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0:00:00.370,0:00:04.520
Under certain artificial lighting situations,
especially when traveling internationally

0:00:04.520,0:00:09.360
or shooting high frame rate — slow motion
— video, we can get flicker in our shot;

0:00:09.360,0:00:13.299
we’ve all seen it. Now as many of us know,
to eliminate it, our camera must have the

0:00:13.299,0:00:17.960
“right” combination of frame rate and
shutter speed or shutter angle. And I’ve

0:00:17.960,0:00:22.010
known HOW to eliminate it for a long time,
but until recently, I never truly understood

0:00:22.010,0:00:26.680
what was actually happening, or the math behind
the solution, or even why there are limits

0:00:26.680,0:00:31.180
to how high of high speed video you can shoot
in artificial light — but now I do, and

0:00:31.180,0:00:37.760
I want to share that with you.

0:00:37.760,0:00:41.719
Filmmaker Griffin Hammond made a video called
“Three math hacks every filmmaker should

0:00:41.719,0:00:45.100
know” (I’ll link to it at the end of this
video; you should definitely watch it) where

0:00:45.100,0:00:48.809
he talked about avoiding light flicker from
artificial lighting and he said something

0:00:48.809,0:00:53.920
that really clicked in my head… that between
shutter angle (or shutter speed) and frame

0:00:53.920,0:00:58.859
rate, you only had to get one of them right
to avoid the flicker. Now, fundamentally I

0:00:58.859,0:01:02.850
already knew this; if you’re shooting at
30 frames per second in North America, or

0:01:02.850,0:01:06.650
25 frames per second in most of the rest of
the world, then you can shoot at any shutter

0:01:06.650,0:01:10.590
angle or speed you want and light flicker
won’t be a problem. It’s when you bring

0:01:10.590,0:01:15.550
an NTSC camera to a PAL region or vice versa,
OR when you start shooting high frame rate

0:01:15.550,0:01:20.030
for slow motion video, that this flickering
becomes a problem. AND there’s a limit to

0:01:20.030,0:01:24.820
how many frames per second you can shoot before
it becomes impossible to avoid the flicker.

0:01:24.820,0:01:29.540
To fully explain this, there are two fundamentals
we need to be clear on. First, why shutter

0:01:29.540,0:01:34.300
speed or shutter angle have limits when shooting
video, and second, how the power system and

0:01:34.300,0:01:36.050
therefore electric lighting works.

0:01:36.050,0:01:37.607
I recently did a video explaining shutter
angle vs shutter speed, and the creative reasons

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you may want to choose one angle or another,
so I’ll link to that at the end of this

0:01:38.607,0:01:39.607
video as well. For now though, let’s just
talk about what it is.

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Shutter angle is a term that comes from shooting
movie film, where instead of shutter opening

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and closing like it does for still cameras,
a disc would spin in front the film as it

0:01:45.530,0:01:50.580
moved past the lens, rotating one full revolution
per frame of film. That disc would have an

0:01:50.580,0:01:56.040
opening in it, which let light through to
expose the film. A disc is of course 360 degrees,

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and the opening in that disc was expressed
as a shutter angle. A 180 degree shutter is

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a disc that has a 180 degree opening, or exactly
one half of the disc. You can choose a much

0:02:05.800,0:02:10.129
smaller angle, or a larger one, but of course
the shutter angle can’t be any bigger than

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360 degrees, or open all the time. This is
an important point; remember that. Converting

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shutter angle to shutter speed requires that
you know the frame rate, (or frames per second)

0:02:20.590,0:02:26.870
as well. A 180 degree shutter at 30 frames
per second means the shutter is open for 1/60th

0:02:26.870,0:02:33.359
of a second. 180 over 360 equals 1 over 2,
or half. At 30 frames per second, the longest

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an exposure can possibly be is one thirtieth
of a second, and half of that is 1/60th.

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Since the shutter can’t be open for longer
than that frame is being recored, your exposure

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duration is limited. Just like the shutter
angle can’t be more than 360 degrees at

0:02:46.639,0:02:51.219
any frame rate, the shutter speed can’t
be more than one over the frame rate. If you’re

0:02:51.219,0:02:56.189
shooting 30p that’s 1/30th of a second.
Shoot 60p and that drops to 1/60th, and so-on.

