gamal shehab

you can just mesure any filter factor by using an expojer meter and mesuring the the light from any source of light with and without the filter and the difference in exp. is the filter factor

( i think that u expose on a point between the backlight and the filler so tha back light still have its shiny like to make the backlight over 1stop and and the filler under 0.5stop so u have the nice texture of the water and not so shiny because the water has a highly reflector and shiny surface so its better to be under from 0.5stop 0r 0.75stop to dont bee too shiny and losses its textur and also for the backlight it is better to dont be be so high for the texture of the water