LESSON 84
The 180-degree shutter in depth
Motion blur, measured in degrees.
IN THIS LESSON
- Where the degrees come from
- Turning degrees into shutter speed
- Narrow and wide angles
- Practice: when to break the rule
Where the degrees come from
Film cameras use a spinning half-disc in front of the film gate. While the open part passes, light reaches the frame; while the solid part passes, the film is pulled down to the next frame. The size of the opening is measured as an angle, and that angle is a share of the full frame interval: an angle of 180 degrees means the film is exposed for half of each frame's time. A setting of 180 degrees is treated as normal, and the blur it records is what audiences have come to accept as natural movement in a film.
Turning degrees into shutter speed
The relation is simple: exposure time equals the angle divided by 360, multiplied by the time one frame lasts. At 24 frames a second one frame lasts 1/24 of a second, so 180 degrees gives 1/48. At 25 frames it gives 1/50, and at 30 frames it gives 1/60. Most stills cameras cannot set 1/48, so at 24 fps people use 1/50, which works out at about 173 degrees, close enough that nobody can tell. The table below gives the common pairs, worked out from that formula.
Narrow and wide angles
A tighter angle shortens each exposure, so moving things are sharper and motion starts to look choppy and staccato. War and action films have used tight shutters to make flying dirt and debris look crisp, and Janusz Kaminski used special shutter tricks on Saving Private Ryan to echo the look of wartime newsreels. A wider angle lets in more blur and makes movement smear and float. Digital sensors have no spinning disc, so they can expose for almost the whole frame, and some can even gather light across several frames, which film cameras could never do at sound speed.
Practice: when to break the rule
This is working practice. Stay at 180 degrees for dialogue, walking and most drama, so that movement looks the way viewers expect. Go narrower, 90 or 45 degrees, when you want tension, rain that glitters or a fight that feels sharp and nervous. Go wider, up to 360 degrees, in very low light when you need every bit of exposure, or for a dreamy smear. Remember that the shutter also controls light: halving the angle loses a stop, which you must win back with aperture, ISO or lamps.
| Frame rate | 180 degrees | 90 degrees | 360 degrees |
|---|---|---|---|
| 24 fps | 1/48 s | 1/96 s | 1/24 s |
| 25 fps | 1/50 s | 1/100 s | 1/25 s |
| 30 fps | 1/60 s | 1/120 s | 1/30 s |
| 50 fps | 1/100 s | 1/200 s | 1/50 s |
Try it today
- 01Set your camera to 25 fps and 1/50, then film a hand waving.
- 02Repeat at 1/200 and at 1/25, keeping exposure equal with ISO or aperture.
- 03Watch the three clips side by side at full size.
- 04Describe in one word how each one feels.
- 05If your camera offers a shutter angle mode, switch it on and compare.