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So again, remember that limitation.

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Now let’s talk about electrical power. Unlike
the light from the sun, which is constant,

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electric light requires a power source, and
the AC power system isn’t constant. In fact

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it’s in the name; AC stands for Alternating
Current. It delivers power in a very fast

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fluctuation that we measure as a frequency.
In North America, we have a 60 cycle per second

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frequency, called 60Hz, and most of the rest
of the world uses a 50 cycle, or 50Hz system.

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The power fluctuation follows a sine wave,
with two peaks per cycle, so a 60Hz system

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has 120 peaks per second, and a 50Hz system
has 100 peaks per second. This Cycled Power

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Delivery Source means that a light it is powering
actually gets turned all the way up to max

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brightness, then drops towards zero, briefly
passing zero before moving back to full power

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again, 100 or 120 times per second, introducing
a flicker. This flicker is too fast for the

0:03:51.480,0:03:55.519
human eye to detect, so we don’t normally
see it, but it’s happening.

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When shooting video under this kind of lighting,
it’s critical that the shutter speed, or

0:03:59.110,0:04:03.919
shutter duration, lines up with the sine wave
perfectly. When the exposure starts and ends

0:04:03.919,0:04:08.029
at the same part of the sine wave, then the
amount of light exposing the film or sensor

0:04:08.029,0:04:12.859
is always the same. However if the exposure
pattern gets out of sync with the sine wave,

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then the exposure isn’t constant, and we
record darker and brighter frames, which we

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see - as a flicker.

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Now things get really interesting. Let’s
go back to the statement that only one of

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the two factors, frame rate or shutter speed
-slash- shutter angle, have to be right to

0:04:27.580,0:04:31.750
avoid flicker. I’ll start with frame rate.
When you’re shooting at 30 frames per second

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under 60Hz lighting, you can use absolutely
any shutter angle or shutter speed you want.

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Why? Because the frequency and the frame rate
are evenly divisible, meaning that each frame

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starts recording at the same point on the
sine wave, which means no matter the duration

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of the exposure, it’ll always end at the
same point of the sine wave on every frame.

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Because every exposure starts on the same
point of the sine wave, the shutter speed

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can be anything you want (that fits in that
time limitations we discussed earlier of course)

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and you’ll have an even exposure.

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The math is actually really simple if you
go the other way and just divide your power

0:05:04.100,0:05:08.720
frequency by whole numbers, allowing you to
find all of “easy” frame rates. Remember,

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60Hz is 120 peaks per second, which is ultimately
what we’re concerned with, so 120 divided

0:05:13.930,0:05:19.310
by 1 of course is 120, meaning you can shoot
easily at 120 frames per second. 120 divided

0:05:19.310,0:05:25.650
by 2 is 60 frames per second, by 3 is 40,
4 is 30, 5 is 24 fps, and so-on. If you’re

0:05:25.650,0:05:29.379
shooting in a 50Hz environment, then you can
shoot flicker free at any shutter angle or

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shutter speed at 100 frames per second, 50,
and 25.

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By the way, a quick side note… NTSC, our
standard broadcast format in North America,

0:05:39.300,0:05:45.080
is 29.97 frames per second, not true 30. When
you see a camera that says it shoots 30fps,

0:05:45.080,0:05:51.380
it’s actually 29.97. And 60p is actually
59.94. So technically, these don’t line

0:05:51.380,0:05:55.990
up perfectly with our 60Hz power. But in reality
they’re so close that there’s no perceptible

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flicker. OK, moving on…

0:05:58.360,0:06:01.689
Now that we know what frame rates are easy
to shoot at, what happens when you want to

0:06:01.689,0:06:07.009
shoot a frame rate that’s NOT evenly divisible,
like 105 or 75 frames per second, or more

0:06:07.009,0:06:11.270
likely, you need to shoot a “common” frame
rate for you but you’re shooting outside

0:06:11.270,0:06:16.220
of your home region — for example, shooting
30p in a 50Hz region. Well remember, to avoid

0:06:16.220,0:06:20.050
the flicker you only have to get ONE of the
two factors aligned, and since frame rate

0:06:20.050,0:06:24.310
is not in alignment, that means that shutter
speed MUST be perfect.

