1
00:00:00,016 --> 00:00:03,133
The light may be changing
faster than you notice.

2
00:00:03,133 --> 00:00:06,960
The camera can collect different portions
of those same changes.

3
00:00:06,983 --> 00:00:09,967
How can the same light look different
through a camera?

4
00:00:09,967 --> 00:00:13,567
Rapid changes can blend
into an apparently steady light.

5
00:00:13,567 --> 00:00:16,179
Different sensor rows
begin their exposures

6
00:00:16,219 --> 00:00:17,999
at slightly different times.

7
00:00:18,000 --> 00:00:21,667
Those differences become bands
across the recorded image.

8
00:00:21,667 --> 00:00:25,195
What you notice depends on the light
and viewing conditions.

9
00:00:25,200 --> 00:00:28,826
Some lights are dimmed
by switching rapidly on and off.

10
00:00:29,085 --> 00:00:32,275
Shorter pulses can reduce
the average light output.

11
00:00:32,446 --> 00:00:34,586
This is one way to dim a light.

12
00:00:34,785 --> 00:00:38,685
Other lights have different patterns,
including much steadier output.

13
00:00:38,918 --> 00:00:41,308
We will follow the pulsing example.

14
00:00:41,467 --> 00:00:43,939
Now consider how the camera
collects light.

15
00:00:44,306 --> 00:00:46,916
Start with just one row of its sensor.

16
00:00:47,017 --> 00:00:51,057
During an exposure, that row
collects light for a short interval.

17
00:00:51,146 --> 00:00:55,136
The light collected depends on
what happens inside that interval.

18
00:00:55,332 --> 00:00:59,052
Here, the exposure falls entirely
inside a bright pulse.

19
00:00:59,349 --> 00:01:03,199
Move it between pulses,
and the row collects much less light.

20
00:01:03,405 --> 00:01:06,995
An exposure can also overlap a pulse
only partly.

21
00:01:07,169 --> 00:01:10,869
That produces a value
between the bright and dark examples.

22
00:01:11,094 --> 00:01:15,114
These are different possible timings
for the same exposure duration.

23
00:01:15,404 --> 00:01:19,644
Across successive frames, that timing
can shift relative to the pulses.

24
00:01:19,961 --> 00:01:22,531
Frame rate sets how often
new frames begin.

25
00:01:23,445 --> 00:01:26,535
Exposure duration sets how long
each row collects light.

26
00:01:26,681 --> 00:01:30,421
But why can one frame contain
both bright and dark bands?

27
00:01:30,700 --> 00:01:32,950
Many cameras use a rolling shutter.

28
00:01:33,093 --> 00:01:35,713
Different sensor rows
begin their exposures

29
00:01:35,753 --> 00:01:37,533
at slightly different times.

30
00:01:37,623 --> 00:01:40,243
Their exposure intervals can overlap.

31
00:01:40,497 --> 00:01:43,457
Some rows collect more of a pulse
than others.

32
00:01:43,627 --> 00:01:47,307
Those differences become bands
across the recorded image.

33
00:01:47,480 --> 00:01:49,526
The light changed over time.

34
00:01:49,566 --> 00:01:52,356
The camera turned that change
into a pattern.

35
00:01:52,601 --> 00:01:55,081
Now make each exposure last longer.

36
00:01:55,205 --> 00:01:59,252
Let each row collect one complete,
repeating light cycle.

37
00:01:59,418 --> 00:02:04,154
Every row now collects the same amount,
despite starting at different times.

38
00:02:04,194 --> 00:02:05,484
The pulses continue.

39
00:02:05,869 --> 00:02:08,979
Their differences no longer appear
as bands here.

40
00:02:09,097 --> 00:02:11,567
Real lights and camera settings vary.

41
00:02:11,788 --> 00:02:16,138
A different exposure duration can reduce
flicker without changing the light.

42
00:02:16,339 --> 00:02:18,979
So return to the light
that looked steady.

43
00:02:19,120 --> 00:02:22,470
Its rapid changes may blend together
in your vision.

44
00:02:22,587 --> 00:02:26,427
The camera can collect different portions
of those same changes.

45
00:02:26,561 --> 00:02:30,641
That is how a steady-looking light
can become a flickering recording.