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KnowHOW: How to avoid flicker on camera

KnowHOW: How to avoid flicker on camera

KnowHOW: How to avoid flicker on camera

John Black looks into what causes flicker and how to prevent capturing it in the first place

At the time of writing this article, many countries around the world have instituted stay-at-home programmes as they struggle to contain and control the spread of Covid-19. Houses of worship, once gathering places both large and small, have closed their doors, scrambling to create new mediums and methods through which to reach worshippers stuck in their homes. While larger organisations may have already been broadcasting services live or on-demand, many smaller organisations have had to adapt quickly to the new reality.

In the transition to engaging their congregations in an electronic format, it may be that some have experienced a fairly common production issue when newly approaching video production: light flicker. This is possible regardless of if you are shooting video on a camcorder, DSLR, production camera or iPhone, and can be frustrating, especially if it isn’t noticed prior to a recording session and is later discovered when starting the postproduction process for editing and then delivery.

In this article, I’ll try and explain what causes flicker on camera. After all, understanding the cause behind it is the first step in avoiding it in the future. We’ll then look at some things that you can do to help prevent capturing flicker on camera in the first place.

Hopefully, by the publishing of this piece, Covid-19 might be behind us and you will have worked through any flicker issues in your services through trial and error or other resources. If not, this article will help you improve the video you capture and ultimately the experience for your electronic congregation. So let’s dive in.

What causes flicker on camera?

To understand what causes flicker on camera, one needs to understand the way in which light sources work to produce light. Camera flicker is an issue in situations using an artificial light source (in other words, not the sun). If you only shoot outside under natural light, you should not encounter camera flicker because the light source is consistent (the sun is always ‘on’).

With most artificial lighting, however, the light being emitted from a light source is actually turning on and off at a very fast rate. This ‘flicker’ is not perceived by the naked eye, but cameras can actually capture this phenomenon due to frame rates that are out of sync with the light source.

Why does this happen? Most lighting is powered using an AC (alternating current) power supply rather than DC (direct current). The result is that the filament in the lamp is turning on and off while powered ‘on’. The rate that this happens depends on where you are in the world, generally at either 50Hz or 60Hz (50 cycles per second or 60 cycles per second, respectively).

To put it simply, when shooting at certain frame rates matching the cycle of the powered light source, this flicker isn’t noticeable. However, as cameras have improved their quality and options to allow very high or specialised frame rates, it is possible that inconsistencies occur where the number of cycles a light source goes through are not the same for each camera frame captured. Therefore, flicker occurs.

Can you avoid flicker on camera?

The good news is that this is avoidable. Powered lights running on AC power will cycle consistently once they are powered on and set. By the way, I use ‘set’ because if you are using dimmable tungsten lights, this flicker may be more noticeable the less bright the lamp is set at. The frequency of the cycle is staying the same, but the voltage is decreasing. As a result, the filament doesn’t glow as bright, meaning that it also cools down (the ‘off’ stage) faster.

The great thing about shooting with most modern cameras is that you are able to control the frame rate at which the camera is shooting. If you are experiencing flicker capture, this is one of the first things that you can adjust to avoid it. In general, so long as your camera is set to a frame rate that is easily divisible by the power cycle rate (or frequency) in the location you are shooting (typically 50Hz or 60Hz), then chances are you will not experience flicker. For example, if shooting in a location where the power cycle is 60Hz, if you shoot at 30fps, 60fps or 120fps for slow-motion work, you shouldn’t notice camera flicker.

You may be wondering about 29.97fps or 59.94fps that cameras may actually be using. These frame rates allow footage to be used more easily in broadcast environments and came out of the initial need for live TV broadcasts to remain in sync with household TV sets. This changed in the move from black-and-white broadcasting to colour broadcasting. Without getting into the finer details, these new rates were altered to allow for backwards compatibility (to allow live, colour broadcasts to work with black-and-white TVs). This was the birth of drop frame timecode. If interested, Frame.io has an excellent article discussing these details titled Timecode and Frame Rates: Everything You Need to Know (https://blog.frame.io/2017/07/17/timecode-and-frame-rates).

