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I have tried searching online with a broad variety of keywords, all of which turned up results for "shooting with both cameras on the same device".

Searching Video Production Stack Exchange also didn't help.

I am making a physics experiment and I will borrow iPhones for their high-quality graphics and built-in slow-motion. I will be recording two angles, so how can I get both iPhones to start filming at exactly the same time? Human error is not allowed so I can't get a helper and say "3, 2, 1, go!" so is there anything such as software, wires, or even a separate control from a third device that I can use?

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    Why not starting the videos one after the other and then editing them and cutting them to be the same length?
    – Thinkr
    Apr 1 at 20:03
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    @Scientology same idea, it's human error with timing that's the problem
    – Aya Noaman
    Apr 3 at 9:23

2 Answers 2

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This is a solved problem for professionals by starting all recordings and using a clapper board. I'm not seeing any way to sync this with shortcuts or other mechanism, so you may need to control for this in post production.

A simple clap or other sharp visual / audio artifact is all that's needed to get the streams aligned and as long as the internal clocks aren't drifting much you hopefully don't need to control for clock skew on two iPhones.

Most people won't detect a sync error of less than 22 ms.

Depending on the margin of error you expect in your experimental measurement, you may need to control for the speed of sound or use a strobe or light flash for an accurate sync.

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    I will be using slow-motion anyway so the sound will be even more precise to detect, and I could also use a helper with a flash of light like you said which is easier to see in frames than sound.
    – Aya Noaman
    Mar 29 at 23:21
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    To add to this, Tom Scott did a video that talked about this. youtube.com/watch?v=yWYkoZKHLfg
    – ErniePC12
    Mar 30 at 3:18
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    The sound from a clap [professional board or just two hands;) is very easy to line up on - in fact video editing suites tend to have the facility to do it automatically these days [look up multi-cam shoot in your manual].
    – Tetsujin
    Mar 30 at 7:24
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    I love that OP was looking for a high-tech solution and the one that was presented was a tried-and-tested method that is over 100 years old. +1.
    – Darren
    Mar 30 at 10:01
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    @9072997 Great point about measuring a moment vs position! I'd be careful with frame-doubling, though -- because those doubled frames are "lying" about the state of whatever they're photographing (at the reported time), you could end up with pairs of frames which claim to be totally in sync but show the world being in two different states at the same time. Whether that's a problem depends on the experiment, I guess...
    – A C
    Mar 31 at 23:19
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Treating this as a an experimental setup problem rather than an iPhone problem (;-) )...depending on the angles you want and the volume you are filming, you may be able to use one or more mirrors to supply the additional point(s) of view, eliminating the second camera. On the other hand, if I were using two cameras, I would include in both fields of view a way of making "clock ticks", such as an electric fan with one of the blades marked.That way, I could make up for any timing errors introduced by video compression in the two video streams.

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    Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
    – Thinkr
    Apr 1 at 19:50
  • The "clock ticks" tool is similar to @bmike's clapperboard/flash suggestion. The angle idea I tried and it didn't work, because one of the things I'm filming is an LCD monitor which is like one of those digital clocks that you can't really read from an angle. The mirror idea seems interesting, I'll see if it doesn't have the same problem
    – Aya Noaman
    Apr 3 at 9:31

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