Friday Film Quiz: Super Synch, Phony Film, or Frame Rate?

If you’re a regular reader, you know that I am on-the-record with a deep belief that photography and filmmaking are not all about speeds and feeds or how big your megapixels are compared to mine. Nonetheless, I do have a healthy respect for the technical side of the craft — and for those who dive in deep.

Like the guy who made this video, for example.

This optical illusion is “purportedly” made possible by synching the camera shutter speed with the rotation of the helicopter’s blades, giving the latter the appearance of “staticity.” Some cry hoax. Others say it’s real. Those who believe it is real have engaged in lengthy debate about how it was achieved. The two sides’ arguments break down like this:

SS: “As the title of the video suggests, the filmmaker synched his shutter speed with the rotation of the helicopter blades to make it appear as it does.”

FR: “This is a matter of frame rate, not shutter speed. The frame rate has to be synched such that with each frame exposure the blades are in the exact same position.”

So here’s the quiz – what’s your take? Real or fake? Shutter speed or frame rate?

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I think it’s a combination of freezing the blades (high shutter speed) but as mentioned you wouldn’t have them in the same position like they do in this video without syncing the frame rate.

LameBMX says:

100% real. First of after the heli idles up, ie rotors are up to speed lift is not created by blade speed but by the pitch angle of the blade. They also compensate for additional power needs while changing the pitch angle to prevent the motor from being bogged down. Next up, you can mute the sync some by using a dividend of they rotor speed. Lastly the tail looks proper since it rotates at a ratio of the rotor speed, it too changes force by changing the pitch on the blades. Its rotational speed looks nice and constant. Lastly, a cheap tool used to check a radio controlled helicopters head speed aka rpm of the blades, is nothing but a shutter glass, you change the shutter rate until the blades appear static and it displays your head speed in RPM’s.

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Absolutely real, this is plain physics and relies on the framerate. Of course the shutterspeed has to be fast to avoid blurry rotors, but the framerate does the trick :)

J says:

Frame rate does not matter. That’s a function of the video file not how it’s captured.

Shutter speed is how many times the shutter opens per second. That is totally possible to sync. Like the person earlier said. 200 rps can translate to 20.

What people don’t seem to get is that shutter angle effects exposure speed. Meaning a 90 degree shutter is twice as fast as a180. this effects motion blur. But motion blur is perceived differently in film and video anyway. Go watch an action movie frame by fame if you don’t believe me.

Pete says:

I’m going to have to with fake as true blades would have curve to them due to the CMOS’ progressive capture method. If it was recorded with an CCD then it might be plausible but how many people are using these cameras these days?

HD Cam Team says:

REAL.

The rear blades rotate faster in a helicopter, and they move “slowly” in this video.

Both sync are required to achieve this: Frame rate and shutter speed.

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It’s basically a much fancier version of filmmakers having to watch their shutter speeds when filming cars driving. If the rate is wrong, the wheels/rims will appear to spin in reverse relative to the car.

Adam W says:

It’s real and pretty simple to understand. Not sure why everyone thinks this is fake.

See this video of the strobe light on a Technics 1200 turntable for a practical application of a similar effect:

http://www.youtube.com/watch?v=KbdK94vl_Bs

a tiny chopper, hanging from a atring. Hey, that’s my educated guess and i’m sticking to it!

Adam says:

I have seen video like this with a tap flowing in front of a speaker. The water stood still as the camera moved around.Its to do with the shutter sync and the sound from the speaker. Can’t remember the details but looks cool.

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Pete says:

I agree with it is both, why? Take a still photo of a tv at 1/60th of a second. What do you get? A picture with a dark band. Now how would you keep that dark band in the same spot in a video? By selecting a frame rate that is a multiple of 60 hertzs. Same idea different example.

