The Moving Image

The Moving Image

A film or video camera has a lens that focuses an image of the world onto a light-sensitive piece of film or a light-sensitive electronic chip. This part is much like the still camera.  But how do we capture movement? The impression of continuous movement in a motion picture is an illusion. A film or video records a sequence of still images (frames) in rapid succession. In film, the standard frame rate is 24 frames per second, written 24 fps. When the images are then displayed one after another on a screen in a theater, TV, or a device, if the frames in the sequence change from one to the next quickly enough and the differences between them are not too great, the brain perceives smooth, realistic motion.  This effect brings the magic of motion to film, video, or flipbooks.

The full picture of how the brain and eye perceive motion is still under debate.  We know that for a realistic viewing experience, we need to create the illusion of smooth motion and consistent illumination. If the images change too slowly from one to the next, the illusion falls apart.  Instead of smooth motion and consistent illumination, the screen may appear to flicker and get brighter and darker as the image changes.

Traditionally, this illusion has been explained by the persistence of vision, which is based on the idea that the eye retains an impression of each frame slightly longer than it is exposed to. According to this theory, when e a new frame is displayed, the eye blends it with the afterimage of the previous frame, creating a smooth transition between them. It is important to understand that this explanation is imperfect, as the aftermath image moves with your eyes. They don’t stay in one place if you look left or right.

A perceptual illusion called beta movement describes one situation in which viewers interpret successive still images as motion.  A static shot of a ball flashes on the left side of the screen, then on the right side, and viewers see it moving from left to right.  Think of a lightened ticker tape-style sign in a store, which seems to scroll from right to left across the display.