Avoid Video Flicker: Understanding PAL (50Hz) & NTSC (60Hz)

How to avoid video flicker in film an video.

We were recently hired to produce a short documentary piece intended for broadcast in China. The project is to be shot and edited here in Canada, and later sent to China. The shooting specs for this project are as follows: 1080i50 in a .mov wrapper. Seems simple enough. We’ll go ahead and switch our camera’s mode from NTSC (60Hz) to PAL (50Hz), and we’re ready to roll! Right?

Unfortunately, it’s not that simple. Because we’re shooting for PAL in an NTSC area, it is important that we understand why NTSC and PAL standards exist in the first place. Until recently, I thought PAL and NTSC standards were simply a matter of preference, sort of like America’s Imperial system vs. UK’s Metric system. However, that is not the case at all. As it turns out, there is a very tangible reason for the opposing NTSC and PAL standards.

50Hz vs. 60Hz, a Little History

In the UK, Africa, Australia, most of Asia and Russia, the frequency of AC (alternating current) running through electrical mains is 50Hz (Herts). While in North America, and a few other countries (Japan uses both), the AC frequency is 60Hz. Why? There are many historical factors responsible for this divide but, in a nutshell, it comes down to economics. At the turn of the 21st century, to avoid competing with one another, manufacturers in America focused on producing 60Hz equipment while manufacturers in the UK focused on producing 50Hz equipment. Each region established its own monopoly and the rest is history.

It’s worth mentioning, 50Hz & 60Hz are not arbitrary frequencies. They were chosen for very specific reasons that go beyond the scope of this blog. But, to provide some perspective into it, certain lights tend to flicker when a low frequency current is run through them. If the frequency is sped up to 50Hz, the flickering, although still present, goes unnoticed by the human eye. This is one of the contributing factors to the popularity of 50Hz.

How does Household Electrical Frequency affect Video?

As mentioned, at 50Hz the flickering effect goes unnoticed by the human eye – the key word being human. The camera’s eye (its sensor) can still see this flicker. The sample footage below was shot in PAL, 1080i50, 1/50 shutter speed, at a school in Edmonton, Canada with 60Hz florescent lighting. While our camera operator was seeing a clean, flicker-free image, the camera itself was seeing something entirely different. Notice the grain and flickering effect over the dark blue curtain. Thankfully it was just a test shot!

What caused this flickering effect? Blame the hertz! (not really). The hertz is a unit of frequency that defines cycles per second. So, if the lighting in the school is running at 60Hz, this is just a fancy way of saying that the electricity flowing into the fluorescent lighting is cycling ON-OFF 60 times per second. While the human eye does not detect it, there is a subtle dimming of the light during each of these ON-OFF cycles. It is this dimming that our camera is seeing, and recording.

How to Prevent Flicker in your Video

It all comes down to synchronization. If you synchronize your camera to the electrical frequency of the lighting, you are basically telling your camera to only take pictures of the ON portion of each cycle within the frequency. So how do we do achieve synchronization? There are two options:

1. Change Your Camera’s Frequency Setting (NTSC or PAL)

Thankfully, many cameras today have the option to change frequencies between 50Hz (PAL) and 60Hz (NTSC). If your camera has this option, simply match your camera’s frequency with the electrical frequency of your environment. Once set to the correct frequency, you can safely use any of the frame rates or shutter speeds your camera offers. If you’re not sure what the frequency is where you are, here’s the Wikipedia link!

2. Change your Shutter Speed/Angle

If you can’t change your camera’s frequency, or if your client requires you to shoot in a specific frequency, as is the case for us, there is a workaround. You can sync your shutter speed to the electrical frequency of your environment. I created a couple tables below with some commonly used frame rates and corresponding safe shutter speeds/angles. If you don’t see what you need in the tables, you can use this handy shutter speed/angle calculator courtesy of the team at Red.

Shooting under 60Hz Lighting (North America)
Your Frame RateSafe Shutter SpeedSafe Shutter Angle
50p/50i1/60, 1/120300, 150
25p1/40, 1/60, 1/120225, 150, 75

Shooting under 50Hz Lighting (most of Europe & Asia)
Your Frame RateSafe Shutter SpeedSafe Shutter Angle
30p1/33.3, 1/50, 1/100324, 216, 108
24p1/33.3, 1/50, 1/100259.2, 172.8, 86.4

The sample footage below was shot on the same day, under the same 60Hz lighting, as the sample footage above. The camera was still in 50Hz mode (1080i50), however we adjusted our shutter speed from 1/50 to 1/60 to match the frequency of the fluorescent lighting. As you can see, the results are much better!