recently i have been taking closed up with velvia.low light plus bellow factor,i frequently encountered exposure time 1 to 20 sec. all the literature i have don't really tell how to correct this for color film.my questions are: 1.Does the film speed decrease in the same manner as B&W film?So at least I can get correct exposure. 2.To which direction does the color shift? 3.What do yo do? Thanks