I understand the premise of pre exposure... take a multi exposure shot once on a grey card, the other on the scene. My question is, how much will this increase the exposure lattitude of Velvia? For now, I see Velvia at about 4 - 4.5 stops lattitude before detail is lost. What is the max. amount this can be increased using pre exposure technique... hopefully 2 full stops more? Any good reading material for this on color reversal film? Any input would be helpful.