I have found the following to be accurate:
Grain |
Finer grain, smoother |
Slightly coarser but still fine |
Sharpness (Acutance) |
High (balanced with fine grain) |
Higher (more edge contrast) |
Tonal Range |
More compensating, softer highlights |
Less compensating, brighter highlights |
Best Film Speed |
ISO 100–400 |
ISO 400 and above (but works across speeds) |
Look |
Balanced, classic tonality |
Crisp, modern, high acutance |
Usage |
Ideal for prints and detailed enlargements |
Ideal for scanning and high-contrast subjects |
I've used FX-39 only recently, on several different films in both 120 and 35mm formats. I compared a roll of Delta 400 developed in both PMK and FX-39 (the same roll, cut in two and each half developed in each developer), and found that the FX-39 negs indeed gave the impression of better acutance, but compared to PMK, the grain was much more coarse, and the tonality overall was more "graphic" (contrasty, with abrupt transitions between values). It wasn't a look I much cared for, but I can imagine for some it would be the cat's meow.
Previous experience with home-made FX-37 was challenging: most listed development times were far too long and gave harsh, overdeveloped negatives. But when I reduced development times from - for example - 8 minutes down to 4.5 - 5 minutes (for FP4), the results were much more palatable. FX-37 does provide a speed boost of up to 2/3 of as stop, so for me, it allowed me to use films at their rated speed and get very good negs.
I found that FX-37 didn't deliver the nicest results with the 400 ASA films I tested, but that was more about personal taste than the developer's suitability for faster films. It was simply too grainy for my liking and it didn't play nice with the highest values, rendering them somewhat abruptly.
Keep in mind that my point of reference for "the ideal negative" is work made using PMK Pyro, which is very effective for maintaining a pleasing rendering of high values (tonal separation without blocking up) and it suppresses the appearance of grain thanks to the pyro stain.