Author Topic: Why do car wheels look like they go backwards?? (Read 9019 times)

mike2niner4 · **on:** 23/02/2009 17:26:54

Hey,

Just wondering why car wheels look like they are going backwards when they get faster??

Thanks

Mike

graham.d · **Reply #1 on:** 23/02/2009 17:54:52

Hi Mike. I don't think they do look like they are going backwards except on film or TV (or somewhere where they are being illuminated with strobed lighting). The effect is due to the regular parts of the wheel (spokes for example) passing a point at a similar frequency to the frame rate of the film (or TV scan). When the frequencies are the same the wheel will look stationary, but if not quite matched the wheel can seem to be rotating slowly in either direction depending on whether the frame rate frequency is higher or lower than the frequency the spoke is passing by.

mike2niner4 · **Reply #2 on:** 23/02/2009 17:56:19

Thanks!!

I was watching Richard Hammand's show last night and i noticed it and thought i would ask on the porche... Interesting to know it's only on film, never noticed that

LeeE · **Reply #3 on:** 23/02/2009 21:01:48

graham.d - you don't need strobed illumination to get the effect - the frame-rate on it's own will cause it if the shutter speed is fast enough to prevent total blurring and allow enough detail to be seen to show that the wheel is actually rotating.

For another fun camera effect:

mike2niner4 · **Reply #4 on:** 23/02/2009 21:16:37

Thats Awesome!

DoctorBeaver · **Reply #5 on:** 23/02/2009 21:37:23

Good pic, LeeE

LeeE · **Reply #6 on:** 23/02/2009 22:26:28

Heh - you can tell from that picture that the camera had a vertical focal plane shutter.

This was the image I was really remembering though, and it shows rather well the effects of a horizontal focal plane shutter

Make it Lady · **Reply #7 on:** 23/02/2009 22:28:06

(on helicpter pic) His rotor blades have stopped, why isn't he falling from the sky?
(on car pic) I think the wheels look elongated not backwards.

chris · **Reply #8 on:** 23/02/2009 22:31:13

"Plane" shutter - but it was a picture of a helicopter...

graham.d · **Reply #9 on:** 24/02/2009 09:24:26

Hi Lee, I thought I had said that it was frame rate OR strobe lighting, but I see that putting the stobe light bit in brackets made it ambiguous.

LeeE · **Reply #10 on:** 24/02/2009 19:04:57

graham.d - my mistake/misunderstanding - it seems less ambiguous reading it again now and the references to frame rate are clear enough.

techmind · **Reply #11 on:** 24/02/2009 21:50:58

The answer to the original question has already been pointed out - you see the wheels stop, slow down, change direction on TV/film or when strobe-lit when the shutter (or strobe) frame-rate no-longer captures the position of the wheels fast enough to accurately represent (or sample) the motion of the wheel. It's only fancy words, but this is an example of "aliasing".

The effect is vivid only when the shutter of the camera has a short "duty cycle" ie the exposure is short compared to the frame rate (sometimes for historical reasons this is called the shutter angle, where 180degrees equals 50% duty cycle). If the exposure time was equal to 1/frame rate then the wheel simply becomes a blur when it gets too fast, rather than appearing to go backwards.

Mobile-phone cameras typically have a top-to-bottom "rolling shutter" (purely electronic) and this causes weird shearing artifacts if you pan the camera horizontally, or stretch-shrink artifacts if you wiggle it vertically. Have fun!

lyner · **Reply #12 on:** 25/02/2009 22:09:45

Aliasing due to temporal sub-sampling.
End of.

Naked Science Forum

Author Topic: Why do car wheels look like they go backwards?? (Read 9019 times)

lyner