03 June 2012

Time-lapse: what is it?

When we think of a time-lapse clip, we think of seeing action at high speed. Plants growing, clouds passing overhead, buildings being constructed, vegetables rotting. What we are seeing is a disparity of time; the rate of playback of the action is different to the rate it was shot. Action that really takes hours or days or months is being seen in a few seconds or minutes, and it can be captivating in a way unlike live action footage.

I'm writing a series of posts about time-lapse. How it is shot, some techniques and why to use them, how to achieve certain effects during shooting and in playback, and so on. Hopefully it will be useful to introduce time-lapse to new fans but go over things thoroughly enough to be of interest to experienced photographers.

I work for a time-lapse production company, Site-Eye, who films projects from any duration of half a day to several years. We use sophisticated camera systems, and my job is to help build them and install them, and occasionally I edit the footage when a project is finished.

A question of time

In regard to this disparity of time, time-lapse is related to slow motion. Both time-lapse and slow motion shoot at a given rate and playback at a different rate. If you're unfamiliar with regular filming (which I refer to here as live action), this is very unusual; a clip captured at a given rate is almost always played back at the very same rate, in order to recreate the motion as it was performed in front of the camera.

For time-lapse the shooting rate is very slow, capturing relatively few images of the action, and playback is at normal speed hence we see fast action; for slow motion the shooting rate is high, capturing more images of the action than usual, and playback is at normal speed hence we see slow action.

Precariously-stretched metaphor:
Imagine keeping a diary, but you only wrote a short sentence each week, just a few words. Ignoring the dates of the entries and reading a few years' worth at a time, the reader might presume you lead a non-stop frenetic life full of hundreds of activities a day.

These differing rates are measured as frame rates, usually in frames per second (fps) [note1]. (The techniques are often confused due to some overlap of their terminology; 'slow motion' refers to the slow playback of the action, but in order to capture this action the filming is run at high speeds, so it is also known as 'high-speed filming', which some mix up with time-lapse. For simplicity's sake we'll use 'slow motion'.)

What counts as time-lapse

The definition of time-lapse is therefore difficult to pin down to specific criteria. How is it different to a regular film clip that is speeded up in post-production? How slow does the shooting frame rate have to be, in order to be considered as time-lapse? [note2.] For my own purposes I consider time-lapse as a shooting process where abnormal time has lapsed between shots, which therefore excludes the practice of speeding-up a live action clip (see photography notes below). But what is defined as 'abnormal time'? Live action shooting (i.e. shot at a regular frame rate with the intention of playback at the same rate) takes pictures at a fast rate, just a fraction of a second between each; 24fps means a picture every 24th of a second, or "1/24th sec." So is abnormal time considered to be just a bit slower, say 1/10th sec? In my personal opinion I would say time-lapse is shot at 1fps or slower; 1fps yields a 25x speed-up during playback at 25fps. Of course the shooting rate can vary wildly depending on the subject and the scene; clips of fast movement, such as people walking nearby, would need to be shot at 1fps in order to capture relative movement at all (as opposed to people appearing and disappearing, or flickering, from frame-to-frame), alternatively slow action such as a building project might be shot at one frame every ten minutes, which for comparison is a 15,000x speed up during playback at 25fps.

A brief note on cameras

Most time-lapse shooting of any scale (i.e. to record events that are longer than a few minutes, which is almost always), is achieved using a stills camera. While it is possible to shoot a live action clip and speed it up during the edit, using a stills camera offers a range of advantages, such as efficient use of memory card space. A timer gadget fires the shutter at an interval of the photographer's choosing, so the filming is automated to some extent.

Coming up

In future posts I will be going into more detail about shooting time-lapse, covering how to choose time-lapse variables to suit the subject, such as frame rate, shutter speed and exposure, and also how to process the footage in post-production. If you have any questions along the way, give a shout in the comments below.

Note 1. Frames per second is a measurement used throughout film making. The general assumption is that the playback of a clip will be at the same frame rate as what it was shot at, for example clips shot at 24fps will be played back at 24fps. For slow motion clips, specialist camera equipment is often used to shoot at up to 1000fps (but some consumer cameras can shoot 60fps). But the clip will be played back at 24, 25 or 30fps (depending on the region and the distribution format). The term 'frame rate' takes on a different metric when discussing time-lapse shooting; instead of frames per second, it is more often measured in seconds per frame, or in some cases minutes per frame. Strictly speaking this term should be seconds between frames, i.e. the time lapsed between frames, commonly known as 'the interval' which will be discussed in a future post.

Note 2. Time-lapse is shot at a very low frame rate. Consider that persistence of vision (the phenomenon of the optic nerve that tricks the eye into seeing 'moving pictures' instead of a series of distinct still images) can be achieved at a playback frame rate of 18fps or higher; I'm drawing a line in the sand for the purposes of this discussion — any rate between 18fps and the usual playback rate of 24-30fps cannot be termed time-lapse, since it was not shot at a rate faster than that of a convincing moving picture.

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