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So what does that actually mean? Let’s use
shooting 30 fps under 50Hz lighting as our

0:06:29.039,0:06:34.580
example. When you divide 50 Hz or 100 peaks
by 30 frames, you end up with three and one

0:06:34.580,0:06:39.170
third peaks per frame. So that means each
frame does NOT start its exposure on the same

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part of the wave. This uneven starting point
means that for most exposure durations — shutter

0:06:43.500,0:06:48.479
speeds — the amount of light hitting the
sensor during each exposure will be different

0:06:48.479,0:06:52.830
every time, resulting in an uneven exposure
across frames, which is where the flicker

0:06:52.830,0:06:53.950
comes from.

0:06:53.950,0:06:58.349
But it turns out that if the exposure starts
AND ENDS at the same PART of the wave — the

0:06:58.349,0:07:02.599
same point along that curve — then the total
light reaching the sensor will in fact be

0:07:02.599,0:07:06.979
the same in every frame! Each frame can have
different starting point — as long as it

0:07:06.979,0:07:09.969
ends in the same position that it started
on.

0:07:09.969,0:07:14.569
So to get consistent exposures across every
frame, the exposure itself EITHER has to start

0:07:14.569,0:07:19.759
at the same point of the wave for EVERY frame
of video and then can end anywhere, OR the

0:07:19.759,0:07:24.150
exposure has to start and end at the same
position on the wave — but that can be anywhere

0:07:24.150,0:07:26.460
along the wave, and different for every frame.

0:07:26.460,0:07:31.199
So, how do we figure out what that shutter
speed should be? That’s just math, and there

0:07:31.199,0:07:36.219
are plenty of tools out there to make it easy.
I like the website red.com/flicker-free-video

0:07:36.219,0:07:40.440
and the app FLICKERfree on iOS — I’ll
link to both of those below. You just punch

0:07:40.440,0:07:44.659
in your frame rate and choose 50 or 60Hz and
it gives you the shutter angle or shutter

0:07:44.659,0:07:49.100
speeds to safely shoot at. But because I really
wanted to understand this, I dug up the formula,

0:07:49.100,0:07:52.879
and made my own calculator. Here’s the formula.

0:07:52.879,0:07:59.020
One over the frame rate, times the shutter
angle over 360, equals N (an integer, or whole

0:07:59.020,0:08:01.550
number) over the frequency.

0:08:01.550,0:08:06.580
Continuing with our 30p in a 50Hz region example,
that means available shutter angles are one

0:08:06.580,0:08:14.680
over 30, times A over 360, equals one over
100, so A equals 108 degrees. The next integer

0:08:14.680,0:08:19.889
is 2, so divide by 2 and you get 216 degrees,
then by 3 and it’s 324 degrees, and the

0:08:19.889,0:08:25.680
next would be 432 however we can’t have
more than a 360 degree shutter, so 108, 216

0:08:25.680,0:08:27.759
and 324 degree shutters are our options.

0:08:27.759,0:08:32.839
Great! Now, how about converting those shutter
angles to shutter speeds? Ironically, as much

0:08:32.839,0:08:36.529
as I appreciate shooting in shutter angle,
when it comes to avoiding flicker, it’s

0:08:36.529,0:08:40.350
actually a LOT easier to just work in shutter
speed because no matter the frame rate, the

0:08:40.350,0:08:45.589
math always reduces back to the same few shutter
speeds. The hard math is to multiply the shutter

0:08:45.589,0:08:52.290
angle as a fraction, like 108 over 360, by
one over the frame rate, like 1 over 30, and

0:08:52.290,0:08:56.940
you get a shutter speed. But no matter the
frame rate, the answer will always be one

0:08:56.940,0:08:57.970
the same numbers.

0:08:57.970,0:09:05.700
For 60Hz power sources, those numbers 1/120th
of a second, 1/60th, 1/40th, 1/30th, and 1/24th.

0:09:05.700,0:09:13.640
For 50Hz sources, they’re 1/100, 1/50, 1/33⅓,
1/25 and 1/20th.

0:09:13.640,0:09:16.580
How do we arrive there? Well… pretty dang
easy.

0:09:16.580,0:09:24.300
120 peaks divided by one is… 120. 120 divided
by 2 is 60, by 3 is 40, by 4 is 30, by 5 is

0:09:24.300,0:09:27.450
24.
100 divided by 1 is 100, by 2 is 50, by 3

0:09:27.450,0:09:31.320
is 33⅓, by 4 is 25. Nice and easy.

