[rollei_list] Re: Rollei neg scan practical comparison


On 23 Mar 2011, at 17:47, CarlosMFreaza wrote:

Hi John:
            It could be an explanation considering Newton rings are
formed due to light rays reflections-refractions between a flat
surface and a lens (the neg or part of it would be the lens) through
an air layer; where the material working as a lens and the flat
surface have contact the rings appear, perhaps the silver (neg with
right exposure) contributes to diminish the film surface acting like a
lens and this way diminishing risks to form Newton rings, I think the
film base reflectivity has to do too.

Newton rings are a very secondary problem to me since I rarely scan on
the glass directly, but I'm giving practical samples on this issue, in
the other hand the photograph about the teen fishing that appears
first in my Flickr gallery was also scanned on the glass directly,but
it did not get a minimal Newton ring, that is a neg with a right
exposure.-

Carlos

Interference effects, as seen in the scans of the negatives, can occur whenever there are multiple possible paths for the light to travel. In this case, we are combining light reflected from front surface of the glass followed by the undersurface of the negative with light which has passed directly through the both the negative and the glass.

Because the light is poorly correlated the path difference has to be small for these effects to be seen. Placing the negative surface directly on the glass allows such small path differences to occur.

Also the surfaces have to be smooth, as the light and dark bands seen correspond to path differences of half the wavelength of the light concerned. If the surfaces are rough on a microscopic scale, as I would expect occurs when lots of developed silver grains were present, then the local variation in path length as one moves laterally across the negative will make the effect invisible.

The ideal solution is to support the negative as a flat surface above, and not in to close ( <0.5mm ) proximity to the glass. A good negative holder will do this. The negative holders that were supplied with my epson scanners don't do this well. The film always seems to be curved to some extent.

A alternative is to reduce the intensity of the reflected image. This is done by putting a fluid whose refractive index is close to that of film into the gap between the film and the glass. This can be done, but appears to be both time consuming and messy. The results are good however.

One final comment. The light source in many scanners seems to be pseudo white light made up of an number of narrow close to monochromatic bands, this is the nature of the fluorescent sources used in many scanners. For reasons which are probably to tedious to go into here, this makes such a source more susceptible to such interference effects. If a manufacturer made a scanner which used an incandescent light source, this might be less prone to this problem.
All the best
Laurence Cuffe



2011/3/23 John Wild <JWild@xxxxxxxxxxxxxxxxxxxxxx>:
Carlos, just a thought...

If Newton rings are more of an issue with underexposed negatives, could it
be that these negatives, having less silver are either less rigid or
alternatively the base, having no silver in larger areas, becomes prone to sticking to the glass. If this is the case, the silver may act as a barrier
to the formation of Newton rings. Different film bases may behave
differently though...

I will have a look at some of my glass mounted transparencies to check how
they behave.

John
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