Knowing how many frames to shoot can be tricky. We recommend that you consider the action in terms of how long it takes to happen in reality and in the finished clip. Some basic computations are needed to work with the following variables.
- Real time: Start with the real time. How long is the event that needs to be recorded? An hour? 4 hours? Days? Break it down into minutes and then seconds for easier calculations later on.
- Screen time: How long does the shot need to be for the finished video clip? 5 seconds? 10? 30 seconds?
- Frame rate: At what frame rate do you need the finished file to play back? Are you editing in 24p? 25? 29.97? The number of frames you need to fill one second of video will affect the number of frames you need to capture.
- Interval between frames: How quickly do you want time to pass? Do you want to show the passing of minutes or weeks?
All these considerations will determine the number of frames you will need, which will also indicate the time interval between each frame (or burst) from your camera.
For example, consider this scenario (and forgive us for sending you back to math class): You are trying to record the sun moving across the sky, edge to edge in your frame. From your sun path calculations you conclude it will take 4 hours to cross the frame you have set. You want the shot to last 12 seconds onscreen. How many frames do you need? And at what interval do you record them?
Here are the calculations:
- 4 hours equals 240 minutes, or 14,400 seconds.
- The project will eventually be screened at 30 fps.
- Your shot goal is 12 seconds. Multiply 12 by 30 fps to get a length of 360 frames.
- Divide the total number of seconds (14,400) by the number of frames needed (360) to give you your frame record interval. In this case the answer is 40.
- Therefore, every 40 seconds you make 1 exposure for a 4-hour period, which will result in approximately 12 seconds of screen time.
A note of caution is warranted here if you’re trying to be too precise. You may only need 12 seconds of screen time, but you want that time to be exactly 12 perfect seconds! You don’t want any bumps or bogies walking through your frame; fogging of your lens; unwanted clouds, airplanes, animals; or just plain lack of activity for a portion of your shot. Good luck!
Consider a fudge factor and pad your record time if possible. Why? Things go wrong, so make sure you have the flexibility to lift out the best portion of the shot—just when the clouds were perfect or the traffic was at its most frenetic. If your glitches are only momentary, you have the benefit of eliminating the offending frames in post and smoothing over the jump with a proper edit.
This post was co-authored with Jim Ball. You can find more about time-lapse, stop motion, and DSLR video in the book – From Still to Motion: A Photographer’s Guide to DSLR Video.