0:09:31.320,0:09:35.829
The problem is most cameras don’t accommodate
all of those shutter speeds. Most cameras

0:09:35.829,0:09:42.730
give 1/125th of a second, not 1/120. You’ll
probably find 1/100th, 1/60th, 1/50th 1/40th

0:09:42.730,0:09:47.810
and 1/30th, but not 1/33 and a third, and
finally you’d have to choose 1/25th instead

0:09:47.810,0:09:52.950
of 1/24th. If your camera is capable of working
in shutter angles though like the LUMIX GH5

0:09:52.950,0:09:57.881
and GH5S, you can be more accurate by selecting
shutter angle instead of shutter speed. But

0:09:57.881,0:10:00.940
for most uses, relying on shutter speed is
perfectly fine.

0:10:00.940,0:10:04.860
OK! There’s ONE MORE point I want to make,
and that’s why you can’t shoot really

0:10:04.860,0:10:10.560
high frame rate under AC powered light. Sort
of. Remember in the beginning I said “the

0:10:10.560,0:10:19.180
shutter angle can’t be any bigger than 360
degrees, or open all the time. This is an

0:10:19.180,0:10:22.790
important point; remember that.” Or that
the shutter speed can’t possibly be more

0:10:22.790,0:10:27.940
than one over the frame rate. At 30p, an exposure
can’t be longer than 1/30th of a second,

0:10:27.940,0:10:32.760
or it’d overlap into the next frame. Under
60Hz lighting, with 120 peaks per second,

0:10:32.760,0:10:36.540
we now understand that we can’t have less
than a full wave exposing a frame without

0:10:36.540,0:10:41.551
introducing flicker. The minimum exposure
duration is going to be one full wave. Anything

0:10:41.551,0:10:45.430
less would mean we wouldn’t be starting
and Ending the exposure on the same position

0:10:45.430,0:10:49.760
of the wave. Which means we can’t shoot
anything faster than 1/120th of a second.

0:10:49.760,0:10:53.130
Which means we can’t have more than 120
frames per second.

0:10:53.130,0:10:56.940
The good news is there are ways around this,
IF you have control over the lighting. You

0:10:56.940,0:11:01.070
could power your lights with DC power instead
of AC. That’s battery instead of from the

0:11:01.070,0:11:04.850
wall. DC is “direct current” instead of
“alternating current”, and the whole flicker

0:11:04.850,0:11:10.440
issue goes away. LED lighting shouldn’t
flicker. It still can if it’s dimmed, but

0:11:10.440,0:11:17.100
at full power LED should not flicker. But
it turns out there’s at least one more trick.

0:11:17.100,0:11:21.610
I reached out on twitter to Gavin of The Slow
Mo Guys and asked how they work around it,

0:11:21.610,0:11:25.900
because they shoot extremely high frame rate
slow motion video and not all of it is outdoors.

0:11:25.900,0:11:29.880
He responded “There are a lot of options
for high speed lighting. Good rule of thumb

0:11:29.880,0:11:33.560
is the bigger the light, the less it flickers.”

0:11:33.560,0:11:37.300
I had to dig into this answer a bit but what
I’ve learned is that essentially really

0:11:37.300,0:11:41.930
big lights, as you can imagine, burn really
hot. Light bulb filaments get bright because

0:11:41.930,0:11:45.290
they get hot; the flicker is happening because
they are gaining and losing power through

0:11:45.290,0:11:50.000
the sine wave of power delivery, effectively
heating up and cooling down 120 times per

0:11:50.000,0:11:54.810
second. Really big bright bulbs do the same
thing of course, but since they burn SO hot

0:11:54.810,0:11:59.240
they don’t cool down as much, and therefore
the flicker is barely noticeable. Plus I would

0:11:59.240,0:12:03.150
imagine that shooting at 2,000 frames per
second means the flicker becomes much less

0:12:03.150,0:12:07.550
noticeable since a flicker we’d normally
see over a single frame is stretched out to

0:12:07.550,0:12:12.290
almost two seconds so the variation would
become much less noticeable. If anyone watching

0:12:12.290,0:12:15.010
this can expand on that in the comments, I’d
love to learn more.

