There was a recent question about long exposures in available light. I'm rather new to this subject. The longest exposures I've done were about 12 seconds. I wo rk with Fuji Velvia. I've often read that for longer exposure times in color a c olor correction filter (CC) is a must. There a plenty of them, however. How do I know what filter I need for Fuji Velvia at a certain exposure time? Thanks for your help.