Re: filmscanners: Scratched Negs & Home C-41 processing

[Photoscientia <photoscientia@photoscientia.co.uk>]

Hi Hart.

> ......Collimated light, such a from a point source and then run through an
> enlarger condenser increases the contrast ............

I'm pretty sure that the contrast isn't increased with colour materials, for the
following reason:

The contrast increase in B&W printing is due to the so-called Callier effect, 
and
is caused by the diffusion properties of the negative increasing with the silver
density of the image.
Light through the clear film base is hardly diffused at all, and most of it goes
on to hit the enlarging lens, but in denser parts of the negative, some light is
scattered by the silver, misses the entry pupil of the lens, and so can't
contribute to the printed (or scanned) image.
That's how the contrast is increased, by the varying Callier co-efficient
(co-efficient of light diffusion) of a B&W negative.
However, the situation is different with colour materials, since they don't
contain any silver.
The dyes that make up colour images don't impart much turbidity to the negative
or slide, and the light diffusion is much more constant across the density 
range.
In any case, the mask of colour negative film means that the dye density is
fairly heavy, even at it's most transparent.
The Callier effect is therefore almost non-existent with colour materials, and
the contrast varies very little with collimation of the light source.

Of course, light scattering from a scratch will still occur with either a dye or
a silver image, and a collimated light source will definitely increase the
visibility of surface defects.



> but also increases the visibility
> of any negative defects.  Collimated light essentially is where all the
> light rays are parallel to each other.  Diffuse light (different than a
> diffused image) was preferred, particularly from a so called "cold light"
> source which is what I have had in my Beseler enlarger for many years.
> [Something more learned from Ansel Adams, who printed with a cold light].
> Even Adam's negs were not perfect, by any means! In later years, he had an
> assistant whose job was to retouch his prints before they were matted and
> framed.  He *never* exhibited an unretouched print.
>
> Hart Corbett
>
> ----------
> >From: Roman Kielich® <panromek@bigpond.com>
> >To: filmscanners@halftone.co.uk
> >Subject: RE: filmscanners: Scratched Negs & Home C-41 processing
> >Date: Wed, Jan 24, 2001, 2:13 AM
> >
>
> > At 07:18 23/01/2001 -0800, you wrote:
> >
> >>         In one case, I picked up some negatives which demonstrated a very
> >>long scratch across several frames which didn't show up in the prints
> >>(which I use as pseudo-proofs).  The significance of the scratch was
> >>it should have showed in the prints, and my conclusion was the scratch
> >>occurred during the printing process.  One method of avoiding this is
> >>to use a service which puts the sticky laminate on the film after
> >>processing, but before printing.  However, the laminate has it own
> >>downside ... primarily being a hassle to remove before scanning.
> >>
> >>shAf  :o)
> >
> > if they accepted your argument, they deserve to be punished. Most printers
> > use diffused light which "masks" fine scratches. The same neg printed with
> > a point light source would look terrible. Saying that, I'd rather look for
> > scratches which are not parallel to the edge. It is definite sign of a film
> > abuse.
> >
> >
> > "Don't worry about the world coming to an end today. It's already tomorrow
> > in Australia".
> >
> >