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Wagon Wheels: Newsletter Challenge (03/22/05)

Posted March 22, 2005 9:13 AM

The question as it appeared in the 3/22 edition of Specs & Techs from GlobalSpec:

"You come home to find — wonder of wonders — your son and daughter quietly watching an old western movie on cable TV. (No fighting — how can that be?) "In addition to making fun of what they're watching ("this is so hokey, Dad"), they point out the movie is really lame because the stagecoach wheels are turning backwards. How do you explain this to them? (You can even use this as an opportunity for an impromptu lesson on data acquisition.)"

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Participant

Join Date: Apr 2005
Posts: 2
#1

Wagon Wheels rotating

04/12/2005 11:05 AM

Two possible answers: 1. They are rewinding a taped movie 2. The images are made up of discrete frames. As such, the frames are not syncronized with rotations and the images will, particularly in relatively slow rotating wheels, have a lead that is long enough to show more than a 180 deg change. Hence the rotation will appear backwards merely because subsequent frames show >180 deg positions.

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Participant

Join Date: Apr 2005
Posts: 2
#2
In reply to #1

Re:Wagon Wheels rotating

04/20/2005 11:07 AM

Films shot with a typical motion picture camera expose one frame every 24th of a second... Let's take a simple wheel with only 1 spoke. The wagon is going left to right so the wheel goes clockwise. In frame 1 the spoke is at the noon position. If the wagon only travels enough of a distance to put the spoke at the 11 o'clock position on frame 2, 10 o'clock on frame three and et cetera, then the wheel will appear to be going backwards. Adjusting the shutter on the camera can minimize or maximize this effect, as can longer focal length lenses. Quickly panning the camera (which means left to right movement on a tripod head) with a long lens can introduce the "picket fence effect." An object will jump from position A to position C without a B position in-between. Cheers! Steve.

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Participant

Join Date: Apr 2005
Posts: 2
#3

wagon wheels and cameras

04/20/2005 12:44 PM

Films shot with a typical motion picture camera expose one frame every 24th of a second... Let's take a simple wheel with only 1 spoke. The wagon is going left to right so the wheel goes clockwise. In frame 1 the spoke is at the noon position. If the wagon only travels enough of a distance to put the spoke at the 11 o'clock position on frame 2, 10 o'clock on frame three and et cetera, then the wheel will appear to be going backwards. Adjusting the shutter on the camera can minimize or maximize this effect, as can longer focal length lenses. Quickly panning the camera (which means left to right movement on a tripod head) with a long lens can introduce the "picket fence effect." An object will jump from position A to position C without a B position in-between. Cheers! Steve.

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Friend of CR4

Join Date: Dec 2004
Posts: 1995
Good Answers: 35
#4

And the answer is...

05/12/2005 3:01 PM

As reported in the 03/29 edition of Specs & Techs from GlobalSpec:

This is due to an effect known as aliasing, whereby the frame rate is not fast enough to capture the correct direction of rotation. Each frame (1/24 second apart in most American movies) is captured when the wheel has gone approximately 7/8 of the way around. This results in the wheel appearing to have traveled backwards 1/8 of a turn in each successive frame. This is an important concept in data acquisition, for which the Nyquist theorem states that the data sampling frequency (in this case, the movie frame rate) must be at least twice the acquired signal frequency (i.e. the stagecoach wheel rotation) for meaningful data.

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Anonymous Poster
#5
In reply to #4

Re:And the answer is...

06/22/2006 9:23 AM

one word : aliasing. QED

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Anonymous Poster
#6

Re: Wagon Wheels: Newsletter Challenge (03/22/05)

03/22/2007 8:19 PM

How to explain to kids?

Get a variable frequency strobe light and show them while you explain it.
Let them play with it. They'll figure it out.

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