In motion pictures and video, why do car wheels seem to be revolving in...

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SteveJ

Member
Hi there.
The eye can be deceived as well - it is an effect called persistence of vision.
Your eye takes a length of time to react to what it sees. When it locks on a point on a rotating wheel, then the wheel can move so fast that by the time the eye replaces its original recognition with the next one, the point followed has done NEARLY a rotation - your brain interprets this as the point going backwards, as that is the most logical explanation in nature.Each refresh will see that point a little further back.
The brain builds up and stores a picture of your environment and refreshes at its set rate, being programmed to spot changes first and to leave everything else in place unless a change is noticed.
Cheers, Steve.
 
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barticus

New member
Yes. It's something called "aliasing".

I'll try to explain this with a very simple example. Suppose you're filming a wheel that has no features except for the air valve, which is at the 12 o'clock position at the start. If the wheel turns clockwise at 24 revolutions per second (rps) and you film it at the rate of 24 frames per second. In a case like this, the air nozzle won't seem to move at all.

Now suppose the wheel turns a bit slower, say clockwise 23 rps. Then the film will show the air valve moving CONTER-clockwise at maybe 1 rps. That's the reverse turning you're talking about. If, on the other hand, the wheel turned clockwise at 25 rps. They you'll see the air valve turning clockwise at about 1 rps.

When you see wheels turning in reverse, you're probably seeing the wheel lug nuts or cut slots in the rims. That's a bit more complicated because the you're probably not tracking the same feature. In other words, if a wheel has 5 slots, A through E, and is turning clockwise really fast, you're probably not seeing the same slot from one frame to the next. If A is at 12 o'clock in frame 1, B might be just short of 12 o'clock in frame 2, then C a little short of that in frame 3, and so on. The wheel is indeed turning clockwise, but it's moving so fast the camera can't keep up with it. Since all the slots look the same, it looks like the wheel is turning in reverse.

Hope that helps!
 
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PokemonFan1

New member
Okay, I'll give you an easy example-
You see the clock at 12:00 pm
Then, you see it again at 11:00 am the next day
Again, you see it at 10:00 am the next day, and so on
If you notice the clock only on these times, then it appears to
be moving anti-clockwise
Now you get it?
In those movies, the speed of the camera frame was less, so everything appeared to be
moving fast, and the frame at which the wheel was caught kept changing
 
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