Question

formulae for exposure time of lens

Answer 1

A simple mathematical formula to calculate the exposure time of lens is

tND = T0 *2ND

where ND is the stop value of the ND filter

t0 is the base shutter speed in seconds (without filter attached)

tND is the final exposure time

for example:

ND = 10 f - stops, t0 = 1/30 secs  -->  tND =34 secs