Flickering in video and film is a common problem resulting from certain frame rate and shutter speed combinations under artificial lighting. This article gives an overview of why this happens, along with recommending settings for minimizing the chances of this happening in the first place.
Although light sources might appear continuous to our eyes, many actually illuminate using high frequency pulses of light. Flickering results when the combination of shutter speed and frame rate captures different fractions of these pulses each frame:
This appears as distracting flashes in video and is almost always undesirable. The example below was captured under mixed lighting, so the flickering appears in regions which are primarily lit by artificial light:
The key is that many types of artificial lighting flicker at twice the rate of the power source. This is most pronounced with fluorescent lighting, but also happens to a lesser extent with incandescent. Common power sources are 50 Hz (in Europe and most of Asia) and 60 Hz (in North America), which cause 100 and 120 pulses of light each second, respectively.
Other than changing the lighting, one has two options for overcoming this problem:
Frame Rates. Choose a frame rate equal to the lighting pulse rate divided by some integer. This is often the simplest option since most corresponding shutter speeds will appear flicker-free.
Shutter Speeds. Choose a shutter speed equal to the lighting pulse rate divided by some integer. This option often isn't as straightforward, and can constrain one's ability to control motion blur. On the other hand, this option typically minimizes flicker under a broader range of lighting and power types.
For example, in North America, the standard 24 and 30 fps settings work with most shutter speeds (since 30x4 and 24x5 = 120).
Click here or the image above to go to the flicker free tool.
Results may vary depending on the characteristics of your particular lighting. For example, artificial lighting may flicker independent of the power source with some neon and LED lights, and when the bulbs are near failure or improperly seated. Also try to avoid high frame rates (greater than the lighting pulse rate) when possible, or use artificial lighting that provides continuous illumination. Regardless, always capture test footage to verify that everything appears as intended.
Another complication is that power doesn't always cycle precisely as expected. The frequency can vary by up to about 1% depending on power plant load, in which case there may be no shutter speed or frame rate that is a perfect divisor of the lighting pulse rate. In those situations, using a flicker-free shutter speed will usually provide the best results.
RED® CAMERA SETTINGS
With RED, two shutter controls are available: shutter speed and shutter angle. Shutter speed is expressed as a fraction of a second (when less than one second). The shutter angle is a term unique to motion capture, and describes the shutter speed relative to the frame rate.
Depending on the power frequency and frame rate, specifying a flicker-free exposure duration may require a non-standard shutter speed. For example, if one specifies an unusual 7 fps under 60 Hz lighting, the flicker-free tool above will suggest a 189° shutter angle. However, 189° does not have a standard fractional shutter speed equivalent (since it equals 1/13.333... seconds). In those cases, one can navigate to the Exposure menu, and manually enter an angle instead.
TIPS & ALTERNATIVES
One can often eliminate flickering by switching between the regional standard 24 or 25 fps recording frame rate (while keeping the playback rate unchanged). For example, when shooting in the UK for playback in the US, one could shoot at 25 fps (thus permitting any shutter speed), but then have this played back at 24 fps in the US. The only disadvantage is that the footage will appear roughly 4% slower, but this typically isn't noticeable.
Even if flickering cannot be eliminated entirely, its severity can usually still be reduced. In general, the intensity of flickering can be decreased by using a slower shutter speed. Similarly, the flicker frequency can sometimes be decreased by using a shutter speed which is as close as possible to a known safe shutter speed. Ultimately though, the only sure-fire way to eliminate flickering is to shoot under natural lighting or continuous artificial lighting.