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?
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.
I’ll go with what u laid out here, I got about 90% of it. Toight analysis man. Respect!
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.
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!?!
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!
Great answer with the correct explanation. Kudos.
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.
And Anonymous comes in for the win!
False. Helicopters have a constant rotor speed in flight. Only thing that changes is the pitch of the blades.
Or if helicopters don’t work the way you think they do.
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