With slide film, exspecially such difficult ones as Velvia 50 I need a center filter. As far as I know the right one for the SA 90mm /8 is the Schneider Centerfilter IIIa (older model) or IIIb (newer model). I have now used my SA 90mm /8 on 4x5 with a Centerfilter III what is orriginally designed for the 75mm SA and had very good results.