Tips - Time Lapse Flicker
Flickering is a common problem in time-lapse or stop motion footage captured
with a digital still camera. Frame by frame variations in lighting or exposure
lead to perceptible brightness fluctuations, or flicker.
Cause #1 - Artificial Lighting
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.
The most obvious is fluourescent lighting with flickering that is apparent to
the naked eye. 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. "
It's also true for incandescent lamps. 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.
Cause #2 - A Camera's Mechanical Aperture
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. We 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 we
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 digital camera is not completly electronic. It's shutter is electronic but its
aperture 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.
Each curve has a degree of random variation. Curiously, for this camera there is
less luminance variation for smaller diameter apertures (higher f-stops). This
suggests that one way to reduce flicker is to user a higher f-stop.
Cause #3 - Subject Failure (Bright Blue Skies)
Histogram with Blue clipping
In our years of doing time-lapse, we have often wondered why scenes with a lot
of blue sky or bright white clouds tend to have some flicker.
The histogram to the right is for what the camera thought was a properly exposed
image (it was shot in Program Mode where the camera determines the shutter
speed and aperture). However, you can see that the blue channel does not go
down to zero value on the right side of the histogram. This is called
"clipping" and it means that a sizeable area of the image has pixels with the
maximum possible blue value. The image will not be an accurate representation
of the scene, because many pixels will have had their blue values truncated.
This is reminiscent of the term "Subject Failure" from film photo processing.
Have you ever taken a roll of film to the drug store to be processed and then
been disappointed with the color a week later when you saw the prints. The
photo lab's equipment was set up to properly develop an average photo and your
photos weren't average, so it was your fault - i.e. "subject failure?.
"Subject failure is the term used for any subject
which does not have the average distribution of tones giving an overall
reflectance of 18%."
For a digital camera, apparently having a lot of blue is like having "Subject
Failure". The standard luminance calculation weights the colors differently to
match the perception of the human eye. The weights are: 59% for green, 30% for
red, and 11 % for blue. Since blue is less important in the camera's exposure
calculation, large areas of blue in the image will lead to an incorrect
exposure. Bright white clouds are a problem, too, because white contains blue.
Cures
There are a few things you can do to eliminate or minimize time-lapse flicker...
-
Avoid Flourescent Lighting
- It's just too prone to flicker.
-
In artifical light, use a long exposure
- If you have to use artificial light, make sure to use a shutter speed that is
much longer than one power cycle (i.e. 1/60 of a second). Use a shutter of 1/2
second or longer so your exposure captures many power cycles and the
fluctuations average out.
-
Avoid clipping
- Pay attention to the histogram before you start. If it is anywhere near
clipping, back off the exposure by -1/3 or -2/3 EV. Make sure the blue channel
isn't clipping.
-
Use a higher f-stop
- Your camera might give less variation at higher f-stops.
-
Remove the flicker later - Sometimes,
your best efforts fail and you wind up with some flickering. That's when it's
time to use GBDeflicker, an Adobe Plug-in for After Effects and Premiere. Watch
this short video to see a flicker example and how GBDeflicker can fix it.
