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Old 04-01-14, 04:52 PM   #1
TallRider
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spokes appear weirdly curved in sprint-finish photo

Here is a picture of Peter Sagan accidentally winning the opening stage of Three Days of De Panne.



I'm curious why the spokes all appear to be curved here, and particularly why they are all curved away from the ground-contact point.
I think the major variables are:
a) camera shutter
b) wheel rotation
c) forward motion

The spoke lined up with the ground has the least total velocity (angular+linear), which probably accounts for its being the point of visual symmetry here.

But I'd love to better understand what's going on here.
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Old 04-01-14, 05:12 PM   #2
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. . .
a) camera shutter
b) wheel rotation
c) forward motion
. . .
All of the above. "Photo finish" cameras use a vertical slit shutter and moving film. Most importantly, they capture a time derivative of the scene instead of the usual location derivative. A vertical spoke is captured all at once (along its length) while the others are captured over a time.

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Old 04-01-14, 05:17 PM   #3
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Here is a picture of Peter Sagan accidentally winning the opening stage of Three Days of De Panne.



I'm curious why the spokes all appear to be curved here, and particularly why they are all curved away from the ground-contact point.
I think the major variables are:
a) camera shutter
b) wheel rotation
c) forward motion

The spoke lined up with the ground has the least total velocity (angular+linear), which probably accounts for its being the point of visual symmetry here.

But I'd love to better understand what's going on here.
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Old 04-01-14, 05:19 PM   #4
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All of the above. "Photo finish" cameras use a vertical slit shutter and moving film. Most importantly, they capture a time derivative of the scene instead of the usual location derivative. A vertical spoke is captured all at once (along its length) while the others are captured over a time.
Damn, that is really interesting.
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Old 04-01-14, 06:13 PM   #5
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April Fool?
Not at all. His description as to how "photo finish" cameras work is correct.
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Old 04-01-14, 06:44 PM   #6
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Not at all. His description as to how "photo finish" cameras work is correct.
I know, but I hadn't seen any AF reference on BF yet today. Seemed like a good opportunity. Plus I wrote it before the explanation posted.
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Old 04-01-14, 06:46 PM   #7
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Not at all. His description as to how "photo finish" cameras work is correct.
True, but the same spoke effect can be noticed in plain old digital camera photos because it still takes time for the scan to cross the sensor.
The shortening effect of the faster riders compared to the slower riders is a photo finish exclusive though.
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Old 04-01-14, 07:02 PM   #8
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True, but the same spoke effect can be noticed in plain old digital camera photos because it still takes time for the scan to cross the sensor.
The shortening effect of the faster riders compared to the slower riders is a photo finish exclusive though.
Yes, and you used to see the same effect in film cameras with focal plane shutters. Large 4x5 "Press Cameras" really showed it because the shutter had to travel so far and took appreciable time.
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Old 04-02-14, 09:44 AM   #9
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True, but the same spoke effect can be noticed in plain old digital camera photos because it still takes time for the scan to cross the sensor.
...
AKA a "rolling shutter". https://www.google.com/search?q=roll...=1553&bih=1017
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Old 04-05-14, 07:40 AM   #10
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Yes, and you used to see the same effect in film cameras with focal plane shutters. Large 4x5 "Press Cameras" really showed it because the shutter had to travel so far and took appreciable time.
4X5 press cameras were Speed Graphics and did not have a focal plane shutter. The shutter was built into the lens.
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Old 04-05-14, 07:51 AM   #11
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4X5 press cameras were Speed Graphics and did not have a focal plane shutter. The shutter was built into the lens.
No, not all. Some lenses for the Speed Graphics did include diaphragm shutters but many did not and the camera bodies had focal plane shutters. The focal plane offered a wider range of shutter speeds up to 1/1000 second while the diaphragm shutters were limited to 1/500 at best.

Back in the 30's and 40's newspaper and magazine pictures of things like race cars (regular pictures, not finish line photos) often showed them with "ovalized" wheels due to the focal plane shutter's distortion.
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Old 04-05-14, 08:13 AM   #12
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I would imagine it depends upon the ratio of the slit width to the total width of the sensor/film. Narrower slit = more distortion.
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Old 04-05-14, 08:35 AM   #13
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What is up with the guy at the top who has the seat in his stomach.
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Old 04-05-14, 08:41 AM   #14
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What is up with the guy at the top who has the seat in his stomach.
Trying to jut the bike out in front of himself before the finish line. Same idea as (running) sprinters when they thrown their upper bodies forward at the line.
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Old 04-05-14, 08:55 AM   #15
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Trying to jut the bike out in front of himself before the finish line. Same idea as (running) sprinters when they thrown their upper bodies forward at the line.
Works especially well against premature celebrations.

