Persistence of vision

persistence-vision-1Before we get on to the nitty-gritty of animation timing it might be useful to briefly cover the principle that underpins film and animation, and to understand how the illusion of movement is achieved where none is actually present. This marvellous phenomenon is known as the persistence of vision and it is through this that we experience moving images made up of individual frames on a film strip.The secret of this illusion is to be found in the remarkable capability of a part of the human eye, the retina, of momentarily retaining any image it receives. Imagine, if you will, a light being shown into the eye only briefly and appearing on the retina as a bright spot. This bright image would appear to remain for a brief period even after the light had been turned off. It’s this slight period of retention or delay that allows for separate sequential images, if seen in quick succession, to appear as a moving image, and it’s upon this principle that film and video projection works. Although this phenomenon had been observed in ancient times, it wasn’t until the systematic experiments in 1765 by the Frenchman, Chevalier Darcy  that it was established that this retention period was approximately one-tenth of a second. The early optical devices that were developed and began to appear in the first half of the nineteenth century clearly demonstrated this effect. What started out as serious scientific investigation soon found a practical application for entertainment through the use of such devices as the thaumatrope, the zoetrope, Joseph Plateau’s phenakistoscope and Emile Reynaud’s praxinoscope. Variations of these quickly began to appear as popular parlour toys in the homes of the upper classes throughout Europe.


Frames per second
All animators, irrespective of what discipline they work in (2D classical animation, stop-frame animation or computer animation)24 Frames and despite what work they undertake (commercial, studio-based or experimental animation), all use the same basic raw material to create their work – time. They use this in much the same way as a painter uses paint or a sculptor uses stone, and while this raw material shapes and defines their work, what they choose to do with it is another matter. The use of time varies from animator to animator, just as paint does with a painter, and it’s this varied approach and use of time that helps make the variations in stylistic execution of animation, be it funny or tragic, naturalistic, cartoon or abstract. The basic unit of time we deal with as animators and film-makers is determined by the recording and projection rate of the individual film frame or video image. This rate is commonly known as frames per second (fps).

While this playback or projection rates have varied since the development of cinematography, the standard recording and playback rate for film and video usually equates to:

● Film – 24 frames equals 1 second.

● Video – 25 frames equals 1 second (PAL).

● Video – 30 frames equals 1 second (NTSC).

These figures become critical in achieving the illusion of motion. As we have already established, the persistence of vision retention rate is around one-tenth of a second – much longer than the fps projection rates for film or video. If the projection rate was below that of the retention rate on the retina, the sequence of images would appear jerky and as individual images, and the illusion of movement would be lost. Because animators determine the speed of action of all they animate by creating animation timing, rather than recording movement as in live action, it is possible to achieve all of the variable animation timings they could desire. Working within these normal fps rates does not constrain the creative potential for animation, other than on purely technical issues, and is generally not noticeable to the eye. By comparison, live action film-making depends upon ‘recorded’ time as opposed to ‘constructed’ time. The timing of action is recorded not ‘created’ and is an automated process dictated by the film equipment; therefore, it is often necessary to use other techniques involving variable fps rates to achieve the desired effect. By recording/filming at one speed and playing back or projecting at another, it is possible to achieve slow motion or speeded-up actions.