0:12:15.010,0:12:19.570
Well there you have it. Now you know WHY the
shutter speed has to be so precise, and what’s

0:12:19.570,0:12:23.550
actually happening that causes the flicker,
and how to avoid it. And, why you can’t

0:12:23.550,0:12:28.740
shoot over 120 frames per second slow motion
video under AC lighting (or 100 frames in

0:12:28.740,0:12:31.959
a 50Hz region) without taking total control
of the light.

0:12:31.959,0:12:35.839
Hey if you learned something today, I’d
love it if you liked and subscribed, but also,

0:12:35.839,0:12:40.680
do me a favor and share the video! Just hit
the share button below, post it on Twitter

0:12:40.680,0:12:44.470
or forward it to a friend. And also be sure
to check out the pinned comment below; if

0:12:44.470,0:12:48.370
there are any updates to this video, that’s
where you’ll find them. Next up, watch the

0:12:48.370,0:12:51.420
live Q&A that followed this premiere, or check
out the videos I mentioned, in the playlist

0:12:51.420,0:12:51.540
below. See you next time!
 

Under certain artificial lighting situations, especially when traveling internationally or shooting high frame rate — slow motion — video, we can get flicker in our shot; we’ve all seen it. Now as many of us know, to eliminate it, our camera must have the “right” combination of frame rate and shutter speed or shutter angle. And I’ve known HOW to eliminate it for a long time, but until recently, I never truly understood what was actually happening, or the math behind the solution, or even why there are limits to how high of high speed video you can shoot in artificial light — but now I do, and I want to share that with you.

Filmmaker Griffin Hammond made a video called “Three math hacks every filmmaker should know” (I’ll link to it at the end of this video; you should definitely watch it) where he talked about avoiding light flicker from artificial lighting and he said something that really clicked in my head… that between shutter angle (or shutter speed) and frame rate, you only had to get one of them right to avoid the flicker. Now, fundamentally I already knew this; if you’re shooting at 30 frames per second in North America, or 25 frames per second in most of the rest of the world, then you can shoot at any shutter angle or speed you want and light flicker won’t be a problem. It’s when you bring an NTSC camera to a PAL region or vice versa, OR when you start shooting high frame rate for slow motion video, that this flickering becomes a problem. AND there’s a limit to how many frames per second you can shoot before it becomes impossible to avoid the flicker.

To fully explain this, there are two fundamentals we need to be clear on. First, why shutter speed or shutter angle have limits when shooting video, and second, how the power system and therefore electric lighting works. 

I recently did a video explaining shutter angle vs shutter speed, and the creative reasons you may want to choose one angle or another, so I’ll link to that at the end of this video as well. For now though, let’s just talk about what it is.

Shutter angle is a term that comes from shooting movie film, where instead of shutter opening and closing like it does for still cameras, a disc would spin in front the film as it moved past the lens, rotating one full revolution per frame of film. That disc would have an opening in it, which let light through to expose the film. A disc is of course 360 degrees, and the opening in that disc was expressed as a shutter angle. A 180 degree shutter is a disc that has a 180 degree opening, or exactly one half of the disc. You can choose a much smaller angle, or a larger one, but of course the shutter angle can’t be any bigger than 360 degrees, or open all the time. This is an important point; remember that. Converting shutter angle to shutter speed requires that you know the frame rate, (or frames per second) as well. A 180 degree shutter at 30 frames per second means the shutter is open for 1/60th of a second. 180 over 360 equals 1 over 2, or half. At 30 frames per second, the longest an exposure can possibly be is one thirtieth of a second, and half of that is 1/60th. 

Since the shutter can’t be open for longer than that frame is being recored, your exposure duration is limited. Just like the shutter angle can’t be more than 360 degrees at any frame rate, the shutter speed can’t be more than one over the frame rate. If you’re shooting 30p that’s 1/30th of a second. Shoot 60p and that drops to 1/60th, and so-on. So again, remember that limitation.

