FP4 and HP5: ISO values and development times in DD-X.
Thanks to some advice from Thomas Bertilsson on another thread, Iíve started investigating the relationship between film ISO values and development times. Up to now Iíve been using HP5 film (box speed 400) and DD-X developer. I thought Iíd branch out into FP4 (box speed 125) so as to have an alternative film.
Iíve read in various sources that HP5 can be exposed at 400, 260 and 160 (according to light conditions and required shadow detail) and development times can be reduced accordingly. I assumed that similar arrangements applied to FP4. Now hereís the interesting thing: both the packing of FP4 and the info supplied with DD-X show that FP4 can indeed be exposed at ISO 50 as well as 125. On the other hand neither the packing of HP5 nor the DD-X info show dev times for HP5 exposed below the box speed (although adjusted times are given for higher ratings up to 3200).
This leads me to the following questions:
a. For HP5 lots of experienced photographers have worked out satisfactory exposure/dev combinations for ISO values lower than the box speed. Has Ilford got any particular reason for not mentioning these?
b. Ilford mentions only an approximate halving of the ISO for FP-4 from 125 to 50. Can one take it a further step lower to e.g. 32 as well (analogous to going down to 160 for HP5)? If not, why not?