Apple Final Cut Express HD - Manual (Page 1049)

Appendix B Frame Rate and Timecode 1049
XIII
Early television systems used a different approach for the same result: increased flicker
without increasing the necessary electronic bandwidth. Interlaced scanning fills a
television frame with only half the video lines of a frame (this is known as a field), and
then fills in the remaining lines (the other field). A field effectively fills the television
screen with an image, even though it is only half-resolution, and it does so in half the
time it would take to draw the full frame. The result is a perceived frame rate which is
double the actual frame rate. For NTSC, the frame rate is 29.97 fps, but the perceived
frame rate (the field rate) is 59.94 fps. This causes less flicker. PAL, which has a lower
frame rate of 25 fps (or 50 fields per second) has a slightly more noticeable flicker.
Frame Rate Limits: How Many Frames per Second Is Best?
When recording an object in motion, there are practical reasons to limit the camera
frame rate:
 The limit of human perception: There is no reason to show more frames per second
than the viewer can perceive. The exact limit of human motion perception is still up
for scientific debate, but it is generally agreed that there is an upper threshold after
which people can’t appreciate the difference.
 Media cost and size: Film and video tape stock cost money. Higher frame rates require
more footage, and are more expensive to shoot. Editing and media management
become more difficult as the amount of raw media increases.
Recording High Frame Rates for Slow Motion Effects
Despite the increased cost and effort, there are cases where shooting higher frame rates
is useful. Slow motion effects are created by recording hundreds of frames per second
and then playing the same frames back at a slower rate. For example, a bullet shattering a
light bulb may take only a fraction of second, seeming almost instantaneous to anyone
watching. If a camera records the light bulb a thousand times per second and then a
projector plays the frames back at 24 fps, the movie on screen will take almost 40 times
as long (1000 fps / 24 fps = 41.6 seconds). The higher the frame rate, the more temporal
(time) resolution your footage has, which means it can be slowed down to show detailed
moments that would otherwise be a blur. Shooting at high frame rates also requires more
light, because there is less time to expose each frame.