Now let’s talk about electrical power. Unlike the light from the sun, which is constant, electric light requires a power source, and the AC power system isn’t constant. In fact it’s in the name; AC stands for Alternating Current. It delivers power in a very fast fluctuation that we measure as a frequency. In North America, we have a 60 cycle per second frequency, called 60Hz, and most of the rest of the world uses a 50 cycle, or 50Hz system. The power fluctuation follows a sine wave, with two peaks per cycle, so a 60Hz system has 120 peaks per second, and a 50Hz system has 100 peaks per second. This Cycled Power Delivery Source means that a light it is powering actually gets turned all the way up to max brightness, then drops towards zero, briefly passing zero before moving back to full power again, 100 or 120 times per second, introducing a flicker. This flicker is too fast for the human eye to detect, so we don’t normally see it, but it’s happening. 

When shooting video under this kind of lighting, it’s critical that the shutter speed, or shutter duration, lines up with the sine wave perfectly. When the exposure starts and ends at the same part of the sine wave, then the amount of light exposing the film or sensor is always the same. However if the exposure pattern gets out of sync with the sine wave, then the exposure isn’t constant, and we record darker and brighter frames, which we see - as a flicker. 

Now things get really interesting. Let’s go back to the statement that only one of the two factors, frame rate or shutter speed -slash- shutter angle, have to be right to avoid flicker. I’ll start with frame rate. When you’re shooting at 30 frames per second under 60Hz lighting, you can use absolutely any shutter angle or shutter speed you want. Why? Because the frequency and the frame rate are evenly divisible, meaning that each frame starts recording at the same point on the sine wave, which means no matter the duration of the exposure, it’ll always end at the same point of the sine wave on every frame. Because every exposure starts on the same point of the sine wave, the shutter speed can be anything you want (that fits in that time limitations we discussed earlier of course) and you’ll have an even exposure.

The math is actually really simple if you go the other way and just divide your power frequency by whole numbers, allowing you to find all of “easy” frame rates. Remember, 60Hz is 120 peaks per second, which is ultimately what we’re concerned with, so 120 divided by 1 of course is 120, meaning you can shoot easily at 120 frames per second. 120 divided by 2 is 60 frames per second, by 3 is 40, 4 is 30, 5 is 24 fps, and so-on. If you’re shooting in a 50Hz environment, then you can shoot flicker free at any shutter angle or shutter speed at 100 frames per second, 50, and 25.

By the way, a quick side note… NTSC, our standard broadcast format in North America, is 29.97 frames per second, not true 30. When you see a camera that says it shoots 30fps, it’s actually 29.97. And 60p is actually 59.94. So technically, these don’t line up perfectly with our 60Hz power. But in reality they’re so close that there’s no perceptible flicker. OK, moving on…

Now that we know what frame rates are easy to shoot at, what happens when you want to shoot a frame rate that’s NOT evenly divisible, like 105 or 75 frames per second, or more likely, you need to shoot a “common” frame rate for you but you’re shooting outside of your home region — for example, shooting 30p in a 50Hz region. Well remember, to avoid the flicker you only have to get ONE of the two factors aligned, and since frame rate is not in alignment, that means that shutter speed MUST be perfect. 

So what does that actually mean? Let’s use shooting 30 fps under 50Hz lighting as our example. When you divide 50 Hz or 100 peaks by 30 frames, you end up with three and one third peaks per frame. So that means each frame does NOT start its exposure on the same part of the wave. This uneven starting point means that for most exposure durations — shutter speeds — the amount of light hitting the sensor during each exposure will be different every time, resulting in an uneven exposure across frames, which is where the flicker comes from.

But it turns out that if the exposure starts AND ENDS at the same PART of the wave — the same point along that curve — then the total light reaching the sensor will in fact be the same in every frame! Each frame can have different starting point — as long as it ends in the same position that it started on.

So to get consistent exposures across every frame, the exposure itself EITHER has to start at the same point of the wave for EVERY frame of video and then can end anywhere, OR the exposure has to start and end at the same position on the wave — but that can be anywhere along the wave, and different for every frame.

So, how do we figure out what that shutter speed should be? That’s just math, and there are plenty of tools out there to make it easy. I like the website red.com/flicker-free-video and the app FLICKERfree on iOS — I’ll link to both of those below. You just punch in your frame rate and choose 50 or 60Hz and it gives you the shutter angle or shutter speeds to safely shoot at. But because I really wanted to understand this, I dug up the formula, and made my own calculator. Here’s the formula.

