# Wheels appearnig to rotate backwards on film

**URL:** <https://boards.straightdope.com/t/wheels-appearnig-to-rotate-backwards-on-film/107712>\
**Category:** Factual Questions\
**Created:** [May 7, 2002, 1:29pm UTC](https://boards.straightdope.com/t/wheels-appearnig-to-rotate-backwards-on-film/107712 "2002-05-07T13:29:35Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\
**Post date:** [May 9, 2002, 11:03am UTC](https://boards.straightdope.com/t/wheels-appearnig-to-rotate-backwards-on-film/107712/21 "2002-05-09T11:03:56Z")

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> [@](#):
>
> \*Originally posted by MonkeyMensch \*  
> \*\*[hijack quiz]  
> Well, I once saw the effect in braod daylight. Can any of you guys figure out how this might have occured?  
> [/hijack quiz] \*\*

Your eyes also have an image sample rate, of about 30 times/sec. So a tire rotation that almost matches that (or a multiple of that) will also produce the effect.

This is why motion pictures are at 24 frames/sec. It’s so close to our natural frame rate that the images appear to be moving. Filming at more frames per second would just be a waste of film, since it wouldn’t make a difference in appearant image motion.

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**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\
**Post date:** [May 9, 2002, 11:16am UTC](https://boards.straightdope.com/t/wheels-appearnig-to-rotate-backwards-on-film/107712/22 "2002-05-09T11:16:02Z")

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> [@](#):
>
> \*Originally posted by AWB \*  
> **Your eyes also have an image sample rate, of about 30 times/sec. So a tire rotation that almost matches that (or a multiple of that) will also produce the effect.**

I’ve never heard this before, and it goes against what I know - I must ask for a cite. The image may persist in your eye for 1/30 of a second (actually I thought it was usually longer…). But for the aliasing effect to occur, the eye must become sensitive and insensitive at a rate of 30 cycles a second. This means that if you set a strobe light at 30Hz and adjust the frequency and phase, you won’t see the light at all. I’m sorry, I don’t believe it.

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**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\
**Post date:** [May 9, 2002, 2:40pm UTC](https://boards.straightdope.com/t/wheels-appearnig-to-rotate-backwards-on-film/107712/23 "2002-05-09T14:40:40Z")

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> [@](#):
>
> \*Originally posted by AWB \*  
> \*\*
> 
> Your eyes also have an image sample rate, of about 30 times/sec. So a tire rotation that almost matches that (or a multiple of that) will also produce the effect.
> 
> This is why motion pictures are at 24 frames/sec. It’s so close to our natural frame rate that the images appear to be moving. Filming at more frames per second would just be a waste of film, since it wouldn’t make a difference in appearant image motion. \*\*

If this were true watching TV would be alost impossible - similar to watching computer monitors on video.

Motion pictures are at 24fps because tests have shown that, at reasonable distances, your eye cannot detect flicker above 47Hz. Movie projectors use a double shutter so each frame is seen twice, giving a flicker rate of 48Hz. Older movies were shot at 16fps and used a triple shutter for projection.

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