Why is more aggitation different from longer development?

Actually I have got two related questions:

1)Why is a longer develpment time different from more aggitation of the developer?

2)Why is a stronger delution of developer different from longer develpment?

Okay I know grain tends to be more visible with more agitation than with a longer develpment time while it both increases the contrast. Does the same go for the developer delution, and is there more to it?

cheers!
Quinten

With many developers, agitation is directly linked to highlight density. More agitation, the higher the highlight density RELATIVE to the overall density of the negative. Agitation has little effect on the shadow densities, which is determined by the length of time in development. The apparent grain increase of longer development or stronger concentrations of developer is linked to the proportion of agitation relative to the overall time, or concentration.

There is a minimal difference in the results of development in different dilutions IF the time and agitation are compensated. This can be seen clearly with Rodinal.

However, with developers like D-76 or Microdol X / Perceptol, dilution tends to increase the shadow density.

These are rather sweeping generalities.

You can count on being able, with Rodinal, to a slight reduction of the apparent grain by reducing the agitation from 5 seconds every thirty seconds to 5 seconds per minute. Agitation every 5th minute has a greater effect. Furthermore, you can increase the effective densities of the shadows by increasing the time in development (Pushing) while restraining the highlights (Pulling). The price you pay is an increased devlopment time.

The same technique works well with Aculux 2, FX39, dilute Xtol, and dilute D76.

I hope this helped a little.

I won't pretend to explain WHY it happens. It does work consistently, however.

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Think about what happens in the developer when you don't agitate the solution. The developer becomes partly exhausted and weaker in the areas where development is strongest - where it does the most work. Also, bromide and iodide, restrainers, are released in these areas. When you agitate, you bring fresh developer to all areas of the film, so development is more even. This affects contrast, grain, and acutance. Convection currents in the developer and other things that cause migration of the developer constituents moderate and change these behaviors, too. Agitation is generally needed to produce reliable, even development.

A similar thing happens with increased dilution. The developer becomes exhausted more easily in local areas where development is greatest. The other components, such as sulfite, are also reduced in concentration and have lesser effects. One of the most important effects of greater dilution is lower contrast. While that is also true of reduced development time, the effect does not seem to be exactly the same.

I'm sure this is not the entire story, especially for dilution, but it is a start.

These are theories that work with some developers and not others, and to different extents with those developers with which they do work. Actually, you have asked a couple of two part question. The first part in both questions is "Why" and the second parts are "Is more agitation different from longer development" and "Is stronger dilution different from longer development". Philosophers have tried for many centuries to answer the first part without success. The answers to both second parts are the same: sometimes. I couldn't resist paraphrasing an old comedian's shtick.