People these days have short attention-spans, so although time-lapse videos can be very appealing, most of us would probably prefer to watch one for 30 seconds rather than ten minutes. Thanks to a clever feature which Apple simply refers to as “dynamically selected intervals,” almost any time-lapse video you shoot in the default Camera app in iOS 8 will end up as 20-40 seconds, whether you shoot for 10 minutes or two hours …

The secret was revealed by Studio Neat, the people behind the Glif tripod mount for the iPhone.

Turns out, what Apple is doing is quite simple, and indeed, pretty clever. What Apple means by “dynamically selected intervals” is 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.

Studio Neat compiled the results of their tests into a simple table:

What this means is that a five-minute time-lapse will be captured at two frames a second and end up 20 seconds long at 30fps. A 40-minute time-lapse will be captured at one frame every four seconds and thus also end up 20 seconds long.

You don’t need to know in advance how long your time-lapse will be: the Camera app will automatically switch frame-rates as the recording time increases, dropping frames from earlier sections to match.

If you haven’t yet tried time-lapse photography, you can get some inspiration through examples, then check out our how-to guide for everything you need to know.

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