Agitation is used to control the highlights of a negative.
More agitation raises the highlight density, less agitation lowers highlight density.
In this system, development time places your shadows. Exposure places your midtones.
It is effective to visualize comparative curves pivoting at Zone V rather than rising out of film base fog as is conventional in various systems. Introducing agitation as the third variable allows us to define any density as a 'speed point'.
This system of exposure and development was common in the early 20th century, and was often referred to as 'standing', or 'minimal', or 'tank ' development. The principle has often been called 'compensation', although I observe that compensation is due less to special properties of developers and due almost entirely to certain films like Tri-X and TMY2 which are designed to hold information when given a great deal of extra exposure.
NB. Standing agitation is a misnomer. Few old timers ever witheld agitation completely over long periods of time, as is often attempted today. The necessity of agitation was well documented and understood. The use in this test of 5 minute resting cycles is safe, in my experience with Rodinal, for 35mm and 120 negatives and steel reels. Some experts limit their cycles to 3 minutes. Little is gained, I have found, by using longer resting cycles while the risk of negative defects are increased.
In the illustration are two curves which demonstrate the effect of using agitation, time, and exposure to shape a tone curve. Rodinal was diluted 1+50, and used at 68°. The blue curve shows TMY2 developed for 11 minutes, and receiving 10 seconds of agitation every minute. The red curve shows TMY2 developed for 16', agitated for 10 seconds at the beginning, 10 seconds at the 5th minute, and 10th minute.
The curves have been adjusted left to right to represent matching mid tones by varying exposure. I am a portraitist, and mid tones are essential to me. You may match curves however you wish.
The red curve shows greater shadow detail, and slightly higher highlights, although in practice, the highlights print identically by adjusting the print developer or filtration with variable contrast paper. The extra shadow density is the object of the exercise.
Rodinal is an excellent developer with this technique. You may use many other developers with this method. Pyrocat, FX2 and dilute XTOL are some of many good choices.
http://www.apug.org/forum/index.php?resources/shaping-the-tone-curve-of-a-rodinal-negative.50/
(Aggiornamento del 2017/04/09 - Su apug c'e' qualcuno che usa questa tecnica per sviluppare la Fomapan 100. Dice: "Fomapan 100(135) came good with Rodinal 1+50(300ml + 6ml) but the agitation was according to the article. 15 mins: Three inversions at the beginning and three at every 5th minute @20° C and this prints very well on MCP 310 with Focomat Ic. Note: I shot mostly during day time in Italy with Yellow-Green filter and my incident reading was EV 14 @ISO 50(filter compensation)." Devo provare! http://www.apug.org/forum/index.php?threads/best-tamming-of-fomapan-100.94633/#post-1258510 )
(Aggiornamento del 2017/04/10 - Altro utente di apug che usa questa tecnica per la Fomapan 100. Dice: "I guess I'll chime in here. I use Rodinal at 1+100 with Arista Ultra iso100. Set the timer for 18 minutes. Constant gentle inversions for the first 60 sec, then one gentle inversion only at 15, 10, and 5 min. Nice midtones, no streaking. I find HC110 at 1+100 gives VERY similar results, without as much accutance. BTW-all with 120 film and 500ml of solution." Devo provare! http://www.apug.org/forum/index.php?threads/rodinal-agitation-the-definitive-thread.43906/page-2#post-627906 )