Flicker

"I've just looked at my first shot and it's flickering! What happened?"

Flicker is one of the banes of stop motion. Flicker, in case it isn't obvious by the name, is unintended variation within the frame over the course of a shot, typically of exposure, so an occasional frame is darker or lighter than those surrounding it.

Isolating the cause of flicker can be puzzling. Sometimes the fix is easy, other times it's very difficult.

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Possible Cause: Camera Settings on "Auto"
Most cameras these days offer the option of automating everything. Some of them don't even offer the option of controlling functions manually. This automation can cause trouble for animators, because the cameras and their lenses may not be consistent. Still cameras are designed for producing acceptable still images, not for making movies frame by frame, and consumer-level video cameras are designed for easy, continuous use in varying environments. In still cameras, automated apertures don't close down to the same point every time. Meters may be fooled by different things in the frame. White balance can also shift depending on what dominates the frame at any moment. And focus is subject to the same variation.



In the above sequence, everything on the camera is set to automatic: ISO, aperture, shutter speed, and focus. Variations in exposure and focus are obvious.

Fixing the Problem
Turn every automatic option in the camera to "manual." Lock down the ISO, aperture, shutter speed, white balance, and focus to a single setting. If you need to change focus during a shot, make sure you have a camera with which this is easily accomplished. If you cannot manually control these critical parts of the camera, then you'll either have to live with the results or get a new camera.

Some camera systems—notably Nikon DSLRs—make it difficult to turn everything to manual. What people have done successfully with Nikon cameras is to mount the lens on the camera, but not completely engage the lens. In other words, they leave the lens slightly "unscrewed" so that the automatic aperture mechanism is not allowed to engage. You probably wouldn't want to do this for day-to-day photography in the field, but in the relatively controlled environment of a stop-motion shoot, it seems to be safe enough.

Possible Cause: Voltage Variation
If the circuit a light is on is shared by other electrical devices, when one of those devices comes on or goes off, it can change the voltage feeding your light, thereby affecting the brightness of the light. Your camera will record this variation, even if you don't notice a change.



In the above example, the lights (which are normal household tungsten lamps in this case) are on the same circuit as the small heater in the frame. Also visible is a light meter and a voltage meter connected to the same circuit. When the heater is turned on, voltage on the circuit decreases and light level drops.

Sometimes nothing else is on a circuit except the lights, and still flicker from varying voltage occurs. That's because we live in a big, wide world and we have to share the electrical grid with turbines of industry and folks who like their air conditioners set on max. In other words, the power you get from the power company may not be consistent.

Fixing the Problem
There are various ways of dealing with this problem.

One possible solution is to ensure that the lights don't share the circuit with anything else in the studio/home that may be operated during the shooting period, and then monitoring the voltage supply to determine if outside sources may be affecting the electricity. Some people simply stop shooting during unreliable periods. Voltage meters are relatively inexpensive (and have other uses around the studio).

Others install a variable resistor, a.k.a. a variac, which allows them to control the voltage going to their lights to a limited degree. They will set the variac voltage lower than their main power (i.e., if their normal supply is 120 volts, they'll set it to 105 volts), which gives them a buffer. When the main supply dips, they will crank up the variac; when it surges, they'll crank it down. In any case, they need to have a voltmeter attached to their circuit at all times while shooting and to be diligent in monitoring their voltage. Variacs capable of handling the amperage required by high wattage incandescent lights can be expensive.

Another possible solution—a more costly one—is to install a power conditioner, which evens out the voltage before it gets to your lights. Some such devices work by converting the incoming A.C. power to D.C., charging an internal battery, then using the battery as a source that gets converted back to A.C. to operate your lights.

Possible Cause: Unstable Animator or Environmental Inconsistency
When in the midst of animating a puppet or other object, we typically move back and forth from camera to subject. Objects within the scene may reflect offscreen items, and that can include us! Sometimes we may be on one side of a puppet, sometimes on the other. The camera records what it sees, and that may include flickering shadows or reflections in the objects on screen.



In the above scene, the animator wore a bright white shirt. Sometimes he stood close to the objects being animated, other times he was standing next to the camera. You can see flickering, especially as caused by the reflections on the right side of the the lens barrel.

Notice also in the example that the backdrop wavers, causing flicker.

Another possible source for lighting variations is an unshuttered or unshaded window. The light of day varies tremendously, and if you allow that light into your studio while you're shooting, you're also allowing in a potential source of flicker.

Fixing the Problem
Make sure your environment is appropriately light-shielded. Ordinary aluminum foil over windows is cheap, easily taped in place, and completely opaque.

Lock down anything in the environment (such as a backdrop) so that it cannot be knocked easily or moved by air currents.

Make sure you and other people in the studio are dressed in dark clothing that won't reflect light onto your set.

And very importantly, the animator should get away from the set and into shade before taking a frame. People in the studio should aim to return to exactly the same spot every time before the frame capture is triggered.

Possible Cause: Shutter Speed Too High
Household A.C. electrical power cycles at a certain frequency, typically 50 or 60 cycles per second depending on where you live. Such cycling is usually not noticeable to the naked eye, especially under incandescent lights. But if your shutter speed is faster than 1/50th or 1/60th of a second, and especially if you are shooting under fluorescent lights (in which the cycles are more evident), then the camera may catch the lighting at a dimmer point in the cycle. This will translate to flicker.

Long shutter speeds can help buffer potential flicker caused by brief voltage changes. They have the added benefit of requiring that the camera lens be stopped down, which increases depth of field. As long as your camera is mounted on a tripod, long shutter speeds are usually a good thing.

Fixing the Problem
Slow down your shutter. It's not uncommon for an animator to shoot using a shutter speed of a full second. When you slow down your shutter, you'll either have to decrease your ISO or, better, stop down the aperture.

Possible Cause: Failing Lights
As household tungsten bulbs get older, their performance changes. They can become warmer in color temperature and dimmer in output. Though such relatively slow changes aren't as likely to cause a problem for animators, it can be a concern.

Some fluorescent fixtures, especially older ones, are more prone to flickering problems. The bulbs themselves eventually go bad, or the ballast may wear out, or the connections between bulb and socket may become dirty or otherwise faulty. All these things can lead to flicker. A good indication that a fluorescent lamp is wearing out is darkening at the ends of the tube. The CFLs (compact fluorescent lights) that are taking over the role of the common lightbulb cycle much more rapidly, so seem to be less prone to flickering problems.

Sometimes, even switches used to operate the lights can be a source of problem if they are dirty or have loose connections.

Fixing the Problem
Make sure your lighting fixtures are in good working order. Clean contacts, no aged bulbs, etc. Note that a single bad fluorescent bulb in a multi-lamp fixture can adversely affect them all. Be careful though: ALL MAINTENANCE SHOULD BE PERFORMED WITH THE FIXTURES DISCONNECTED FROM THE POWER SUPPLY!

There are, without doubt, other possible sources of flicker, but these are among the most common.

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