Hi - Whilst shooting today in full sunlight, I hit the ceiling in terms of shooting wide open with my PL 25mm F1.4. (not that I always want to shot wide open) I'd like to buy a ND filter but using a filter is new to me. Any recommendations for brand and also amount of stops. If i were to get just one - which would it be in terms of ND4, ND 8 etc? Thanks in advance.