Shutter Angle vs Shutter Speed

Angle vs speed

A film camera's shutter is a spinning disc with an opening cut into it, and the size of that opening in degrees is the shutter angle. A 180° shutter is open for half of every frame, whatever the frame rate. Shutter speed is the same thing stated as a time: at 24 fps, half a frame is 1/48 second.

Cinema cameras default to angle because it keeps the look constant when the frame rate changes. Ramp from 24 to 48 fps at 180° and each frame is still exposed for half its interval, so motion blur per frame looks the same; the exposure time halves, though, and you lose a stop. Stills-derived cameras default to speed. Either way, you need to convert between the two, and you need to know which settings will make the lights flicker.

The conversion

Shutter angle to shutter speed

Exposure time for common angles. The fractional rates (23.976, 29.97, 59.94) give fractional speeds, shown to one decimal place.

Shutter speed to shutter angle

The angle a given shutter speed needs. A dash means the time is longer than one frame, which no camera can do; 360° itself is out of reach on most digital cinema cameras (the ALEXA 35 stops at 356°).

Choosing an angle

The 180° shutter is the long-standing convention, and audiences read its motion blur as normal. Moving away from it is a creative choice with an exposure cost or gain.

• Narrower (90° or 45°). Less blur per frame, so fast motion looks crisp and staccato. 90° costs one stop, 45° two.
• 172.8° and 144°. Almost 180° to the eye. Used at 24 fps to make the exposure a whole number of light pulses: 1/50 on 50 Hz and 1/60 on 60 Hz.
• Wider (270° to 356°). More blur and more light: up to about one stop more than 180°. Useful in low light, or for a smeared, dreamlike movement.
• Very narrow (under 20°). Freezes motion for VFX plates or sports, but needs a lot of light: 22.5° at 24 fps is 1/384 second, three stops under 180°.

Each halving or doubling of the angle is one stop, so the exposure math is the same as for any other control. The exposure triangle guide shows how to trade a shutter change against the stop, the EI or ND.

Why lights flicker on camera

Mains power alternates, and many lamps brighten and dim with it, peaking twice per cycle. Light from them pulses at twice the mains frequency: 100 times a second on 50 Hz power and 120 times a second on 60 Hz. Fluorescents dip to about 35% between peaks; some LEDs dip almost to zero. Incandescent filaments flicker less because they stay hot between peaks.

The eye does not see 100 or 120 Hz, but a camera can. If each frame catches a different part of the pulse cycle, the frames differ in brightness and the picture flickers or rolls. There are two ways to stop it:

1. A frame rate that divides the pulse rate. 24 and 30 fps on 60 Hz (120 ÷ 24 = 5, 120 ÷ 30 = 4), or 25 and 50 fps on 50 Hz. Every frame starts at the same point in the cycle, so most shutter settings are safe.
2. An exposure time that is a whole number of pulses. 1/120, 1/60 or 1/40 second on 60 Hz; 1/100, 1/50 or 1/33.3 on 50 Hz. Every frame collects the same amount of light whatever the phase. This works at any frame rate below the pulse rate and tolerates small drifts in the supply.

Flicker-free angles on 60 Hz

For North America and western Japan, among others: 120 light pulses a second, so exposures of 1/120, 1/60, 1/40 and 1/30 second. Angles of 359.6° and 360° are left out: they are above the 356–358° maximum of most digital cinema cameras.

At 23.976 and 29.97 the pulse rate is almost, but not quite, a whole multiple of the frame rate. The phase drifts by 0.12 pulses a second, so with an angle that is not on the list, brightness can rise and fall in a slow cycle about every 8.3 seconds. A listed angle avoids it; 144° at 23.976 is 0.1% longer than 1/60, which is close enough.

Flicker-free angles on 50 Hz

For Europe, most of Asia, eastern Japan and many other places outside North America: 100 light pulses a second, so exposures of 1/100, 1/50, 1/33.3 and 1/25 second. Check the supply wherever you shoot: Japan alone has both. As above, 360° is left out.

This is why 172.8° is the usual 24 fps setting in 50 Hz countries: at 180° each 1/48-second exposure catches 2.08 pulses, and the leftover fraction lands at a different point on every frame.

Next steps

• See the shutter angle, T-stop, ISO or ND that compensates any change in stops with the free exposure calculator, in your browser.
• Trade a shutter change against the stop, EI or ND in the exposure triangle guide.
• Work out the ND to hold your stop once the shutter is set, with the ND filter chart.
• Plan high-frame-rate shots and their playback speed in the slow motion guide.

CineMath's free Exposure calculator has 172.8° among its shutter presets and shows the angle that compensates any change in stops, so you can check the cost of a flicker-free angle before you set it.