When Night Falls - A Brief Introduction to the math of time lapse

Time lapse was once a cumbersome process requiring a timing device attached to the camera. Many digital cameras now come equipped with the timing device built-in. Termed an intervalometer, settings for the time interval, how many images to take, and whether to start at a specified time or immediately can all be set in-camera. The equipment for timelapse photography is now no more complicated that what equipment is required for the subject matter, though a good sturdy tripod is a must.

Setting the time interval (T in seconds) and the number of frames (F) to shoot is a matter of knowing
1) the length of the assembled video (L in seconds)
2) the compression factor (CF)
3) the video image rate (R in frames per second)

For example, if you show your video using an image rate of 20 frames/second (R = 20 fps) and want to speed things by a factor of 40, then you will be shooting one frame every 2 seconds. In equation form, T =CF/R.

If your finished video will be one minute in length (= 60 s), then you will need 20 fps X 60 s = 1200 frames, or
F = RxL

Alternately,
F = LxCF/T = (60 s)x(40)/(2 s/frame) = 1200 frames


When Night Falls was shot with a 6s interval between images and used 550 frames. The video image rate is 10 fps. So R = 10 fps, T = 6s, and F = 550 frames. Using the equations, the compression factor, CF = RT = 10x6 = 60 and L = F/R = 550/10 = 55 seconds in length.

In the finished video, watch for events you may not normally notice in either straight video or single images.