In addition to checking the capture frame rate on your camera, another way to avoid flicker on camera is to look at what type of light source you are using. I mentioned earlier tungsten lights similar to what you would find in a stage lighting system in any house of worship. In those situations, capturing video uses frame rates that work with the local power frequency. However, if you are working with variable frame rates or at very high frame rates, you may need to consider the actual light sources themselves.

You may find that in many houses of worship, fluorescent light sources are in use. These light sources have a very lower flicker rate. This is because the light in a fluorescent lamp is created by an arc (the term ‘arc-source’ is given to lamps that operate in this manner regardless of fixture type) that produces a plasma inside the glass tube and causes the phosphor coating to glow. Therefore, if you are running into flicker problems and you have fluorescent light sources in use, try changing out those light sources with tungsten lamps.

The on-camera screen of a Nikon D5 digital SLR.
The on-camera screen of a Nikon D5 digital SLR.

What about LED?
After all, most LEDs don’t operate on AC power, but rather operate using DC. Though this doesn’t always guarantee flicker-free operation (due to troublesome power converters used for converting the AC to DC, for example), you are more likely to have success. The challenge that comes with LEDs is the ability and method used to dim them (something important when using LEDs in stage lighting). Even to the naked eye, when slowly dimming LEDs, some products will visibly ‘step’ or ‘jump’ between intensity levels. There are two main methods used for dimming LEDs: pulse width modulation and switch mode regulation.

Pulse width modulation works by introducing periods of time where the LEDs are ‘off’ (receiving no power). Over time, this makes them appear dimmer, but ultimately results in a pulse that can be detected as a flicker on camera. Switch mode regulation operates at a very high frequency and therefore operates virtually flicker-free, regardless of the frame rate you are shooting at.

When you are purchasing LEDs for your stage and you know that video is or will be incorporated into your services, be sure that the LED products you are looking at can dim flicker-free. You want a fixture that will allow you to create the looks you want to the naked eyes of the live congregation, as well as provide a high-quality visual product to your online congregation.

A Genaray LED-7100T 312 LED variable colour on-camera light.
A Genaray LED-7100T 312 LED variable colour on-camera light.

If you are regularly shooting on-location in a variety of settings, it may be worth looking into building your own lighting kit with ‘flicker-free’ lighting fixtures that you can take with you to ensure that, regardless of the location, you have a lighting kit available that you are comfortable working with to prevent flicker on camera. There is a large number of products available on the market at all price ranges that claim flicker-free operation at full intensity and during dimming.

Alternatively, if you shoot on-location but aren’t able to take a lighting kit with you, be sure that you do some testing to ensure that your frame rate is set to avoid flicker with whatever lighting is available. If you need help, RED has an excellent web tool called Flicker Free Video that will help you figure out what frame rate and shutter speed combinations should be safe to work on their website (https://www.red.com/flicker-free-video). They also have a free mobile app called RED Tools that is useful. 

Finally, a source of flicker you may not be thinking about is a TV, computer or other type of screen used in your video shot (perhaps as sermon notes or a digital whiteboard for a speaker). It’s important to remember that these screens are essentially light sources as well and have their own refresh rate. While this isn’t necessarily an issue with newer screens, if you do find that the screen is flickering in your camera capture, adjusting your shutter speed by slowing it down slightly will typically take care of that. Don’t forget that slowing your shutter speed will also affect the capture of everything else in the camera frame, so this must be done in small increments and checked.

Conclusion

I hope this has provided a bit of background insight into flicker captured on camera as well as provided you with a couple of tricks to try and avoid it. As a general rule that I try to follow, always test things out before you commit. This can prevent having to find time to redo potentially hours and hours of work.



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