Apple Shake 4 - Manual (Page 193)

Chapter 6 Importing Video and Anamorphic Film 193
This effect occurs because video fields are recorded one after the other, just like frames.
When a moving subject is recorded using an interlaced video format, the subject is in
one position when the first field is recorded, and in another position when the second
field is recorded. When the video is played back, and each field is played in the correct
order, the image appears normal. However, looking at both fields together as a single
frame, the change in position can be seen to occur within the one frame.
The following images show field 1 and field 2 of a single frame as separate images.
Notice the black lines across the images—these are the lines that are filled by the
opposing field. This example clearly illustrates that each field contains only half of the
total image data in a given frame.
The following illustration depicts a close-up of the interlaced frame, with both fields
combined. Because of the subject’s change in position from one field to the next, the
fields appear offset.