Essogas says:

Actually, it’s a matter of neither shutter speed or frame rate. It’s photographed with a camera that has the capability to adjust shutter ANGLE, like the Arri Alexa.

louie aguila says:

show lcd of the camera used while being filmed, if its pure logic then it shouldn’t be that hard to do it again but this time with another camera filiming the camera used to achieve this so called sync… im a doubting thomas so to see is to believe.. not hating or anything.. just show tangible proof so the questions are answerd.

Dean says:

Watch this clip … http://youtu.be/-yorvhbzs48?t=1m36s same effect … not fake

Dean says:

This is totally correct. The effect has been done in many ski movies already. I have been at the shots and have seen the Heli leave … then watched the video after. The sure as hell didn’t fake the chopper leaving a backcountry lodge.

Evan says:

Of course it’s real. Anyone who’s done any kind of cinema work should be familiar with the concept of aliasing.

The rotors turn, let’s say, 300 times per minute, or 5 times per second. That means, given 5 blades, the rotor would appear identical to this 25 times per second. If one were shooting at 25 fps, which is a pretty standard frame rate, boom, done.

Evan says:

Also worth noting: 25 FPS is not the only rate that would work with these example numbers, though it is the fastest. 25/2, 25/3 25/4 25/5 – all these would work as well.

faisal says:

Its real and its frame speed.

Caleb says:

Nice hedge!

Felix says:

It is both, shutter speed to freeze the motion, frame rate to sync the blades.

The speed of the rotor is constant for the most part. Go out and get a toy helicopter, the cheap ones vary the speed of the blades where the more expensive ones change the pitch of the blades. Which one is easier to control? The one where the pitch of the blades are changed.

I believe there used to be an example of a similar situation at the Chicago Museum of Science and Industry.
The exhibit demonstrated why circus wagon wheels would appear still at certain speeds (ever watch a western and notice the wagon wheels appear to move at different speeds compaired to the wagon?), The exhibit had a strobe that the viewer could adjust the strobe frequency flashing on a spinning wheel. By changing the strobe rate, you could make the wheel appear to spin, freeze, and even go in reverse.
I am going to say this is shutter speed dependent as a camera shutter would equal the strobe of the light from the exhibit.

vins0n says:

Looks real. Again I don’t know why someone would fake it. Also the rotors do move a little and the pitch changes. The tail rotor is moving and if you were going to fake it wouldn’t you just fake that as well?

dvape says:

Why someone would spend the time creating a fake of this bewilders me about as much as why I spent 20 minutes geeking out about it’s validity.

Tadeo says:

+1 to the last comment ;) they should also tether the space shuttle and say it´s an OVNI lol

Eddie Lee says:

Or maybe it is hooked up to another chopper above. That would explain why the camera never goes wide angled.

Tadeo says:

This phenomenon even has a name: temporal aliasing. it´s basically the same thing as using stroboscopic lights to “freeze” movement and has to do with both shutter speed and frame rate. shutter speed as was said before accounts for a reduced motion blur and frame rate is what syncs the motion in place. if your temporal sampling ( frame rate) is higher, the blades would look moving in one direction, if it´s lower it would look as moving the other direction. same thing happens when you look at a car accelerating, at a time the wheels seem to move to one direction, let the car move faster and you literally see how the wheel seem to reverse (which is of course an optical illusion called again: aliasing)

Joe says:

+1 for aliasing. That’s exactly what’s happening, and it’s because the sampling (frame) rate is too low to accurately reproduce the signal (position of blades).

Joel says:

1.) You mean you can’t see the skyhooks? You know, what holds the helicopters and airplanes up in the air? ;-)

2.) The semantics of it all is that the method to sync is probably framerate. This would put me in the framerate crowd, but I honestly think it’s just semantics.

As others have mentioned, the blades appear to be slowly moving, the back rotor is moving slowly, the pitch of the helicopter blades changes which results in speed and direction. You do this with an extremely high framerate.

3.) Nice model helicopter. ;-)

David says:

I’ll be the first to say.
Who cares, looks cool.
That said;
has to be a sync of both FR and SS. The position of the blades do not change over the helo body.
That makes sense as we know the rotation speed is fixed.
The FR matched one of the rotations, and froze the position of the shaft, and the shutter speed froze the blade itself.
Both camps are right.
Or its a toy on a string, with added audio.
keepin’ da options open.

