- The total duration - how long do you want the sequence to run?
- The playback frame rate
- The speed of any moving objects
- The speed of any camera motion
- The kind of results you want
Time-Lapse photography is a lot of fun and the results can be amazing. They can also be disappointing, with unwanted jerky, uneven movement, movement that's too fast to keep up with or something that resembles a slide show instead of a video.
In most cases, these problems can be attributed to one thing: poor interval selection. Of all the aspects of shooting a time-lapse sequence, this seems to be where many photographers struggle the most. That's because there are several variables that need to be considered and no reasonable, single formula is going to yield optimum results every time. There are a few guidelines that will help you get ball-park results, however.
There are 5 basic considerations in setting the interval for any time-lapse:
What to Consider First
Interestingly enough, the last item mentioned above is almost always the first you should consider. Do you want the motion in the final video to be smooth? Some very effective sequences actually create the illusion of objects popping from one location to another. These latter effects will call for longer intervals than the former. Decide what effect you're shooting for first.
Basic Duration Calculations
Let's look next at how duration fits into the overall picture. If you're creating time-lapse videos for profit, this is an important consideration, because your clients will often have a specific duration in mind. A little bit of basic math will help you set the initial parameters. Here's a very basic example:
Assume a client needs a smooth time-lapse of a sunset at a frame rate of 24 fps with a total duration of 12 seconds. You'd like to use a 2 second interval, because it's given you good results in the past. How's that going to work for this assignment? Here's how:
24 fps x 12 seconds = 288 shots 288 shots x 2 second intervals = 576 seconds 576 seconds / 60 seconds = 9.6 minutes total shooting time
That means you're going to have less than 10 real-time minutes of sunset in your sequence. What if you increase the interval to 3 seconds, to get more of the sun's arc in the sequence?
288 shots x 3 second intervals = 864 seconds / 60 = 14.4 minutes
So, you've increased your total shooting time by almost 5 minutes. At the same time, however, you've compressed that much more movement into the same 12 seconds of playback. That movement might be too fast. Reducing the interval to 1 second will smooth out the movement and reduce the shooting time, but will also reduce the amount of sun movement you'll record.
As you can see, both the duration limit and the frame rate are factors in your interval selection. If you could change either, you'd have more options for your interval setting. This is going to hold true for each of the other variables listed above, too.
The rules of thumb are simple enough for object motion and camera motion when using a slider or dolly. Faster movement will generally call for shorter intervals to maintain smooth motion and vice-versa. From that point, you use that first consideration, the result you want, to determine the interval to use. Experience will be your best teacher, so experiment and learn.
Last, but not least: Remember that your exposure settings are going to come into play, too. There's one very important rule in regard to that: Interval time MUST exceed exposure time. Obviously, an interval of 1 second in a situation where your shutter speed may exceed 1 second isn't going to work. Less obvious is the need to allow time for your camera's buffer to finish writing the data to the memory card. As a general rule, you should set intervals 20% to 40% longer than the longest shutter speed to avoid lost data and dropped frames.
Practice, practice, review and practice!
Finally, remember that there aren't many absolutes in this type of photography. You're going to encounter new challenges and unexpected problems as well as pleasant surprises. Get out there and shoot. Keep meticulous notes. Learn by doing. Have fun!