The new Time Lapse camera mode in iOS 8 allows you to take fast-motion videos that display only a few frames, as if separate images were taken at long time intervals and then stitched together to make a short but fast movie; however, you might notice that when you take short time-lapse videos you will get a larger number of frames per second than if you extend your video out to tens of seconds or minutes.

This happens because Apple has implemented a tiered frame counting routine in the time lapse that changes the number of pictures taken in the video, depending on the duration of the movie. Apple describes this process as iOS 8 taking photos at “dynamically selected intervals,” but to outline exactly what this means, Dan Provost at StudioNeat put the new Time Lapse mode to test.

Apple’s dynamic interval process is actually quite simple, where movies start out at 2 frames per second for the first ten seconds of video, with this rate dropping by half every time the movie doubles in duration:

This boils down to Apple keeping the time lapse videos under 40 seconds in length. That is, a 10 minute recording will run at 2 frames per second, and end up being 40 seconds in duration (10 divided by 15) when replayed at a standard 30 frames per second, whereas one that is 20 minutes long will similarly be 40 seconds, but with half as many frames. The same goes for exposures of 40 minutes, 80 minutes, and so on, where at each doubling of the recording duration, the captured frame rate is dropped in half.

This approach is neat, because it allows you to quickly take time lapse video without having to manage large videos and then adjust the number of frames in your video to get the desired length and speed effect. While this does impose some limitations on time-lapse, it does allow you to take quite acceptable and useful high-speed playback of extended scenes.