Hey -- this timelapse thing should be easy. That's why I made the chart.
Of course you want to have a formula to get the interval-- which I put at the bottom. but when you are on location, say in the middle of Times Square, things change. All of a sudden you don't have 8 hours to shoot but instead 5 or 6. If you want the same number of seconds of footage, then you want to make an estimate on the fly.
Also, often DP's have a favorite interval, which I don't understand, which may not work for the particular shot that they want. Remember this is an analog calculator, it works the way our brains do and as such shows a lot of options right next to each other. Good luck.
By the way, this is my old email on the chart. If you want to contact me you can use LSRIECK@GMAIL.COM
