Frame Rate - Motion

Thread Starter

ben sorenson

Joined Feb 28, 2022
106
Out of curiosity, if there was an extremely fast object and you could physically see that object moving super fast with your own eyes and recorded the moving object with a video camera shot in 1080p @ 400 Frames Per Second, with a playback speed of 60 Frames Per Second.

If during the playback of the video the object that was recorded appeared to move extremely slow, almost in super slow motion what would the significance of that be?
 

WBahn

Joined Mar 31, 2012
27,895
Out of curiosity, if there was an extremely fast object and you could physically see that object moving super fast with your own eyes and recorded the moving object with a video camera shot in 1080p @ 400 Frames Per Second, with a playback speed of 60 Frames Per Second.

If during the playback of the video the object that was recorded appeared to move extremely slow, almost in super slow motion what would the significance of that be?
Is this object making repetitive motions, like a pendulum, or a spinning wheel?

If so, then what you are seeing is almost certainly aliasing. It's the same phenomenon that is encountered with strobe lights.
 

MrChips

Joined Oct 2, 2009
27,686
What you are seeing is the wagon wheel effect on movies when the wheel appears to be turning in the opposite direction.
This is the called aliasing or strobe effect as WBahn mentioned.

There are anecdotal accounts that when fluorescent lights were installed in factories, machinists got their fingers chopped off on machine lathes because they observed that the machines had stopped even though they were still turning. This was on account of the strobe effect of fluorescent lights.
 

MrSalts

Joined Apr 2, 2020
2,620
If during the playback of the video the object that was recorded appeared to move extremely slow, almost in super slow motion what would the significance of that be?
It means the frame rate of your camera was (almost) some nice fraction of the actual speed of the pendulum.
 

Ramussons

Joined May 3, 2013
1,321
Out of curiosity, if there was an extremely fast object and you could physically see that object moving super fast with your own eyes and recorded the moving object with a video camera shot in 1080p @ 400 Frames Per Second, with a playback speed of 60 Frames Per Second.

If during the playback of the video the object that was recorded appeared to move extremely slow, almost in super slow motion what would the significance of that be?
Significance?
If a video is played back at a slower frame rate than recorded at, then all action is slowed down.
Playing back at a faster frame rate speeds up the action.

The first is used to study the intricacies of an action, like the flapping of a Bird's wing or motion of a Body in a car crash ......
The second, germination of a seed, growth of a plant, ....

I see no stroboscopic effect here ........
 

MrSalts

Joined Apr 2, 2020
2,620
Significance?
If a video is played back at a slower frame rate than recorded at, then all action is slowed down.
Playing back at a faster frame rate speeds up the action.

The first is used to study the intricacies of an action, like the flapping of a Bird's wing or motion of a Body in a car crash ......
The second, germination of a seed, growth of a plant, ....
Obviously, but, he is asking why the recorded video plays back extra slow vs the expected slowdown based on frame rate differences. See above answers related to aliasing (wagon wheel effect).
 

Ramussons

Joined May 3, 2013
1,321
Obviously, but, he is asking why the recorded video plays back extra slow vs the expected slowdown based on frame rate differences. See above answers related to aliasing (wagon wheel effect).
Each frame of the original recording will have the image position shifted by a small amount, "x". That "x" will be "400 x" away after 1 second. Playback at a slower frame rate will still display the image shifted by the same "x" per frame but the displacement woukld be only 60 x after 1 second.
Stroboscopic effect will mean that the image display moves "back", or does not move at all. That is not possible in this case.

What you mean can happen only on a speed up of the display frame rate, not on slowing down.
 

MrAl

Joined Jun 17, 2014
9,633
Yes this happens when the capture rate is an integer multiple of the object movement.

An example is a guitar tuner. You can program a microcontroller to put out pulses of light with an LED and when you shine it on the string to be tuned the string will appear to stand still when the vibration is an integer multiple of the pulse frequency. It works really good but you do have to remember that an integer multiple may not be the actual frequency. Luckily the right frequency isnt too hard to determine because the strings would appear too loose or getting too tight if you tried to tune them to a frequency too far from the frequency they should be tuned at.
 
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