Ivan Tadić says:

I think it’s real..

Jason says:

similar to the frozen in space waterdroplets , check this out .http://www.youtube.com/watch?v=_PkgQQqpH2M

Nermal says:

Synchronizing the frame rate ensures that the rotor is in the same alignment with each frame of the video.

A high enough shutter speed is required to ensure the individual frames aren’t overly blurred so that the rotor looks crisp.

Nick says:

http://www.youtube.com/watch?v=_eFPhg5nIrY&feature=share&list=UUNLWT4b_Uc38UvoxQbja9qg

Unfortunately the 4 has a pretty bad rolling shutter, but you get the idea. The above video is simply luck of the right shutter speed. I was more impressed by the fact the the shutter speed remained constant throughout the video.

Chris H says:

I’d say that it’s a combination of shutter speed and frame rate, mostly frame rate.
You need to have the right frame rate to sync with the rotation of the blades. You also need a fast shutter speed to avoid too much motion blur (too much motion blur and you wont get the right effect).

I think the title of the video is a misinterpretation of the term shutter speed. The shutter speed only effects the amount of motion blur, so you need the right frame rate to get the right aliasing effect..

Johan says:

I know that for a lot of helicopters if they need more lift, the angle of the rotors is changed and not the speed of rotation.
So this could be possible.
Key for this effect is the frame rate, that needs to be in sync, Shutter speed must only be high enough to get a clear definition of the blades,

Gerald says:

Definitely real. It is related to both really. The frame rate is synched with the rotors, an effect which is exaggerated by the high shutter speed that freezes the blades in place each time a frame is captured.

ABRphoto says:

It is possible with shutter speed.
Next time it rains go out and start filming it at 1/50 and shift it up to 1/4000. You’ll se a dramatic change and basically the same thing happens here.

David Crowe says:

It is the FR and shutter speed, but mostly the former. As others have said you need a shutter speed high enough that a single shot would appear to freeze the rotors. More importantly the frame rate needs to be an even fraction of the rpm (not a multiple). If the frame rate was exactly 1/10 the rpm then it would grab a quick shot on every 10(h rotation.

The slow rotation of the main blade is proof it’s real, prettty much, or a super high quality fake. Put it this way, doing it for real would be cheaper and easier than faking it this well.

Jakub says:

I think the shutter speed just have to be fast enough to freeze the blades and then it is just about the frame rate…e.g. shoot at 120fps and keep only the frames with the blade at the same position…imho :)

btw. engine speed remains (almost) constant in helicopter flight; vertical thrust is adjusted with collective pitch.

Gert says:

It’s the shutter speed.

shutter speed is apparently very small, but that is essentially irrelevant to the question as it only affects motion blur. and the frame rate needs to a multiple of 1/5 of the rotor speed (five blades, you can’t tell which one it is in each frame). you see that the rotor is not perfectly static, which could be explained by minor variations in rotor speed as the pilot adjusts collective pitch & the engine speed varies under load.

a quick google says helis have somewhere between 100 and 500rpm, so let’s take 300rpm (it’s a sizeable heli, so lowish rpm to avoid the speed of sound barrier at the blade tips). that’s 300/60 = 5 rotations per second. with 25 fps you’d get every 1/5th revolution, which happens to be exactly what we’re looking for.

back to shutter speed: at 5 rotations per second, 1/500 means 1/100 rotation of blur, or 3.6 degrees. so also in a plausible range.

hard to tell with that dinky small video, but i’m tempted to say it’s for real.

JohnnyMilk says:

I’ll go with what u laid out here, I got about 90% of it. Toight analysis man. Respect!

