Re: filmscanners: Re: filmscanners: RE: filmscanners: Re: Rescans and archiving

[Arthur Entlich <artistic@ampsc.com>]

Keep in mind that the dyes in E-6 based films are not colored when they 
are coated on the film.  They have to be transparent and very low 
density when they are in an unprocessed form, so the light can go 
through them.  As it is, the silver halide components block enough 
light, causing each subsequent layer to have to be much more sensitive 
than the previous layer.  So a 100 ASA film, might have 100 ASA 
sensitivity silver halide for the yellow layer, but the magenta layer 
under it might have to be 400 ASA and the cyan layer under than 1000 ASA.

Anyway, the E-6 process allows for the dyes to form around the silver 
granules and become colored as a chemical reaction, and the silver 
granules are then bleached out.  That's asking a lot... getting exact 
color response, exact color, etc, and then to produce a dye that is 
light and age stable.

Kodachrome resolved that problem by not having to produce the color from 
within the film layers.  The dyes don't even have to be organic; they 
are produced as separate chemical entities external of the film and 
added after the fact.  That allows for a lot more flexibility in their 
composition.

Art

Rob Geraghty wrote:

> Harvey wrote:
> 
>>Don't know about the Velvia, but it is my understanding that Kodachrome's
>>
> stability is due to the fact that it
> 
>>starts as a b&w silver based film and the color is added during the
>>
> processing, and is not incorporated in the
> 
>>original film (i.e. not color coupled).
>>
> 
> I've heard that too, although I don't understand what difference it makes
> when the dyes are added!
> (but I can see that it does)
> 
> Rob
> 
> 
> .
> 
>