When you compress time in a video, you are making the duration shorter than real-time. Time compression is at the heart of video editing and is used in most productions to some extent. It is a critical skill — any good editor must understand how it works.
There are many techniques you can call upon to compress time. Here's a look at a few of the most common ones...
Time Lapse (Fast Motion)
The simplest way to compress time is simply to speed up the video clip. This is great for scenes where the action takes place in a single location over a long period of time, for example, a building construction. Fast-motion can also be used with a moving camera, although most editors would tend to use time remapping rather than a single overall change in speed.
As mentioned previously, time remapping allows the editor to smoothly vary the speed of a video clip. When used for time expansion, the video is slowed down. When used for time compression, the video is sped up.
For example, imagine a scene in a travel documentary in which a presenter walks around a tourist attraction. Traditionally the video scene would uses cuts and b-roll footage to get from one part of the attraction to the next without showing the walking between them. With time remapping, the scene could be real-time as the presenter explores one part of the attraction, then as they walk to the next part, the camera keeps recording and the action is sped up. The audience experiences the entire walk-through but much more efficiently.
Careful use of cuts can depict passing time. This is one of the most important time-compression techniques to understand — it is ubiquitous in film and television production.
Using cuts really just involves careful planning to get from one shot to a shot that is later in time, without looking uncomfortable.
For example, let's say the subject of a video sequence is "children getting ready for school". Rather than including shots of every step in the process, the sequence can use half a dozen carefully chosen shots to represent the whole sequence. A shot of breakfast being prepared can lead to a shot of it being eaten — you don't have to include finding cutlery, carrying it to the table, etc. In fact you probably don't need to include the preparation either — if you see someone eating breakfast you subconsciously fill in the preceding part where it was prepared. It is implied and doesn't need to be spelt out.
Place shots of different subjects in sequence to help pass time. For example, a shot of one person waking up is followed by someone else in the kitchen, then the first person walking into the kitchen. The viewer didn't need to see the first person getting out of bed or putting their dressing gown on.
It's a good idea to begin new shots with the subject out of frame. For example, a shot of the mother combing the kids' hair can be followed by an outside shot of the house where the door opens and the subjects emerge carrying their bags. The viewer subconsciously fills in the gap where the comb was put away, the kids put their backpacks on and the mother picked up her handbag.
Editing: Other Transitions
Some video transitions imply a change in time. For example, you can insert a quick dip to black (fade to black and fade back up again) into a shot at a point where you remove a section. This is common in interview shots — the viewer understands that you've jumped forward in time slightly. The dip to white is also common, although this is often used to depict a jump to a different time (e.g. flashback).
A slow fade to black and back up into a different scene suggests that more time has passed than in a quick fade.
Editing: B-roll Footage
B-roll footage (shots other than the main action) can be used to cover up removed time in the same way as a dip to black. For example, in an interview you can remove unwanted sections and cover up the gap with cutaways and noddies. In the "getting ready for school" example, a shot of the kids cleaning their teeth can lead to a cutaway of a clock, then a shot of hair being combed.
Summary & Final Notes
Don't always rely on the same techniques to compress time, to the point where you become predictable or artificial. The video will look silly if every single shot begins with the subject out of frame. Mix it up and use different ways to achieve the same time-compressing effect.
Different situations call for different levels of time manipulation. If you are telling a story that takes place at school, you will probably want to compress the time spent getting ready for school a lot. However if the video is about parenting skills you might want to spend a lot more time showing the tasks involved in getting ready for school. Different details for different audiences.
Of all time-manipulation techniques, the most important and most difficult to master is time compression. Thinking about ways to shrink time and practice different methods. You will probably find a few tricks that you can pull off well and consistently but you do need to strive to have a variety of options. This is one of the defining skills of a video storyteller so make sure you invest the effort to get it right.
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