You can shape your film's tone curve by balancing exposure and development time with agitation.
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.
*** I forgot to mention that I needed to move away from contrasty developers like Dektol to print these slightly higher density negatives.
LPD works perfectly, and the red curve will print on Ilford FB MG just right. Ansco 130 (minus HQ), D52/Selectol, a blend of Detktol and Selectol Soft (David Vestal's Delectol !) all work well.
I have withheld the speed rating of the test film. The intent of this short article is simply to illustrate the principle while not introducing yet another 'magic bullet'. It is better to do this work without a densitometer, and judge the results by making contact prints. After all, if you see a difference, there IS a difference. If you are interested in this method, try it out ...find your own way! You are, after all, your own magic.
For the technically minded, the curves were created by projecting a Stouffer step wedge into a 35mm camera,
then reading the developed strips with a graphic arts baseboard densitometer. I used a Durst L1200 enlarger, an Apo EL Nikkor lens, and a Nikon F4 camera. The film was exposed at 1/125, several negatives were read, averaged, and plotted.
It is a remarkably flare-free system.