Intro to Slow Motion Video
Slow motion video can provide new and interesting views of otherwise everyday events. This enhances their emotional impact, but also pushes the technical abilities of a camera system. In this article we’ll provide a background on how slow motion works in addition to discussing best practices.
Slow motion playback starts with high-speed capture. With early film, this was called “overcranking,” since the camera operator would literally crank the film reel through the camera more rapidly. This footage would then be played back at normal frame rates — stretching time and making motion appear slower:
With digital, slow motion works much the same way, except overcranking is accomplished when the capture frame rate is faster than the playback frame rate (or “time base”). Such a camera is said to be in “varispeed” mode, and the ratio between these rates represents the amount of slow-down. If a subject were captured at 96fps and played at 24fps, for example, then motion would appear at quarter speed:
The key is that true slow motion video requires both faster capture and slower playback. Using slower playback alone, which is perhaps one’s first inclination, would result in sub-standard frame rates and likely stuttered motion. Alternatively, using faster capture alone would create smoother motion, but at real-life speeds.
The degree of slow-down varies depending on application. A 2-4X slowdown is considered standard for replays or artistic effects, and often means a capture rate near 60-90 fps. This speed can also give the appearance of smoother panning and steadier shoulder-mounted footage, since high-frequency movements appear damped.
Capture rates near and above 300 fps are an opportunity to take the viewer to places their eyes have never been. Water splashes, flying insects, sporting moments, explosions and other split-second events can become visually accessible and even other-worldly.
Note how objects move more slowly across the frame in the tenth speed example on the left. For the same action, the more a subject is magnified (either by using a longer focal length or moving the camera closer), the slower that subject will need to be shown for the same effect.
RED® CAMERA SETTINGS
With RED, the suggested playback rate (or “project time base”) can be changed within the project settings menu (using the “Settings > Projects > Time Base”). Similarly, the capture frame rate can be changed using the top menu tab (as shown below). Whenever these two rates differ, the fps value at the top left will turn yellow, and the bottom right will display “Varispeed” to indicate that this mode has been successfully enabled:
ARTIFICIAL SLOW MOTION
Sometimes slow motion can be emulated with post-production software, using a technique where extra frames are interpolated in-between normal frames — enabling slower playback at standard frame rates. Software such as Twixtor can be helpful for this. However, just as enlarging a photo doesn’t create additional detail, slow motion interpolation doesn’t reveal additional high-speed behavior. These techniques are primarily aimed at masking stuttered motion, and ultimately don’t increase temporal resolution. They also require faster shutter speeds and predictable motion for full effect.
OTHER CONSIDERATIONS & BEST PRACTICES
Perhaps the biggest obstacles to high-speed capture are technical. In particular, data management and lighting require even more attention than they would otherwise. For example, higher frame rates increase storage space and media write-speed requirements, which in turn may demand either lower resolution images or higher REDCODE compression. Similarly, higher frame rates mean correspondingly faster shutter speeds, which in turn demands more intense lighting.
Additional considerations include:
- Audio. Sounds are unrealistic if not played back in real-time, so slow-motion footage virtually always needs to be dubbed over with music or sound effects.
Flickering. Artificial lighting is much more likely to flicker at higher frame rates; always try to shoot under bright daylight or continuous studio lighting when possible. See the tutorial on flicker-free shutter speeds for additional tips.
Motion Blur. Viewers typically expect to see minimal motion blur with slow motion, so shutter angles smaller than the standard 180° may be necessary. However, without extra lighting this may require larger apertures and higher ISO speeds, which may come at the expense of sharpness and noise.
Detail. Shooting at very high frame rates typically requires lower resolution. It’s therefore more critical than ever that these images have the highest quality possible. For example, light permitting, stopping the lens down by 1-2 stops will improve sharpness. In post, using the unsharp mask and denoise tools can also help. Most importantly though, use the highest quality lenses available.
Wide Angles. With RED cameras, shooting at lower resolutions means the outer portion of the frame gets cropped out — causing the image to appear as if it were taken with a longer focal length. For example, when shooting at 2K as opposed to 5K resolution, the effective focal length will increase by 2.5X (causing a 20 mm lens to appear as if it were 50 mm). See the interactive RED crop factor tool to see what happens with your specific settings.