[pure-silver] Re: Weak Blacks

Kodak's under the lens Polycontrast filters get cloudy.  I can't speak to 
fading but the cloudiness will affect sharpness.  My set started to go south 
recently and I replaced them w/a set of Ilford PC filters I had lying around 
that I'd never used.  
Those filters cut off more light and seem to have more contrast difference 
between the 1/2 grades than the Kodak ones.  I'm just going to get another 
Kodak set 'cause I don't find the Ilfords as easy to work with.  Hope Kodak 
doesn't stop making those too!
Eric




________________________________
From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
To: pure-silver@xxxxxxxxxxxxx
Sent: Wednesday, January 28, 2009 8:39:54 PM
Subject: [pure-silver] Re: Weak Blacks
 
    If the dichroic filters are made by vacuum depositing a metalic oxide on 
the glass they should not fade, The layer is similar to a lens coating. Gelatin 
filters almost invariably use dyes of some variety. Dyes are all fugitive to 
some extent, some being much more stable than others. Kodak used to indicate 
the stability of the dyes it used in Wratten filters but I don't think any 
variable contrast filter make does so. Some ilford filters are made of a 
plastic rather than gelatin and some are waterproof. I am not sure which. 
Gelatin filters have an advantage when they are used in the image path but not 
when used in the light path. Filters used between lens and image or between 
lens and negative should be as thin and as high quality as possible. Filters 
used in the light path are much less critical of requirments.
    Variable contrast filter sets _should_ be replaced periodically because 
they do change transmission characteristics. The difference may not be visible 
by eye but can still affect the contrast and exposure produced with the filter.
    Some early color head lamp-houses, notably the Omega, used filters colored 
with dyes rather than dichroic filters. These filters are subject to intense 
light and are very short lived: they should be replaced frequently. AFAIK, 
metal film dichroic filters should be very long lived.

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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