There are variables in film development. Conventionally, these are time and temperature.
Liquid developers, like HC-110 and Rodinal add the variable of concentration ( or dilution, if you prefer ).
Before WW2, it was common to view agitation as a variable as well. The 'by-the-book' approach that dominated post war methodology and agitation was generally viewed as a constant: every thirty seconds, or if one was radical and dangerous, once every minute ( because the 'by the book' training of countless photographers in the military required a technical rather than craftsmanlinke approach to photography ).
Over the past 10 years, it has become common to see agitation once again as a creative control. Simply put, compared to a well developed negative with agitation every thirty seconds, reducing agitation to once every fifth minute and extending the length of time, shadows will have a little more density, midtones will be normal, and highlights will have a reuced density.
Rodinal is an ideal candidate for this technique, There are others. Not all developers are good candidates for this. Most films work very well. Some don't seem to care.
Like everything in photography, there is endless speculation, argument and denial. The proof is in the pudding, as the say.
I would suggest agitation your film once per minute just as you develop with HC-110. When you have found a good time for your Rodinal - like developer, try agitation every fifth minute, using 1.4x your 'normal time'. Shoot a test, say with window light portraits, and see the difference.
With Rodinal, as compared to HC-110, you will not see much contrast change over different times: give normal exposure, and simpy develop until you get good shadow detail.
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