[pure-silver] Re: donut solution; enlarger lens problem
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 11 May 2007 08:47:09 -0700
----- Original Message -----
From: "Shannon Stoney" <shannonstoney@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, May 11, 2007 8:30 AM
Subject: [pure-silver] Re: donut solution; enlarger lens
problem
This one is flat. The one I use for the 135mm enlarging
lens is a
recessed cone. I was told by the Devere people that I had to
have the
flat lens board for the 300 mm lens. But maybe they were
wrong. I will
try your suggestions when I get back to Houston. Maybe the
lens would
work with the recessed cone lens board; however, I have a
feeling that
the hole won't fit both lenses. The 300mm lens is huge.
This enlarger (the Devere 108) has a diffusion head.
--shannon
I suspect the problem is not the lens board. The
inverted cone is used for short focal length lenses because
the bellows do not collapse enough to focus it. Many larger
size enlargers use inverted cone mounts for short lenses.
Assuming that the negative can be focused with either
lensboard the position of the lens with regard to the film
will not be affected by the mount.
The doughnut may have been Newton's rings. The rings are
caused by reflection between two surfaces which are very
close together. The number and size of the rings depends on
the distance and the shape by flatness of the surfaces. The
two surfaces must be either exactly flat or have exactly the
same curvature for the rings to be perfectly round. By
making one of the surfaces non-reflecting the rings can be
eliminated. In the case of a glass sandwich negative holder
there are two pairs of surfaces which can cause rings. For
many films the emulsion side is slightly matt so is less
likely to form rings than the support side but some modern
films have emulsions which are quite smooth and reflective.
When "anti-newton" glass is used the rough surface is
normally in contact with the support side of the film. Most
of the Newton's rings problems I've seen show multiple rings
but this doesn't eliminate a couple of large rings. Since
changing the negative holder cured the problem it does seem
to be a Newton's rings problem.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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