Film Style Slow Motion in Digital Times

David Battistella director, editor, www.davidbattistella.com Toronto Ontario Canada ©2011 CreativeCOW.net. All rights reserved.

In this article, David Battistella, respected filmmaker and Creative COW contributing editor and forum host, inspired by questions frequently asked in the COW forums, offers a guide to understanding and re-creating film style slow motion.

FILM STYLE SLOW MOTION WORKS because it is very different from video style slo motion.

Let's start with film, since that is where the concept of slowing down action began. It's important to note that film has a time base of 24 frames per second. This means that a camera and a projector are both "clocked" to capture and playback 24 frames in one second.

The projector has a constant timebase in that the projector always plays back 24 frames per second.

As camera technology began to improve, it was discovered that if you moved the film through the camera faster while recording, say at 48 frames in one second instead of 24, then when it played back on the projector, the footage played back at half the speed. Twice as many frames were being recorded in one second giving you slow motion effects.

Interesting fact: the original standard for film was going to be 48 frames per second but it literally doubles the cost, size and weight of everything, 24 frames was chosen because it had the most tolerable amount of flicker that the human eye can handle. Now, with digital, people are beginning to think about 48fps again for 3D films.

A RED camera treats over cranked footage exactly as film. This is how it works:

• The timebase is 23.98 (film is 24)
• The number of frames you shoot in the time base is the over or under crank. Let's use 72fps as an example.

The RED captures 72fps (in 23.98 timebase) instead of the regular 24 fps in (23.98 timebase) so that when you play back the footage in a 23.98 timeline, you see it plays the 72 frames per second in a 23.98 time base the way film does. For example, if you record for one second at 48 frames per second it takes two seconds to play the footage back. Here is a chart to help illustrate this:



Slow Motion: time in seconds of footage recorded at different fps compared to acquired time in seconds of footage in slo mo.

• 1 second @24 FPS= 1 second clip duration plays back for 1 seconds in a 23.98 timeline
• 1 second @48 FPS= 2 second clip duration plays back for 2 seconds in a 23.98 timeline
• 1 second @72 FPS= 3 second clip duration plays back for 3 seconds in a 23.98 timeline
• 1 second @96 FPS= 4 second clip duration plays back for 4 seconds in a 23.98 timeline
• 1 second @120FPS= 5 second clip duration plays back for 5 seconds in a 23.98 timeline

It is evident that with RED or FILM, over cranking the camera is actually capturing more frames in one second. This is the exact principle the SUPER high speed cameras like Weiss Cam and Phantom use, as well. These cameras are capable of capturing up to 1000 or even 4000 frames in one second. That means (with enough light) they can "see" a bullet flying through the air.

Film works in exactly the same way. It captures more information in the same time base. One second is always one second, you are just telling the camera to capture MORE FRAMES in that one second, so in film and RED you are simply capturing more information and you do not need to do anything other than drop the footage into a 23.98 time base.

What happens in a 5D (or Panasonic HVX 200) is that they use the sensor's 60 frame clock speed to capture the extra frames. While the footage is being recorded, frames are marked for processing. Through a special process, the footage is extracted and the frames playback as slow motion in a 23.98 timebase. (99.9% of all video based editing is 23.98 and NOT 24fps).

There are other methods of extracting slow motion from shot footage which involve post production tools, such as Adobe After Effects with retiming software like Twixtor from RE:Vision Effects. This combination can produce super slow motion effects but it is all with interpolated frames. This is where capturing slow motion and creating slow motion vastly differ. In addition, while Twixtor is a powerful tool, not all footage is suitable for Twixtor because it uses color maps and intelligent pixel interpolation to draw the frames that do not exist.

Another in-camera method is to capture at 59.94. Shane Ross offers an excellent video tutorial podcast at the COW that shows you how to convert 720p60 footage into very smooth slow motion using the CONFORM option of Cinema Tools. My advice when capturing 59.94 is to capture progressive frames; this way the footage can be interpolated back to 23.98, a pulldown can be added and a slow motion effect can be created by telling the software that the footage is 23.98. When it sees 60 frames, it plays them back at 23.98 speed. Here again, the limitation is 60fps slow motion. If you wanted to make that 120fps slow motion, you would only be doubling frames instead of playing back 120 unique frames -- you would be playing back every frame twice to simulate a slow motion effect.

There is a big difference between in-camera Film Style Slow Motion and this method. Remember that with film style slow motion, the camera is capturing every frame, whereas in post-produced slow motion, the frames are being interpolated.

I hope this clears up an often-asked question here at the COW, and helps those who may not have worked with film understand how slow motion works in the digital environment.

My best advice before you go out and shoot: test and consult.


Links -- Check this out! http://vimeo.com/10567176
http://www.youtube.com/watch?v=kM17cKy6SwA
Or this for some under crank: http://www.youtube.com/watch?v=zskO9O3hF78

 

---

 

David Battistella Italy David Battistella is a Writer and Film Maker. Visit www.davidbattistella.com to learn more.