Frank (and everyone else...)
FP4 and Rodinal is my standard combination. After years of tinkering, here's how I do it and why:
At 68-70 degrees, and film exposed at ISO 80-
35mm- Mix 2grams per liter sodium ascorbate and a half gram per liter of borax in water (half the strength Patrick Gainer recommends), then add rodinal at 1+50. Develop for 12 minutes with 3 inversions each 3 minutes.
120- use at 1+50 for 15 minutes, 4 inversions every 5 minutes. I don't add sodium ascorbate or borax for the larger film.
So why all the complication? My enlarger/ paper combination seem to require somewhat lower contrast negatives than most film developing data will accomodate, so using semi-stand development works best.
As for the different combinations with different sizes of film, in 35mm the results are just a little grainier than I like. The ascorbate/ borax makes a very noticeable difference in grain clumping, as well as speeding development. By 645 size, grain is right where I like it without the extra chemicals.
After all this, it took tons of trial by error to come up with a really satisfactory scheme. Done again, I'd stay away from the stuff unless I REALLY like to experiment.
As for HP5 or Tri-x in rodinal, I can't tell them apart except for the strange dark ooze that comes out of the tank with Tri-x, and the extra fixing time required for Tri-x. Results are very grainy in either case.
Specifically for 120 I've had trouble with uneven development with both Tri-x and HP5. I'd suggest running a test roll first.
Hope this is useful!
