- Humans cannot perceive the motion of the hour hand on a clock or the opening of a flower bloom because it is too slow. We cannot perceive things that move too fast either. For example, following a bullet or a baseball pitch may be too fast. The ways we measure speed of motion and our perception of it depend on a variety of factors.
- Our eyes are made up of cells that register the appearance, color, shape, texture and disappearance of a stimulus in our visual field. When progressive groups of cells register a similar visual stimulus moving across our visual field, our brain interprets that as smooth motion, rather than the successive appearance of several independent flashes, like you might perceive in an animation or old movie with a low frame rate (less than 12 frames per second).
- Most animations are set at 12 frames per second, and with video, the frame rate can vary from 30 to around 50 frames per second. There are other factors such as blurring and sampling that can increase the smoothness of recorded movement, but generally, the higher the frame rate, the smoother the motion appears. As the speed of the motion recorded approaches the frame rate of the recording, some motions (like a wheel with spokes) may appear to move backward or flash between only two positions. This has nothing to do with the speed of motion we can perceive.
- Since motion is perceived both by speed and distance, scientists measure our ability to follow and distinguish objects moving past our visual field by the amount of arc they cover in a fixed amount of time. Most people have difficulty registering identifying details about an object that moves faster than 36 degrees per second. Since your visual field is around 180 degrees, anything that crosses in and out of your visual field in less than five seconds starts to blur. Because the cells in your eyes get tired of stimulation after more than two or three seconds, anything that doesn't move significantly in that amount of time will be perceived as stationary.
