Re: APX 25 ISO rated at 50 - development time????

----- Original Message -----
From: "Timo Sund" <timo@palaios.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, February 28, 2005 12:06 PM
Subject: Re: APX 25 ISO rated at 50 - development time????

> Sorry about layout. Maybe this helps a little.
> Rodinal developes everything.
>
> Kate M kirjoitti:
>> Have you got a link to the page Timo cos I can't find it
>> on the
>
> *Compensating developers include Microphen, TMax, and
> any other developers which are specifically formulated for
> push processing
>
>
> --
> Timo Sund
> Palaios Photos
> Finland
> http://palaios.com

    I disagree with this. The definition of a compensating
developer is one that causes a "shoulder" on the film curve.
That is, it compresses the contrast of highlight areas.
Usually, these are rather diluted developers or otherwise
especially formulated for this effect. Neither T-Max, T-Max
RS, or Microphen is a compensating developer, at least not
when used in normal dilutions. Rather all three are
Phenidone based developers capable of delivering full film
speed. All three are also capable of producing very high
highlight densities specifically because they have no
compensating effect.
   These developers, and some others, will yeild about 2/3
stop greater speed than D-76 and about 1.5 stop more speed
than Rodinal. What this means is that the speed is higher
for a given contrast index.
   Push processing does not really increase speed. What it
does is to increase toe contrast along with overall
contrast. This results in the low exposure area, which has
less contrast than the rest of the film curve, having more
easily printable contrast for image which are recorded
mainly in that area. The overall contrast increase results
in very high highlight contrast which can make pushed
negatives hard to print when they contain areas of "normal"
exposure. Pushing also unavoidibly increases grain. At some
point underexposure will be insufficient to record an image
regardless of how much development is increased. Also, at
some point increasing development will begin to increase the
fog level faster than it increases the density of exposed
silver grains.
   Compensating developers are not usually of the fine grain
type and may result in some speed loss due to their
formulation. Rodinal at very high dilutions provides some
degree of compensation.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com