[Leica] Re: Re: Pre Wash??????

[George Huczek <ghuczek@sk.sympatico.ca>]

At 05:12 AM 05/11/98 EST, you wrote:
>Very interesting George.
>
>I am sure you are correct as I have not tried pre-washing yet.  I would have
>assumed that pre-washing would have done just the reverse of what you state,
>allowing the developer to act faster vs. slower.  What has been your
>experience in the development time-reduction, say at 72 degrees(%)?
>
It's quite a common belief that prewashing requires a decrease in
development time.  Quite the contrary.  
   As a starting point, try a 10% increase in development time with
prewash.  With TPX (120) film, this is what I determined was needed with
prewashing.  I no longer follow this prewashing regimen with B&W films, for
reasons explained below.  You have to do your own testing, find what works
best for you, then standardize your results and remain consistent.  Be
careful about relying on anyone else's recommendations.  There are so many
things that will affect your results that you have to establish what works
best for you.  For example, a wide range of things like your metering and
the accuracy of your meters, the quality of your local water supply,  the
accuracy of your thermometers, minor variations in shutter accuracy and
lens speeds, your agitation technique, development tank size, and so on,
will all have an affect.  These variables can offset each other, or else
they can act together to throw your results off in one direction.
Statistically, the overall effect of these kinds of variations is described
using the root mean square law.
   For this reason, I have stopped prewashing entirely.  When I try using
new film/developer combos that are not part of my normal repertoire, I am
never sure if the manufacturer's starting recommendations are reliable with
a prewash.  It is psychologically disconcerting too to see the water come
out coloured after the prewash, especially with films like APX25 and
TPX120.  The colours of the prewash water vary from pale orange to very
deep blue, in most cases due to removing anti halation material from the
film.  The effect that these chemical byproducts have during the
development process would be the basis of some interesting research.  An
strong argument for using a prewash would be to remove these materials
prior to developing, so they do not act synergistically with the developing
agents.  It would seem that if these agents were detrimental to film
development then prewashing would be a standard recommendation.




 _
[o] -GH