Why do car wheels look as if they are spinning backwards on film?

Question

I've noticed this on many TV shows, films and adverts- it always looks as if the wheels are spinning backwards as the car drives forward at speed- why is this?

Answer 1

Hi there.
What you are seeing is a well known optical illusion that only occurs at certain speeds (in real life too, it is just easier to see on film - try it with a spinning top). As a wheel spins faster, it gets too fast for the eye to focus on - with persistence of vision (which is where the brain stores what it has already seen and builds on top of that base image) any spot on the wheel will appear first to stop still and then with an increase of speed will appear to go backwards.

Simply put, the spot on the wheel you are attempting to focus on will not quite reach the same position in the time it takes your eye to notice it has moved - so it appears to go backwards.
Cheers, Steve.

Answer 2

It's all to do with how many frames the camera is taking and how fast the car is going. Every time the video takes a frame it shows the wheel at a certain place but in the next frame the wheel is in a slightly differant place just behind where it was in the previous frame and so on. So when you run the film it looks like the wheel is going backwards.

Answer 3

The "stroboscopic effect" is responsible. When each moving spoke, hole or conspicuous part of the turning wheel passes a reference point (such as a specific spot on the wheel that your eye is glued to instead of tracking the wheel's movement), you discover that there is a frequency (number of times) that the conspicuous part(s) pass your line of sight.

At a certain frequency (or rotation speeds), the moving parts will apear stationary, at other frequencies or rotation speeds the parts will appear to be turning forward or backwards...depending of course on the speed itself.

TV has a role to play in this effect. The rate at which a TV (depending on which color system it uses, I'll explain this later) draws images -using light- also claborates with those rotation speeds I mentioned, to create the effect you are asking about.

If you have a stroboscope (google, Yahoo or Wiki this), you can learn more about it. As for the TVs, heres some info thats relevant to strobe effect:

1). NTSC 3.58 = screen image redrawn at 3.58Mhz. If you can rotate your stroboscope at this same speed, you should be able to get still pictures on your TV.

2). NTSC 4.43 = Same as above but at speed of 4.43MHz.

3). SECAM = System Electroinique Coloeur Avec Memoire. This is french and I don't know how fast screen images are "drawn" by it.

4). PAL = Phase Alternation Line. Screen is divided into -I think about- 625(?) horizontal lines which images are redrawn over again at -I think- 25MHz(?). I can't remember its been such a long time since I played around with this stuff.

Answer 4

Movies are made of a strip of sequentially illuminated, discrete images. We don't see them that way because the images flicker at a rate of 96 times per second on the screen--too fast for our eyes to detect. Were the film speed perfectly in sync with the rotation of, say, a wheel being filmed, the wheel would complete a rotation whenever an image of it occurred, making it appear stationary. As it happens, though, a wheel of a moving vehicle often lags slightly behind the film speed, so that a spoke has not quite returned to its initial position at the moment an image is taken. As we focus on a spoke's original position, the forwardmoving wheel appears on film to turn backward.

Answer 5

It`a all to do with frequencies. Technical, but i`ll try. When the wheel is looks like it`s spinning forward it is spinning less than the frequency of light. When the wheel seems to spin backwards it`s sppinning faster than the frequency of light. You will notice at one point the wheel appears staitinary. This is when the frequencies of the wheel spin and light are the same.

Answer 6

I would guess it is to do with frequency. If you take a fluorescent lit room and place a machine rotating at 50 times per second in it, the machine will appear stationary. Electricity in the UK is at 50 Hz (alternating waves at 50 cycles per second) so in effect the light will flash at 50 times per sec illuminating the machine in the same position each time, so it will appear still. Speeding the machine up slightly would make it look like it was rotating slowly, slowing it down would make it appear to be going backwards. I surmise this is the same principle between TV cameras and the rotating wheels of the car

Answer 7

Lets imagine the wheel is being filmed at n frames per second. If the wheel is rotating at say n-1 revs then detail on the wheel will appear successively retrograde due to persistence of vision. In practice the rev rate of the wheel varies, so what you see is forward and retrograde motion successively.
Try this..turn on the tap in the kitchen to drip ..turn out the lights..turn on a strobe at about 10hz..by varying the flash rate by +/- 1 you should be able to 'freeze' the drops, slow them down, or make them appear to move back up to the tap.

Answer 8

In films like 'the dambusters', scenes with propellor-driven aircraft starting their engines or manoeuvring on the ground show a version of this effect.

As the speed of the prop changes, the blades appear to stop rotating (what you actually see is a flash of reflected daylight from the face of the blade, not the blade itself), and then turn in the opposite direction. This reversal will often happen several times, because the 'standstill' happens every time the speed-of-rotation is a multiple ('harmonic frequency') of the film-frame speed.

Answer 9

I always thought that it was because as most movie making is done under lights to make thing appear 'normal', it is a case of shadows moving backward as the wheel moves forward that we see, this combined with the old frame by frame movie making methods where lots of still pics are run together quicker than the human eye can see, creates the illusion that you see. Does this still happen with digital movies?

Answer 10

Because the speed of the film through the camera ( this is before digital cameras) is faster than the speed of the wheels on the car.