INTERVAL OF TIME/SHUTTER SPEED: Is a arithmetic progression  conventional cameras operate in the fractions of a second range i.e. 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2
The interval of time for the shutter to remain open in pinhole work is more likely to be in the whole seconds range e.g. 1.2.4.8.16.30.60.120.240.480.560.1120.etc. 15 In pinhole practice, rounding interval of time numbers up or down will make the calculations easier, for example, amending 16 seconds to 15 simplifies the numerical progression: 15 . 30 . 60 . 120 . 240. 480 etc. which convert to whole minutes with ease.
