Which shutter won't flicker?

How it works

A lamp on AC power brightens on both halves of every cycle, so the flicker a camera sees runs at twice the mains: 120 times a second on 60 Hz power and 100 on 50 Hz. Tungsten pulses a little; fluorescent tubes, sodium lamps, magnetic-ballast HMIs and many cheap LED drivers pulse a lot.

A shutter adds up the light for as long as it is open. When that time holds a whole number of flicker cycles, every frame catches the same amount of light wherever in the cycle it starts. When it holds two and a half, the leftover half lands on a bright part of the wave in one frame and a dark part in the next, and the picture pulses or rolls with bands. That is why 180° at 24 fps, which is 1/48 of a second and 2.5 cycles on 60 Hz power, is the classic problem in an American office, and why 144° (1/60) fixes it.

The tool counts a shutter as whole when it is within half a percent of a whole number of cycles, and never counts one shorter than a single cycle. Half a percent lets the 23.976 and 29.97 fps rates through at their usual angles, which are a tenth of a percent off, and refuses 1/48 on 50 Hz power, which is four percent off and visibly pulses. This is the rule in Cutvey Meter's own code (FlickerSafety), with the same tolerance and the same choice of the shorter shutter when two are within a degree of each other.

What it does not cover

The rule assumes the light flickers at twice the mains. Plenty of lights on set do something else:

• A flicker-free ballast on an HMI, or a good LED driver, does not pulse with the mains at all. Any shutter is fine there, until someone dims it.
• LEDs dimmed by pulse width (PWM) pulse at whatever rate the driver picked, often hundreds or thousands of times a second. A mains setting cannot describe that, so test on camera.
• A cheap driver that flickers at the mains itself, once a cycle, is safe only at every other shutter in the table: 1/60 and 1/30 on 60 Hz power, 1/50 and 1/25 on 50 Hz.
• Screens and LED walls refresh at their own rate. Set the camera for the wall's refresh rate, whatever the mains is.

So use the table to set a shutter that should be safe, then check it: a slow pan across the light on the monitor, at the frame rate you will shoot, shows banding faster than anything else.

On set

You are shooting 24 fps at 180° in a US office lit by overhead tubes. Type 60 Hz, 24 fps and 180: the tool says 1/48 may flicker and 144° (1/60) won't. The cost is light. Going from 180° to 144° takes away 0.32 of a stop, so open the lens a third of a stop or pull an ND 0.3 and let the residual sit. Shoot the same scene in London at 24 fps and the safe shutter is 172.8° (1/50) instead; switch to 25 fps there and 180° is already safe.