There are two versions of Rodinal:
- Calbe R09,a.k.a FOMA R09, seems to be the very original formula, my Foma bottle is actually black and I have not opened it.
- Agfa Rodinal, a version from the 1950, slighly different dilutions between 1:25 and 1:100, comes almost colourless, will turn reddish brown and after some years darker brown. I guess the bottle in my kitchen was opened 5 years ago...
In an old issue of Leica Photographie 1950-1960 they compared Rodinal zu MQ and other developers and Rodinal was the only system which enabled the user change contrast with dilution without decrease in film speed or excessive developing times.
Actually 1:100 gives a small increase in film speed, like 60 mins for HP5+ at 800.
If I'm in a hurry I use 1:50.
Rodinal used on 35 mm gives good "sharpness" by the grain structure on a 18*24, grain size will be reasonable on film like FP4+ and fine on TMX and very fine on Efke 25/50 films. If enlarged larger, eg. 12 times one will find actually resolution is not very good as grain size has an influence on resolution of fine details.
(When I did experiments with Brenzcatechine similar results: I had high sharpness at 6 times but at 12 times resolution was rather poor)