The general idea of the developing film is that enough light has been supplied to the film for a nominal exposure, and to take the next step, a chemical reaction must take place to get that latent image.
What do you think happens if your developer was half exhausted when you used it? You would only get a partially developed movie.
Therefore, we use a developer mix that contains enough real chemical to make the batch never reach exhaustion. If I had to leave your film in the developer for a period of time, I would end up with highly developed negatives because there is enough developing chemical to keep in the truck.
Then, its controls are converted to temperature (colder == slower reaction) and time (less == less development). Almost all the work for this is done by the manufacturer and published in the massive development chart, the film box, the film specification sheets, etc.
Now, what has all that to do with Rodinal? Simple, in most proportions, Rodinal is like a normal developer in the sense that you have many more chemical chemicals than are necessary for it to develop normally. This is any ratio up to 1 + 50.
With 1 + 100 or more, you end up with a miserable 5 ml of Rodinal in 500 ml of liquid. This maximizes the volume in the smaller Patterson tanks of most people and is so unrealistic that will reach exhaustion during the development. When you get to this point, That becomes the goal! This technique is called stand development. The majority begins with 5 ml of rodinal in proportions 1 + 100 and supports the film for one hour. Personally, I shake mine a little after 30 minutes (semiposido).
Rodinal shows different characteristics in its development based on the chosen proportion.one. There really is little to do except to advise you to experiment. In anything 1 + 50 and below, you will get usable negs following the standard process.
1 + 100 and above is for stand development, should be considered highly experimental (do not risk your precious shots), and a warning about this: Rodinal loses speed at this level. If your plan is to stand, then overexpose your shots from 1/3 to 2/3 when you stop to compensate. Again, however, this is experimental … YMMV.
1: You could write a book on this topic to keep it short: the developer does not "work" at the same pace throughout the film. The highlighted areas are dense and develop first, reaching a local depletion faster than the shadow areas. That's why you stir. Using higher ratios such as 1 + 100 and do not shaking both will force a greater sharpness, will make the grains appear larger and will have a compensating effect (less contrast, helps avoid the blocking of the reflections while the shadows develop). For this reason, most do not recommend Rodinal absolutely for high ISO films and especially do not Stand development.
Delta3200 @ ISO800 at 120 w / 1 + 100 is almost as granulated as I will. And it is very grainy. I like it for the images of the city / street (it comes out as sandy) but I would avoid like the plague for portraits. With 135 movies, the images for me are unusable, no matter how well exposed / developed.
But, this is an art form. Be subjective and you like what you like.