Ilfosol-S is a prepackaged Ilford developer, sold as liquid concentrate to be diluted 1+9 or 1+14. It offers fine grain, good acutance (not "high"), and exellent tonality with FP4+. It also has very poor shelf life, and tends to die on me between films.Originally Posted by Ka
FX-2, on the other hand, is an old(ish) published recipe for a high acutance developer with extremely good shelf life. A concentrate lasts a very long time (I had a ready-to-use batch sitting on my desk for a week in an open beaker with no loss of activity), and it is also exellent for stand development for those pesky situations when you forget what film's in your camera and you end up exposing "sufficiently". It happens to me once in a while - I excuse myself with having 6 old folders... It also gives a slight increase in sensitivity over most other developers. The tonality isn't quite as smooth as with Ilfosol-S, but you have to compare negatives directly to see any difference.
So for someone like me who tends to mix up the developer one day, then go off to work for a fortnight, FX-2 is the perfect developer.