[rollei_list] Re: Rollei Retro Film

----- Original Message ----- 
From: "Jeff Kelley" <jeff@xxxxxxxxxxxxxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, February 25, 2008 10:17 PM
Subject: [rollei_list] Re: Rollei Retro Film


> Richard Knoppow wrote:
> Even though some modern B&W films have very hard
> emulsions not all do and there is still some advantage to
> the hardener in preventing damage to the film when its wet
> (the hardener has little or no effect on dry film).
>
>
> Richard, is it true that staining developers like Pyro 
> harden the emulsion &
> therefore eliminate the need for adding a hardener to the 
> fix or stop bath?
>
> Jeff
>
Pyro and some other developers, hydroquionone under some 
conditions for instance, cause differential hardening of the 
emulsion. While the same reaction products that cause 
tanning are not the ones that cause the stain image they are 
similar in that the effects are proportional to the image 
density. Pyro developers leave a sort of relief image which 
can be seen when the negatives are viewed by reflected 
light. When used on unhardened gelatin Pyro developer can be 
used to form a matrix image of the sort used for dye 
transfer work although I think the developer supplied by 
Kodak for dye transfer used a different developer.
    Because Pyro has relatively little effect on those parts 
of the emulsion bearing a low density image it probably does 
not protect them from wet abrasion.
    BTW, a side effect of the differential hardening is a 
small change of the index of refraction of the gelatin. This 
can produce a small acutance effect at the borders of high 
and low density areas. There is also a variation in the 
swelling of the emulsion which can cause a further acutance 
effect due to a variation in developer penetration, plus 
there can be some slight geometrical distortion of the image 
due to the variation in shrinkage. For the last reason Pyro 
developers designed for aerial photography are usually of 
the non-staining type. Note that Pyro in the presense of 
enough sulfite will not generate enough of the reaction 
products of development to cause either staining or tanning. 
While both effects are considered desirable now they were 
not at one time and a considerable amount of work was done 
in devising non-staining Pyro formulas especially for motion 
picture use where the stain could be misleading in judging 
the contrast of negatives (development by inspection was the 
rule in the silent period) plus the exagerated edge effects 
caused by the tanning could result in quite noticable "edge 
crawl" on prints especially if a duplicate negative was 
used. All ancient history.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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