Time-Lapse Photography - Frame-by-Frame Stop Motion: The Guide to Non-Traditional Animation Techniques - page 59

Time-Lapse Rates and Formulas
Determining the rate of the interval for a particular event may take
some experimentation. This is why it is important to know how long an
event occurs like the rising of the moon. How long do you let time lapse
before you shoot another frame? The moon may take many hours to rise
and if you want to highlight the dramatic rise of the moon then speeding
up that progression is more effective. That means that you want to allow
more time to lapse between frames giving the moon a chance to rise
between shots. If you are shooting people then you may not want to have
too much time between shots. People move relatively fast (certainly in
comparison to the moon). It may take a person 5 seconds to walk across a
large room, so if you shoot a frame every 2 seconds, then you get only two
frames for that action. That is not enough to get a sense of the movement
or direction of the action. You may want to consider shooting a frame
every half second, so you end up with ten frames for that action. This way
you can see the action and direction in what will play out as less than a
half second of screen time. Since the moon is so slow you may consider
a longer lapse, more in the 15 to 30 second range, so the change can be
perceived in a dramatic playback. It is important to consider what the
main focus of your time-lapse film is and set the interval accordingly. If
you want to watch the sun rise at a good steady rate, then your interval
will be longer but you will have clouds that streak by at a fast pace. If the
clouds are your main focus, then the interval will be faster and the sun will
not rise that fast in the sky.
If there are restrictions on the length of the playback and you need to squeeze
a time-lapse event into that length, then there is a formula to help figure out
the interval between shots.
If, through observation, you have determined that the event you want to
make a time-lapse film of is 3 hours long and you have to compress that
3-hour period into a 15-second piece of film, then you can work out the
interval of the shutter exposure through simple math.
H = Length of event in hours
S = Length of time-lapse film in seconds
F = Frame rate
I = Interval rate
60 × (H × 60)/(S × F) = I
60 × (3 × 60)/(15 × 30) = 24
So, to shoot a 3-hour time-lapse event with a playback frame rate of 30
frames per second in 15 seconds of playback time, you must shoot a
frame every 24 seconds.