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Camera Shutter Speed Synchronized with Helicopter Blade Frequency

Derek Sine's picture
Last seen: 4 years 4 months ago
Joined: 12/15/2009 - 7:28pm

Pretty Cool Shutter Speed Sync.

http://www.youtube.com/watch?v=C8_6NRXfRVE



composite1's picture
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Joined: 12/11/2008 - 7:54pm
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Neat how they did that. But the beauty of helo's watchin' them blades whir around faster than you can see. As is that looks like there should be a little kid's hand underneath going, 'eeeeeyaaaaarrrrrrr!'

H.Wolfgang Porter, Composite Media Producer Dreaded Enterprises Unlimited, Inc. www.dreadedenterprises.com



pseudosafari's picture
Last seen: 14 hours 42 min ago
Joined: 01/19/2009 - 2:09am

what's next? stage coaches in a cowboy movie that move along the trail without their wheels turning?



composite1's picture
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Joined: 12/11/2008 - 7:54pm
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"Isn't it the frame rate that is synchronized?"

Roble,

Unlikely. If shot with a non-high-speed video cam, it is more effective to just match your shutter speed. Frame rates for standard video cameras don't get much higher than 60fps (NTSC) or 50fps(PAL). That isn't fast enough to create that effect.

H.Wolfgang Porter, Composite Media Producer Dreaded Enterprises Unlimited, Inc. www.dreadedenterprises.com


roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

No, it's frame rate. Frame rate is the number of times the sensor is exposed per some time period, say 60 times per second for a frame rate of 60fps. Shutter speed is the length of time the sensor is exposed, for example 1/2000 sec. So the units are wrong (frame rate has unit of 1/time while shutter speed has unit of time). The sensor here is probably CCD because there is no evidence of rolling shutter (the blades appear straight, not curved). A typical helicopter rotor spins at about 500 rpm (note same units as frame rate, i.e. rotations per a time period, here rotations per min.). Since the blades are not blurred the shutter speed is probably at least about 1/2000 sec. This is a typical shutter speed obtained with a ordinary camcorder. My Canon HF11 has a maximum of 1/2000 sec. A CMOS sensor is a little different and operates in that the width of the region exposed determines the shutter speed. As this region moves down the sensor one gets the rolling shutter effect. This is similar to the focal plane shutter in film cameras. Here the frame rate is how often this happens.


composite1's picture
Last seen: 2 weeks 4 days ago
Joined: 12/11/2008 - 7:54pm
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Roble,

You answered your own question correctly that I answered correctly without all the math. Well at least you know....

H.Wolfgang Porter, Composite Media Producer Dreaded Enterprises Unlimited, Inc. www.dreadedenterprises.com


Derek Sine's picture
Last seen: 4 years 4 months ago
Joined: 12/15/2009 - 7:28pm

CMOS Sensor

Derek Sine
Trails Ventures, LLC.


roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

Dear Composite 1, Your answer that it is shutter speed that is sychronized is wrong. It is frame rate that is synchronized. Sorry if my previous post was not clear to you.

Robert


XTR-91's picture
Last seen: 8 months 3 weeks ago
Joined: 12/06/2008 - 8:57pm

Sounds a lot like something that might have happened with LANC synchronization, having a configuration box (or connection) somewhere between the camcorder and helicoper. Maybe not, but everyone else seems to indicate that it's nearly impossible by hand.


composite1's picture
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Joined: 12/11/2008 - 7:54pm
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Roble,

Survey says... XXX! (insert angry buzzer sound) Oh I'm sorry but you sir are incorrect! Thank you for playing. Unless the guy had a high frame rate video camera (which he didn't) the only other way he could have done it was with... wait for it... SHUTTERSPEED! And before you attempt to explain something to me that I've been teaching off and on since 1998, read the title bar on the video above. He said it himself that he synced it with shutter speed. If that isn't enough for you, here's some nice articles on the subject:

http://provideocoalition.com/index.php/aadams/story/rolling_shutter_and_flickering_hmis/

http://en.wikipedia.org/wiki/High_speed_photography

http://www.cybercollege.com/tvp019.htm

H.Wolfgang Porter, Composite Media Producer Dreaded Enterprises Unlimited, Inc. www.dreadedenterprises.com


roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

The effect seen with the helicopter blades is the same as that seen in movies where car tires sometimes appear to stop rotating or even rotate in reverse. This is because the frame rate, 24fps for movies, is synchronized with the rotational rate of the tires or some fraction thereof depending on the number of spokes. No high speed photography or exceptionally fast shutter speed is involved.



roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

A helicopter rotor rotates at constant rpm, with lift (load) adjusted by blade pitch. This is why the rotors appear fixed even though the helicopter is maneuvering about. We can calculate the rpm if we assume a frame rate. The rotor in the video has 5 blades. If we take a frame rate of 30fps, typical of a camcorder, then we get 30/5 = 60 rev per second or 360 rpm.

Curious people want to know how things work.

Robert


composite1's picture
Last seen: 2 weeks 4 days ago
Joined: 12/11/2008 - 7:54pm
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Roble,

Dude the maker of the vid said he used 'Shutter Speed' to sync the video. I used to fly in helicopters to do aerial photography and videography (with people shooting at us BTW.) The way we dealt with blade interference was syncing up the shutter speed of the video camera. I'm not saying that you cannot sync up with the camera's frame rate. You're calling the guy who made this a liar by insisting he did it with frame rate when he said it was done by adjusting the shutter speed.

Since you're so 'curious' and 'want to know how things work' when 'people' have told you how, please take a video camera and prove your theory and post it up here so it can be seen. All it will avail you though is to conclude what you were told a number of times from a number of differing sources.

H.Wolfgang Porter, Composite Media Producer Dreaded Enterprises Unlimited, Inc. www.dreadedenterprises.com



roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

To be in sync the rotor has to appear the same frame after frame. The rotor has 5 blades so this rotor will appear the same every 1/5th of a rotation. It would also appear the same after 2/5ths, 3/5ths etc. However typical helicopter rotor speeds are a few hundred rpm. If I assume it rotates 1/5 of a rotation every frame at a frame rate of 30fps, it appears the same every 1/30th of a sec. so it makes a full rotation in 5x(1/30) = 6 rev/sec. or 360 rpm, in the range of typical helicopter rotation speeds. A high shutter speed is necessary to make the blades not appear blurred. If the shutter speed was only 1/30sec then it would be just one big blur, but if the shutter speed was 1/10th this, say 1/300sec then the blur would only be 10%.

Here is a link to a video I made of a passing train. The Canon HF11 has a CMOS sensor where the top of the sensor is exposed first. This was taken at 30fps and a shutter speed of 1/2000sec. Note that the freeze frame is sharp because of the high shutter speed and the leading edge of the car slants back. The train is moving from right to left and the top is recorded first so it appears to slant back.

Canon VIXIA HF11 Shutter Speed Test

http://www.youtube.com/user/roblewis56?feature=moby#play/all/uploads-all/0/tsb6T_4SYJE



Rob Grauert's picture
Last seen: 2 months 2 weeks ago
Joined: 02/16/2008 - 10:47pm

Ok, your math checks out, but you also proved Composite's point. The shutter speed needs to be sped up to reduce the blur so you can see the blades, as you stated. So it is the shutter speed that the camera operator is syncing with the blades in order to achieve this effect.



roblewis56's picture
Last seen: 1 year 2 weeks ago
Joined: 06/07/2009 - 11:59pm

I think I could show the units in my example more clearly.

(30frames/sec) / (5 frames/revolution) = 30/5 = 6 revolutions/sec and x 60sec/min = 360 revolutions/min.

The shutter speed is irrelevant as long as it is faster than about 1/300sec



anindya49's picture
Last seen: 4 years 3 months ago
Joined: 01/15/2010 - 10:26am

I think composite1 and robiewis56 are both right: a high shutter speed is needed to make the rotor blades appear sharp, but the rpm must also match the frame rate so that when each frame is exposed the blades are at the same positions (modulo the number of blades) in the rotation cycle. So, if the first frame has a blade in the 12 o'clock position, each subsequent frame needs to have a blade at the same position for the rotor to appear stationary - really simple, basic stroboscopic effect.

Great find Derek, as always! I think the video would have looked even more interesting with a rolling shutter camera, judging by the still you posted later.