How this calculator works
Playback needs a fixed number of frames, equal to the clip length times the playback rate, so a ten second clip at 30 frames per second needs 300 photos. The camera takes one photo every interval, so the real recording time is the photo count times the interval. If instead you have a fixed window, such as a six hour construction shoot, dividing the window by the photo count gives the interval the camera must use. Storage is the photo count times the file size, and the compression ratio compares the real recording time with the finished clip length. Intervals longer than the exposure time leave the shutter closed, which is fine; intervals shorter than the write time of the camera will drop photos.
Formula
photos = clip length x playback frame rate. recording time = photos x capture interval. capture interval = recording time / photos. storage = photos x size of each photo. time compression = recording time / clip length.
Worked example
A ten second clip at 30 frames per second needs 300 photos. At one photo every ten seconds the camera records for 300 x 10 = 3000 seconds, which is 50 minutes, and the compression of real time is 3000 / 10 = 300 times. At 5 MB per photo the sequence needs 300 x 5 = 1500 megabytes, or 1.5 gigabytes. If the shoot must fit in a 50 minute window instead, the interval is 3000 / 300 = 10 seconds, the same answer.
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Frequently asked questions
What interval should I choose?
Divide the event length by the number of photos you need. Moving clouds and traffic suit intervals of a few seconds, while growing plants and buildings suit minutes or hours. Too short an interval fills the card; too long makes the motion jumpy.
Why is the finished clip so much shorter than the shoot?
Playback compresses real time into a few frames per second of event time. A 300 times compression turns a 50 minute shoot into a 10 second clip, which is the whole point of the technique.
Does the camera battery last for a long time lapse?
Often not. Long shoots usually use mains power, a battery grip or an intervalometer with a sleep mode. Check the storage and power budget before leaving a camera alone for hours.
Should the shutter speed be shorter than the interval?
Yes, with margin. If the exposure plus file writing takes longer than the interval, the camera misses frames and the playback stutters. Leave at least a second between exposures for most cameras.