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← Twenty-Four a Second

Twenty-Four a Second

Three short lessons on the numbers inside film: frames per second, aspect ratios and black bars, and how slow motion is made.

Spark & Whetstone · sparkwhetstone.org/learn/twenty-four-a-second · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. Frames per second

Film shows a run of still pictures fast enough that the eye sees motion. Cinema has used 24 pictures a second for about a century; a ten-second shot is 240 pictures.

Animators often draw "on twos", holding each drawing for two frames, so a second of animation needs 12 drawings, not 24.

Question. An animator draws on twos at 24 frames a second. How many drawings for a 5-second scene?

  1. 240 drawings
  2. 120 drawings
  3. 60 drawings

Say it back. In your own words: why does drawing on twos halve the work?

Lesson 2. Aspect ratios and black bars

A frame’s shape is its aspect ratio, width to height. Televisions are 16 : 9 (about 1.78 : 1); many cinema films are 2.39 : 1, much wider.

Fit the wide film to the TV’s width and its height comes out smaller than the screen, leaving black bars above and below. The bars are the difference between the two ratios.

Question. A 2 : 1 film fills the full width of a 16 : 9 screen. About what fraction of the screen’s height does the picture use?

  1. All of it
  2. About half
  3. About 89%

Say it back. In your own words: where do the black bars on a wide film come from?

Lesson 3. Slow motion

Slow motion is filmed fast and played normally. Film at 120 frames a second and play at 24, and every real second takes five seconds to watch.

The slowdown is the ratio of the two frame rates. Phones that film at 240 frames a second slow things ten times when played at 24.

Question. A camera films at 96 frames a second and the clip plays at 24. How long does a 3-second real jump take to watch?

  1. 12 seconds
  2. 3 seconds
  3. 72 seconds

Say it back. In your own words: how does filming fast make slow motion?

At the table: A sticky-note flipbook

Players: One or more, ages 5 and up with a grown-up. You'll need: A pad of sticky notes, a pencil.

  1. On the last page draw a ball at the bottom. On each page before it, draw the ball a little higher, then a little lower: a bounce.
  2. Before flipping, guess how many pages a one-second bounce needs if you flip about 12 a second.
  3. Flip the pad with your thumb and watch the ball move. Then try drawing the steps closer together to slow it down.

Answers

  1. 60 drawings — 5 seconds × 24 = 120 frames, and on twos that is 120 ÷ 2 = 60 drawings.
  2. About 89% — At width 16, the film is 16 ÷ 2 = 8 high on a screen 9 high: 8 ÷ 9 ≈ 89%, with thin bars top and bottom.
  3. 12 seconds — 96 ÷ 24 = 4 times slower, so 3 seconds of jump takes 3 × 4 = 12 seconds on screen.