Vladimir says:

I’m not 100% sure, but as far as I remember the main rotor blades of a Mi-24 Helicopter rotates within a narrow range of 240 rpm (4 rounds per second) so it could be done, at least in theory. Not sure about the practice though…

If that’s accurate, 4 RPS * 5 blades means there’s 20 times per second that the rotor would be in the position captured here. That gives you a framerate no faster than 20 FPS. Keep the shutter speed fast enough to freeze the blades (so they’re not blurry) in each frame and you’re golden. That shouldn’t be hard on a nice, sunny day.

It is not possible. The frame rate would have to constantly be changing. If you have ever been around a heli taking off you know that the speed of rotation increases as the craft ascends. Think of the blades ramping up as it takes off. The blade wash increases because the blades are going faster. It is the only way for a heli to be able to adjust its height. The only way this video is possible is if the camera had a constantly adjustable frame rate or shutter speed.

Chris H says:

But what makes you think that the rotation speed changes during this video?
The helicopter isn’t taking off nor a-/descending, so it’s likely that the rotation speed is constant, isn’t it!?!

W says:

Sorry, Daniel that’s wrong. The main rotor RPM remains at a relatively constant speed, which is why the blades don’t appear to move much. The main rotors of all helicopters are designed to operate in a very narrow RPM range.

However, if you’ll look at the tips of the main rotor blades, you can clearly see the pitch change as the pilot changes altitude. The swashplate changes the pitch of the blades and the angle of attack allowing the heli to fly forwards, backwards, sideways and up/down.

To answer Chase’s question: it’s very real and it’s a factor of the shutter speed, not the frame rate. The heli is moving around in what appears to be real-time so you’re limited to the real world frame rates of camcorders and DSLRs (typically 30 or 60 frames per second). You simply cannot adjust the frame rate high enough to freeze the motion, unless you are operating a Phantom or similar. But slow-mo is not at play here.

The heli main rotors are only spinning a few hundred RPM, which is well within the capabilities of any camera with adjustable shutter. A shutter speed of say 1/500 would freeze the motion of the rotor blades. You can’t tell if the blades are in the same position from frame to frame or if they are moving around the circle, though.

Seriously, try this yourself with a ceiling fan. Put a sticker on one blade and try running the fan at different speeds while changing your shutter speed. As you increase the shutter speed, you’ll freeze the motion of the blades or they will appear to reverse direction. Or you can watch this video explain it.
https://vimeo.com/19603537

Chase, are these the same people arguing over the way propellers appear on cameras with a rolling shutter? I bet they really blows their minds!

Anthony says:

Great answer with the correct explanation. Kudos.

Anonymous says:

Good answer but you state it’s a factor of shutter speed and not frame rate. Not true, it’s a factor of both. Since each frame has to ensure the blade is in the same position as the last it therefore needs to be in sync with the rpm of the rotar blades. Shutter speed then needs to be fast enough to freeze the blade without too much motion blur within each frame.

Here the rotor has five blades, now lets say the rpm of the rotor is 300. That means, per rotation, a blade is in a specific spot on five counts. That gives us an effective rpm of 1500. 1500rpm / 60secs = 25.

Therefore shooting at 25fpm will ensure the rotor blades are shot in the same position every frame. Each frame then has to be shot at a fast enough shutter speed to freeze the blade for minimal motion blur.

Anonymous #2 says:

And Anonymous comes in for the win!

Anthony says:

False. Helicopters have a constant rotor speed in flight. Only thing that changes is the pitch of the blades.

Evan says:

Or if helicopters don’t work the way you think they do.

Bram says:

I’m no expert (though I did receive my BSc in mechanical engineering) but I think this could very well be possible. I’d say it’s a matter of frame rate. SS to my knowledge will only influence how the blades actually appear in each still (frozen or blurry), while FR determines the position each blade is captured in. Besides, we’ve seen this before, for instance in westerns where the wheels of the wagons would appear to be still or even moving backwards because of the camera’s frame rate.

Hi,

I think this is real and sounds logic the explanation about synrho. If you pay attention the rear rotor spins, but more slow than a normal chopper video and the main rotor spins too… but too slowly, like out of synchro with the frame rate.

nice weekend
ciao

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