got damaged and was ground off. Lens is still quite good (except very 
stiff aperture ring) and I had assumed that the only real detriment was a loss 
of effective spped due to reflection back into the environment from the front 
element (as opposed to an internal uncoated surface creating more reflections 
bouncing around and ultimately decreasing contrast on the film). 

On Wed, 11 Jun 1997, Jim Brick <jim@brick.org> wrote:
>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
>