advance a black sheet 6 inches for 20 seconds exposure 16 times for a total exposure fo 320 seconds. The sheet should advance in front of a inexpensive 4foot by 8 foot insulation foam board painted with 4 inch lines of different colors. even though some black might try to register- the colored lines being brighter will give you 16 different times on a single sheet of film that should not be hard to interpret without the use of a meter and can be applied to different illuminations.--just an idea for the experimentors
