Movie Making Manual/Effects/Slow Motion
Slow motion is the process of remapping time in film. Slow motion is generally used in dramatic or action scenes in Movies or Science and Nature shows on TV.
No matter whether you are shooting slow motion or faking slow motion, some mathematics is first required to calculate the frame rate needed. First, you need to choose the frame rate you are displaying in. For this example, we will choose 25fps. Then you want to to estimate the time of the event, and the time you want the event to be displayed for.
- Projection rate is 25fps.
- Event is 5 seconds long.
- Event will be on screen for 20 seconds.
Plugging this in, we get:
We can see that the frame rate we need to shoot at to meet our requirements is 100 FPS'. From this, the footage can be slowed down, slow motion is achieved.
This equation can also be used for calculating the interval between frames when shooting a time lapse.
Shooting Slow Motion
Shooting slow motion is generally the easiest method to accomplishing slow motion as little to no processing work is needed, however, this is usually the most expensive. Expensive cameras are generally required to shoot higher frame rates as cheaper consumer cameras do not have this capability. Film is still a common option for some as it doesn't require large memory cards however film needs to be reloaded. Some of these digital cameras include;
- Arri Alexa & Amira (200fps @ 2K)
- Red Epic (Up to 300fps @ 2K)
- Vision Research Phantom (Up to 1,052fps @ 1920x1080 OR 4,410fps @ 256x256)
- Slit Camera (Up to 1 trillion fps)
Light is another problem when shooting Ultra Slow Motion as higher shutter speeds are needed. If shooting at 1000fps, a minimum shutter speed would have to be 1/1000th, however 1/2000th is more ideal. For this reason, expensive lighting may be needed to light a scene. Ordinary house lights are not advised as the refresh rates may cause flickering during a video, A diffuser can lessen this effect, but a constant source like sunlight is preferred.
Don't be tricked into thinking higher frame rates are better, if the resolution of each frame is four megapixels at record rates of 1000 frames per second, after one second, you wil have 300mb+ and 40 seconds of data for a lossy compression. For this reason, larger and faster recording devices are needed which cost more money. This is extremely important when shooting at higher resolutions, larger colour spaces and faster frame rates. This is another reason why film is preferred when shooting slow motion.
NB: A slit camera is an expensive piece of science equipment used for medical imaging but has recently been modified to capture photons. For this reason, it is impracticable for movies.
Interpolation is the process of letting the computer draw in extra frames to smooth out video footage. Interpolation has other uses other than slow motion, these include Matrix Bullet Time, animations and can help remove drop frames.
When preparing to interpolate footage, it is helpful to remove motion blur by using higher shutter speeds and provide contrast between the background and subject. This means to film against plain backgrounds rather than complex patterns.
Even though you might follow all of these steps, warping may still appear, which can wreck your footage however if it does work, it is a cheap way to create that slow motion effect.
Software that supports Interpolation;
- Twixtor (Plugin)
- After Effects (Frame blending)
Fake Slow Motion
Fake Slow motion is the acting slow instead of actually filming in slow motion. This cuts out a lot of movements that can be accomplished through slow motion like jumping, but it is the cheapest way to get a slow motion effect with no budget. Avoid shooting with naturally moving objects such as fans, trees or near a road, these can ruin the effect and make your footage look amatuerish.