WEBVTT

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The light may be changing
faster than you notice.

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The camera can collect different portions
of those same changes.

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How can the same light look different
through a camera?

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Rapid changes can blend
into an apparently steady light.

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Different sensor rows
begin their exposures

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at slightly different times.

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Those differences become bands
across the recorded image.

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What you notice depends on the light
and viewing conditions.

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Some lights are dimmed
by switching rapidly on and off.

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Shorter pulses can reduce
the average light output.

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This is one way to dim a light.

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Other lights have different patterns,
including much steadier output.

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We will follow the pulsing example.

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Now consider how the camera
collects light.

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Start with just one row of its sensor.

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During an exposure, that row
collects light for a short interval.

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The light collected depends on
what happens inside that interval.

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Here, the exposure falls entirely
inside a bright pulse.

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Move it between pulses,
and the row collects much less light.

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An exposure can also overlap a pulse
only partly.

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That produces a value
between the bright and dark examples.

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These are different possible timings
for the same exposure duration.

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Across successive frames, that timing
can shift relative to the pulses.

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Frame rate sets how often
new frames begin.

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Exposure duration sets how long
each row collects light.

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But why can one frame contain
both bright and dark bands?

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Many cameras use a rolling shutter.

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Different sensor rows
begin their exposures

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at slightly different times.

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Their exposure intervals can overlap.

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Some rows collect more of a pulse
than others.

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Those differences become bands
across the recorded image.

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The light changed over time.

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The camera turned that change
into a pattern.

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Now make each exposure last longer.

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Let each row collect one complete,
repeating light cycle.

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Every row now collects the same amount,
despite starting at different times.

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The pulses continue.

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Their differences no longer appear
as bands here.

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Real lights and camera settings vary.

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A different exposure duration can reduce
flicker without changing the light.

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So return to the light
that looked steady.

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Its rapid changes may blend together
in your vision.

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The camera can collect different portions
of those same changes.

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That is how a steady-looking light
can become a flickering recording.