In article ,
Richard Knoppow wrote:

There are two reasons Jobo reccomends a pre-soak. One
is that it saturates the emulsion and contols the rate of
uptake of the developer. That insures it is taken up evenly
to eliminate blotching. The development is also slowed
somewhat because the "induction time", that is, the time
between immersing the film in the developer and the time the
image begins to appear, is lengthened. Because of the very
rapid agitation in the drum the development time is quite
short compared to other non-machine methods so that
increasing it a little also tends to insure uniformity. The
second reason is to temper the tank. This is more necessary
for color development done at elevated temperature so that
the temperature of the developer is not suddenly changed
when entering the tank.

I strongly recommend *against* use of a pre-soak when developing
color film in a Jobo. I think if you peruse the various web fora
you will find a number of former Kodak chemists making the same
recommendation-against.

Many modern Kodak materials (at least) are manufactured in such a
way that developer penetration into the emulsion is controlled by
an "overcoat" of non-sensitized material. Among other issues,
pre-soaking defeats this, so that you are not developing the film
as its designers intended and may not obtain the same curves they
publish, even with the same developer at the same concentration. A
nuisance with black and white materials, but a more serious issue
with color ones.

Kodak recommends either the use of temperature controlled hot air
blown into the tank instead of a presoak (I do not know what kind
of rotary-tube machine can do this) or a longer preheat with the
tank in the water bath but no liquid in the tank. I use the latter
method and have always had good results even with processes with very
short times for some steps such as C-41RA.

--
Thor Lancelot Simon
"All of my opinions are consistent, but I cannot present them all
at once." -Jean-Jacques Rousseau, On The Social Contract