Slow Motion with a Smartphone or iPhone Camera

One of the most powerful tools of filmmaking is the slow motion – or slowmo – shot. Like an overt demonstration of filmmaker’s god-like powers to alter or create reality. There’s something about slow motion shots which feels like we’re showing off – I’m showing you this because I can.

A term for creating slow motion film is “overcranking” which come from the days when film cameras were without motors so had been “cranked” by hand. In other words, you had to crank faster thereby exposing more frames per second than normal. And this is a good way to understand the basics of shooting slow motion.

If you were normally cranking at 12 fps and you decided to speed up to 24 fps, what happens when you play back that film at a consistent speed of say 12 fps? Well, the 24fps section is now playing at half the speed it was shot at – so the motion in the film is slowed to 50% of the actual motion, too.

Imagine – as a camera operator you could simply decide on the spur of the moment to speed up (or slow down) the hand cranking as you felt necessary, without stopping the camera or the take. If a modern camera had that capability, we would be hailing it as a mind-blowingly advanced feature…

Slow motion is cool

Why do we want slowmo in our videos? Because it looks cool. What would those action scenes in The Matrix look like without a large quantity of slow motion cinematography? Or Chariots of Fire without those running along the beach shots. Or two lovers running into each others arms in… every romance movie ever made.

Slow motion somehow helps to exaggerate the emotion of a scene. When a scene is a violent one, it romanticizes and perhaps glorifies the brutality. Of course, slow mo is also a key part of televised sports – you can make the action clearer and appear more impressive at the same time.

With slow motion employed, human movement becomes balletic.

How to create slow motion

There are a few ways. One is the way described above – shoot at a higher frame rate and then slow it down.

If we think of 24 fps as a standard, then shooting at 48 fps and slowing down to 24 fps would achieve a subtle slow mo effect. Clearly, the bigger the difference between shooting fps and playback fps, the slower the movement will appear in the final film.

To avoid ghosting in the footage the rule is the shutter speed has to be double that of the frame rate. At 60fps you need to be shooting at 1/125th of a second, for example.

There’s a good demonstration here:

Things to think about

Light – with the shutter open for less time, there will be less light making it through to the camera sensor. This should be ok in broad daylight, but lower light situations might require extra lighting.

Memory – slow motion shots are shooting more frames so take up more space in your phone’s memory. If you are doing a lot of slow mo work you might need to have extra storage available.

Overdoing it – with our god-like filmmaker powers, the temptation to control time just because you can is great. So try to remember you might want people to watch when you’re finished…

Having said that, check out this beautiful Hennessy advert directed by Ridley Scott (although a lot of credit should go to the CGI team). The entire thing is in slow motion. It’s a great example of how slow motion can generate an exaggerated emotion. The moving image becomes more dramatic, balletic and poetic when slowed down.

Other ways of achieving slow motion

You can also shoot at a regular 24 fps and slow down the footage to get slow mo. The less frames per second you shoot, the less smooth the slow motion looks when slowed down. That because those 24 fps are now playing at 12 fps, which is close to that area where the human eye can start to perceive individual frames.

But these days, technology is stepping into the breach and fixing this for you. Current computer software post-processing (with programs like Twixtor) fabricate digitally interpolated frames to smoothly transition between the frames that were actually shot. Eh what?

Well, more simply put – the computer analyses the individual frames and creates a new frame/s between. This way you will get that smooth slow motion effect without having to shoot extra frames.

Of course, you can always combine the two – shoot at 60 fps, slow it to 30 fps and then have your software add extra frames for extra smoothness.

Speed ramping

In some movies, speed ramping is used to add further expression to the slow motion effect. This is a process whereby the capture frame rate of the camera changes over time – just like those old hand cranked camera operators who adjusted their speed as they were filming.

By using this effect, a shot can start as normal 24 fps speed and slowly increase the frames per second up to say 60 fps. This way the final shot shows the audience the motion transforming from normal to slow, as opposed to cutting to a slow mo shot.

You can achieve this effect in editing software such as Adobe Premiere, which has a Time Remapping function. This allows you to adjust the change in speed with a great degree of control.

Well, I’m feeling a bit overcranked right now. Must have been the 2nd coffee…

(no hand cranking jokes please)

Read more about the… Mobile Motion Film Festival.

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