I have just completed developing two rolls of Tri-X (35mm) + Rodinal. In both cases the E.I. has been 400. First time the dilution has been 1:50 (14 min), second time 1:25 (7min).

I was expecting to see some less grain in the second roll (since developing time has been decreased), but I virtually couldn't see any difference. On the other hand, I can see a great variance in graininess amongst different pictures from the same roll, but I couldn't quite figure out what to attribute the difference to.

I should add that I have assessed the grain by evaluating scans from the neg (at 3200 DPI) and trying to use the 50mm as a loupe to read the negative itself. Will bring the negatives to the lab soon to have some sample prints.

So, given a film/developer combo, which are the factors that affect the graininess of the negative?

Developing time?

P.S. I've read that a sudden change in temperature between developer/stop-bath/fixer/wash can cause the grain to cluster, but I would exclude that since I carefully tested my temperatures throughout the process.