[pure-silver] Re: Pushing delta 400?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 2 Feb 2005 14:10:20 -0800
----- Original Message -----
From: "Eric Maquiling" <eric@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, February 02, 2005 10:11 AM
Subject: [pure-silver] Re: Pushing delta 400?
> Quoting "Koch, Gerald" <gkoch02@xxxxxxxxxx>:
>
>> Dilution B is 1+31. For 240 ml of working solution use
>> 7.5 ml of the
>> concentrate.
>
> Right on! Thanks. So for a 2 reel metal tank I'll need
> 500ml of H20 and 15ml
> of concentrate.
>
>>
>> There's lots of interesting information on HC-110 at
>> www.covingtoninnovations.com/hc110/
>
> Wow! That's a lot of info! Need to print that out.
> --
> Eric
An error on this site is the statement that acutance is
related to solvent effect: it is not except perhaps
indirectly. Acutance is a term originated by Kodak Research
Labs for an optical effect that is interpreted by the eye as
sharpness. The eye tends to interpret edge contrast as
sharpness rather than high resolution. On film the contrast
at borders between high and low density areas can be
exagerated by local action of the developer. The effect is
partially related to the diffusion rate at the surface of
the emulsion and also laterally through the emulsion. In
general it is caused by locally existing reaction products
of the developer. Depending on the developing agents these
reaction products can either restrain or accelerate
development. Sufite, which is present in all developers,
although it may be produced indirectly from other compounds,
has several important functions. Sulfite is a solvent for
silver halides. If its present in sufficient quantity it has
an effect on the shape of the developing crystals. But this
effect does not affect acutance. The main function of
sulfite is to prevent oxidation of the developing agents. It
accomplishes this through competing with them for oxygen.
Development agents are all reducers, i.e., prolific
absorbers of oxygen. This action of Sulfite tends to prevent
the generation of developer reaction products. Since these
are the cause of the border and edge effects which produce
acutance, sulfite tends to suppress acutance. Another factor
is the use of two superadditive developing agents, like
Metol and Hydroquinone or Phenidone and Ascorbic acid. The
combination of agents has a regenerating effect, each on the
other. In other words less reacton products are generated so
there is less acutance effect.
Lowering the concentration of sulfite will often
increase acutance effects but its because the protective
action of the sulfite is limited not because of its solvent
effect.
BTW, the solvent effect can either raise or lower film
speed. Where the action is moderate, as in D-76, the solvent
action removes the outer layer of the halide crystals
allowing the developer to access internal development specs,
i.e., it makes more of the latent image available, thus
raising the film speed. Where action is greater, as in D-25
or Microdol-X, some of the latent image will be destroyed,
thus lowering the film speed.
The effect of sulfite depends on the dilution of the
developer as well as the ratio of the components so a low
acutance developer like Microdol-X or D-25, used full
strength, becomes a high acutance developer when diluted
sufficiently (1:3 for these two developers).
Because the scale of acutance effects are fixed they are
of significance mainly for small negatives which are
magnified a lot in printing. Acutance is of little
significance for large-format negatives.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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