One over the frame rate, times the shutter angle over 360, equals N (an integer, or whole number) over the frequency.

Continuing with our 30p in a 50Hz region example, that means available shutter angles are one over 30, times A over 360, equals one over 100, so A equals 108 degrees. The next integer is 2, so divide by 2 and you get 216 degrees, then by 3 and it’s 324 degrees, and the next would be 432 however we can’t have more than a 360 degree shutter, so 108, 216 and 324 degree shutters are our options.

Great! Now, how about converting those shutter angles to shutter speeds? Ironically, as much as I appreciate shooting in shutter angle, when it comes to avoiding flicker, it’s actually a LOT easier to just work in shutter speed because no matter the frame rate, the math always reduces back to the same few shutter speeds. The hard math is to multiply the shutter angle as a fraction, like 108 over 360, by one over the frame rate, like 1 over 30, and you get a shutter speed. But no matter the frame rate, the answer will always be one the same numbers.

For 60Hz power sources, those numbers 1/120th of a second, 1/60th, 1/40th, 1/30th, and 1/24th. For 50Hz sources, they’re 1/100, 1/50, 1/33⅓, 1/25 and 1/20th.

How do we arrive there? Well… pretty dang easy. 

120 peaks divided by one is… 120. 120 divided by 2 is 60, by 3 is 40, by 4 is 30, by 5 is 24.
100 divided by 1 is 100, by 2 is 50, by 3 is 33⅓, by 4 is 25. Nice and easy.

The problem is most cameras don’t accommodate all of those shutter speeds. Most cameras give 1/125th of a second, not 1/120. You’ll probably find 1/100th, 1/60th, 1/50th 1/40th and 1/30th, but not 1/33 and a third, and finally you’d have to choose 1/25th instead of 1/24th. If your camera is capable of working in shutter angles though like the LUMIX GH5 and GH5S, you can be more accurate by selecting shutter angle instead of shutter speed. But for most uses, relying on shutter speed is perfectly fine.

OK! There’s ONE MORE point I want to make, and that’s why you can’t shoot really high frame rate under AC powered light. Sort of. Remember in the beginning I said “the shutter angle can’t be any bigger than 360 degrees, or open all the time. This is an important point; remember that.” Or that the shutter speed can’t possibly be more than one over the frame rate. At 30p, an exposure can’t be longer than 1/30th of a second, or it’d overlap into the next frame. Under 60Hz lighting, with 120 peaks per second, we now understand that we can’t have less than a full wave exposing a frame without introducing flicker. The minimum exposure duration is going to be one full wave. Anything less would mean we wouldn’t be starting and Ending the exposure on the same position of the wave. Which means we can’t shoot anything faster than 1/120th of a second. Which means we can’t have more than 120 frames per second. 

The good news is there are ways around this, IF you have control over the lighting. You could power your lights with DC power instead of AC. That’s battery instead of from the wall. DC is “direct current” instead of “alternating current”, and the whole flicker issue goes away. LED lighting shouldn’t flicker. It still can if it’s dimmed, but at full power LED should not flicker. But it turns out there’s at least one more trick. I reached out on twitter to Gavin of The Slow Mo Guys and asked how they work around it, because they shoot extremely high frame rate slow motion video and not all of it is outdoors. He responded “There are a lot of options for high speed lighting. Good rule of thumb is the bigger the light, the less it flickers.”

I had to dig into this answer a bit but what I’ve learned is that essentially really big lights, as you can imagine, burn really hot. Light bulb filaments get bright because they get hot; the flicker is happening because they are gaining and losing power through the sine wave of power delivery, effectively heating up and cooling down 120 times per second. Really big bright bulbs do the same thing of course, but since they burn SO hot they don’t cool down as much, and therefore the flicker is barely noticeable. Plus I would imagine that shooting at 2,000 frames per second means the flicker becomes much less noticeable since a flicker we’d normally see over a single frame is stretched out to almost two seconds so the variation would become much less noticeable. If anyone watching this can expand on that in the comments, I’d love to learn more.

Well there you have it. Now you know WHY the shutter speed has to be so precise, and what’s actually happening that causes the flicker, and how to avoid it. And, why you can’t shoot over 120 frames per second slow motion video under AC lighting (or 100 frames in a 50Hz region) without taking total control of the light. 

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