Tips on Time-lapse Flicker

Flickering is a common problem in time-lapse or stop-motion footage captured using a digital still camera. Frame by frame variations in lighting or in exposure lead to perceptible brightness fluctuations, or flicker. Flicker is more of a concern when using GBTimelapse for videography than for monitoring.

Some amount of flicker is almost unavoidable. Thankfully it can usually be removed from the image sequence with a tool like GBDeflicker. The algorithm behind GBDeflicker is powerful and technically complex. The software creates a luminance histogram for each video frame, and then adjusts the image histogram, frame by frame, to minimize flicker. GBDeflicker can usually eliminate any flicker as long as there is no significant clipping.

Sources of Flicker

Cause #1 - Aperture flicker

Because the camera iris is a mechanical device, there are differences in the exact size of the aperture for successive actuations. These small differences in aperture result in frame-to-frame luminance variations that show up as flicker when played back. Aperture flicker seems to be less evident with smaller f-numbers (larger apertures).

Split image of two successive frames

The left and right halves of the image to the right are from two successive frames captured three seconds apart with the same camera settings. The right half is noticeably brighter than the left half. I loaded both images into Photoshop and compared the histograms. They looked almost identical, but had mean values that differed by 4.5 percent (77.50 versus 81.01). Although the clouds did move a tiny bit between these images, it is not enough to account for that large of a difference. Nothing passed in front of the sun, and I assume the sun's light output did not vary between the shots.

How can a digital camera produce brightness variations in successive images of the same subject with identical camera settings? You would expect a digital device to give identical results from identical settings; but that is not the case.

A DSLR camera is not electronic. It's iris is mechanical and mechanical devices have intrinsically limited precision and repeatability. For each shutter press, the camera activates a mechanical device that moves the iris to the desired aperture. The aperture cannot be exactly the same size every time because of friction and other factors so there are differences in exposure. You would expect a higher quality camera to have a more precise aperture mechanism.

To further investigate, we shot twenty successive frames of the same scene at different aperture settings and plotted a graph of the average luminance. Each colored curve in the graph below is for a different setting.

Cause #2 - Shutter flicker

DSLR cameras also have a mechanical shutter, so there will be differences in the exact duration of the shutter for successive actuations. These small differences in duration result in frame-to-frame luminance variations and produce flicker. Using a slower shutter speed is better because a small timing error is less apparent with longer shutter times

Cause #3 - Av stepping

Typical camera Av settings are spaced at 1/3 stop intervals (f4, f4.5, f5.0, f5.6). As the Av value is changed, there are small steps in the luminance because of the changes in the amount of light passing through the iris.

Cause #4 - Tv stepping

Typical camera Tv settings are spaced at 1/3 stop intervals (1/10, 1/13, 1/15, 1/20 etc.). As the Tv value is changed, there are small steps in the luminance because of the changes in the amount of light passing through the shutter. Because bulb exposure times can be set with a precision of 1/1000 of a second (2.100, 2.101, 2.102, etc.), smaller, more gradual changes produce less flicker.

Above is a screen shot from GBDeflicker showing the luminance graph during a daylight-to-night AutoRamp timelapse. The first part of the graph shows a “saw-tooth” luminance pattern as the Tv value was stepped down in 1/3 f-stop increments. After switching to Bulb mode, the luminance curve is smooth.

Cause #5 - Natural flicker

A timelapse video of passing clouds may include shadows being cast on the ground. Although the sky may have a slowly changing luminance, the luminance of the ground may fluctuate rapidly because of the shadows.

Remedy: Use GBDeflicker sub-rectangle If there is unnatural flicker in the sky it can be removed by using GBDeflicker to analyze only a sub-rectangle of the sky in each image.

Remedy: Use AutoRamp instead of Program mode Because a camera’s Program mode determines the exposure at the moment the shutter is actuated, variations caused by cloud shadows will affect each exposure. When using GBTimelapse AutoRamp, the exposure is determined from a moving average over a number of past images – this reduces the influence of cloud shadows between successive frames.

Cause #6 - Lighting flicker

Any artificial lighting that is connected to the power grid can produce flicker. The 60Hz (or 50Hz) power frequency produces a corresponding fluctuation in the light output of the lamps.

Fluorescent lamps are the worst. From Wikipedia, "Fluorescent lamps which operate directly from mains frequency AC will flicker at twice the mains frequency, since the power being delivered to the lamp drops to zero twice per cycle. This means the light flickers at 120 times per second (Hz) in countries which use 60-cycle-per-second (60 Hz) AC, and 100 times per second in those which use 50 Hz. "

Incandescent lamps also flicker. The light produced by the lamp depends on the current flowing through the filament. Since the current is varying with a sine wave, the light output will vary with a sine wave too. Lamps with larger filaments have more thermal mass and produce less variation. In general, lamps in Europe show more flicker because they use thinner filaments to match the higher grid voltage.

Remedy - Use long exposure An exposure time much longer than the power frequency will help average out the light fluctuations.

Remedy - Avoid fluorescent lighting The most obvious is fluorescent lighting with flickering that is apparent to the naked eye.

Minimizing Deflicker Problems

GBDeflicker is a tool for removing time-lapse flicker. It is available as an Adobe Plugin or as a standalone application.

GBDeflicker removes flicker by adjust the luminance of each frame in an image sequence to minimize the frame by frame variations. It does this by mathematically shifting the histogram of a frame left or right (darker or brighter).

GBDeflicker works very well as long as there isn’t any significant clipping of the image data. Clipping occurs when some image pixels are at the maximum possible value. This part of the image is likely overexposed resulting in loss of image information.

When pixels values are clipped, the histogram cannot be shifted correctly because there is no way to calculate the corrected value of an overexposed pixel.

Looking at the graph above, it’s not possible to know what the blue channel should look like if it were shifted to the left. The clipped information has been lost.

Avoid Clipping

Use Program Mode with negative exposure compensation – A negative exposure compensation value is less likely to produce overexposure and clipping.

User a lower target luminance in AutoRamp mode. A lower target luminance produces is less likely to produce overexposure and clipping.

Use Neutral Picture Style

Canon cameras have a Picture Style setting that determines how the raw image data is converted to a JPEG image. The Standard setting increases the saturation and may cause unnecessary color channel clipping (particularly the blue channel). The Neutral setting seems to be the best setting to avoid color channel clipping.

Two images below show the same scene shot with Standard and Neutral picture style. You can see from the histograms that with identical exposures, the Standard saturated the blue channel to the point of clipping.

You can see in the above histogram how the “Standard” picture style boosts the value of the blue channel pixels. This is a “nonlinear” process that is difficult to deflicker.

Above you can see how (at the same exposure) the Neutral picture style has not clipped the blue channel. Using the Neutral picture style is very important when shooting timelapse of blue skies. It is less important with more balanced images such as grey clouds.