"Draco" <JPDFDA@[EMAIL PROTECTED]
> wrote in message
news:55d4a795-b00e-45a3-b78c-ff28e63fa80f@[EMAIL PROTECTED]
Mar 16, 5:09 pm, <jjs> wrote:
> The Focomat IIa uses AN glass for the top glass, and plain
> for the lower.
>
> Is this a universal principle? IOW, can I presume the same
> for any two-glass
> carrier?
Yes, as long as it is an anti-newton negative carrier. Plain
glass
carriers will have plain glass on both sides. This would be
the time
you would get the "newton rings" on your print around the
highlights.
Right?
Draco
Newton's rings are an interference pattern where two
beams of light can interfer. The usual cause is reflection
between two reflective surfaces that are very closely
spaced. In the case of an enlarger using a glass sandwich
negative holder the two surfaces are the sup****t side of the
film and the glass next to it. By making the glass slightly
rough the reflection is broken up and the rings are not
formed. Newton's rings can be formed on the emulsion side
too but the emulsion is usually rough enough, or the amount
of reflection from the gelatin low enough, so that rings are
not produced there.
Newton's rings appear as somewhat uneven dark and light
concentric rings all over the image. They follow
approximately any lack of flatness in the two surfaces
involved.
Newton's rings are often used to measure the degree of
conformance of two closely spaced surfaces where one of them
is transparent as when testing the figure of a lens against
a standard.
I think the effect you are referring to is halation.
This is also caused by reflection, usually in film, where
bright areas are reflected back into the emulsion by the
sup****t. All modern film has either a light absorptive
coating on the back or in a layer under the emulsion (the
usual method for color film) to eliminate halation. Halation
was a very serious problem with early glass plates where it
formed quite noticeable halows around bright objects and
reduced the contrast of more extended bright objects. Glass
plates were also coated with anti-reflective layers on the
back. It is usual for the anti-halation layer to be removed
mechanically (as in the Remjet used for Kodachrome and some
motion picture stocks) or for the dyes it contains to be
made colorless by the sulfite in the developer or fixing
bath.
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Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
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