im confused about this lately..
lets say, i use my camera metering to meter a scenery.. its says for perfect exposure i need to expose with 30secs f/2.8.. (maybe its very dark at midnight)
then i wanna put an nd400 filter and use f/22 instead of f/2.8.. so i gotta use bulb..
then come the question.. how long should i expose for it?
wondering if there is a kinda formula to count this?