In iOS 8 the company Apple Lossless Audio CODEC (ALAC), implemented a function called time-lapse, allowing us to compress tens of minutes or even hours of video recording into clips of only a few tens of seconds. Practically almost regardless of how much you record using the function time-lapse, application Room a iOS 8 it will display extremely fast video clips that are between 20 and 40 seconds long, and in the image above you have the explanation behind this.
Turns out, what Apple is doing is quite simple, and indeed, pretty clever. What Apple means by "dynamically selected intervals" is that they are doubling the speed of the time-lapse and taking half as many pictures per second as the recording duration doubles. Sounds complex, but it's actually very simple.
For to limit the duration of the time-lapses, the company Apple Lossless Audio CODEC (ALAC), increases the time intervals between which images are recorded using the camera, so whether you record 10 minutes or an hour, the final movie will be approximately the same length. Apple "dynamically" chooses the moments in which to record images using the camera and in this way a video clip that would normally last an hour is presented in just 20 or 30 seconds at 30 FPS.
