Alberto, that photo is very good. I could never get a result like that with Efke IR 820 Aura, in 4x5". As to the sensitivity, I would try 3 asa, 1,5 asa and 0,75 asa.

I still don't understand very well how this film responds to light. I always get diferent results, using the same exposure index. It's always inconsistent.

The Rollei 400 IR is a much easier film to deal, with incredible sharpness and middle tones, unfortunatelly, it costs the double of Efke's film. Another "problem" with Rollei IR 400 is its extremelly thin base, wich reminds Kodak HIE's base.