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Old 04-05-14, 08:56 AM   #16
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Trying to jut the bike out in front of himself before the finish line. Same idea as (running) sprinters when they thrown their upper bodies forward at the line.
Thanks, I learned something today.
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Old 04-05-14, 09:00 AM   #17
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I don't think modern photo finish cameras use a slit, that's what they used for film cameras. Digital photo finish cameras are just high speed motion cameras running at ~2000fps. The distortion is different between the two methods, the curved spokes is a digital artifact.
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Old 04-05-14, 11:10 AM   #18
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What is up with the guy at the top who has the seat in his stomach.
The term is "Throwing his bike"
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Old 04-05-14, 11:29 AM   #19
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I don't think modern photo finish cameras use a slit, that's what they used for film cameras. Digital photo finish cameras are just high speed motion cameras running at ~2000fps. The distortion is different between the two methods, the curved spokes is a digital artifact.
When you see this kind of background (ie no background) - you know it's not a snapshot - the slit only sees one line of background and it stays the same and the horizontal axis is time, not distance.



Here's what the actual line looked like (from the other side)


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Old 04-05-14, 12:22 PM   #20
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Excellent explanation of the photo finish technology. BTW, I believe the photo in your above posting is the famous Eric Zabel "I won! Whoops..." shot from the '04 Milan-San Remo finish.
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Old 04-06-14, 07:36 AM   #21
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Simply camera aberration Kind of like the wheels turning backward on the stage coach in old western movies.
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Old 04-06-14, 08:00 AM   #22
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Yeah, slit scans show exactly what crossed the line first, all in one image, without having to compare a bunch of high speed photos. It doesn't need a high speed data storage or a really fast lens to capture the whole race.

Faster moving objects will be squashed horizontally, and slower ones stretched.

For instance, see the runner at the bottom right. Her heel was planted right on the finish line, so it stayed in the scan view for a long time. And the winner's foot was moving fast as it crossed the line, so it's narrow.

From this site.



~~~~~~~~
And that's why the spokes at the bottom are bent. They are moving slower than the spokes at top of the wheel, which are going faster than the bike, so they look like 36 spoke wheels.

On the leading edge of the wheel, as the spoke crosses the line, the nipple end of the spoke goes first, and the hub is last. The wheel is still turning, so the spoke looks curved.

It looks like Sagan was slowing as he crossed the line. His wheels are stretched. And his teammate was booking. It looks like he was already even with Sagan a foot past the finish line in the front shot that DiabloScott posted.

I suppose they adjust the display to make the riders look normal. If they used the same scan rate as the runners, the bike riders would be very squashed looking.

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Old 04-06-14, 11:49 AM   #23
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I suppose they adjust the display to make the riders look normal. If they used the same scan rate as the runners, the bike riders would be very squashed looking.
They adjust the speed of the camera to match the estimated finish speed of the cyclists, runners etc.

So if they expect the cyclists to be going 40 mph, then they set the camera "film" to travel at 40 mph inside. Cyclists going 38 or 42 will look stretched (38) or compressed (42).

Sagan's teammate # 14 is going quite a bit faster than Sagan at the line.

I remember many years ago at a stage race, coach Eddy B. was disputing the finish results because when he saw the photo, he thought it was a normal camera picture and that the guy in 4th place might have passed 3rd place by the time they got to the line. It took a bit of explaining that they were all on the finish line at the same time in the photo.
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Old 04-06-14, 12:58 PM   #24
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It took a bit of explaining that they were all on the finish line at the same time in the photo.
Yeah, the whole photo is the finish line, and the red line is just a tool they can slide back and forth along the "time axis"
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Old 08-22-15, 02:47 PM   #25
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What is up with the guy at the top who has the seat in his stomach.
Yes, I see that the thread is ancient...but, you know, it's all new to me.

So...yes, the poor schmuck is trying to jut the bicycle underneath him out in front of him so as to hit the finish line earlier than he otherwise would, if not for the jutting action. This makes it appear as if he is trying to transplant the saddle into his gut. It is, of course, based on the same notion as the track runner who learns his chest forward in order to break the tape a bit sooner than he would without the lean.

Of course, they are NOT AT ALL the same thing. The runner's lean is sensible and works. This is because the finish is, by rule, only official once the athletes chest has crossed the line...and because the chest is a part of the runner's body and that body can be leaned forward, i.e., extended into the future from the waist.

It does NOT, however, work with bicycles. And it is amazing to see these idiots actually trying. See, the bicycle is in motion, just like your automobile...rolling along

There exists no force, attached to the vehicle, than can cause it to jump forward. It cannot extend itself from itself. It can only move faster via moving its wheels faster.

A cyclist can do to nothing to improve his position except pedal. This is as absolute as the soundest laws of physics. Pushing a bicycle forward, while attached to it, is 100% impossible.

That riders so often attempt it is nothing short of displaying their incredible ignorance and unthinking stupidity to everyone. Not to mention the complete morons their coaches, trainers and loved-ones must be to have said nothing sensible enough to them to stop this wholly cartoonish finish-line spectacle. It amounts to nothing more than a fine example of the superstition that drives, still, a great deal of human behavior.

And in the middle of all the modern equipment that the pros have now...all the engineering and bleeding edge tech that has gone into their machines and training. And the incredible vast majority of persons involved in the sport simply can't help themselves from the numb-skull belief that a bicycle in motion could monkey push itself forward via any method other than increasing the pressure on the pedals that rotate the chain that drive the wheel that are its velocity.

The levity lies in the fact that, because they go through a pushing motion, riders only end up pushing themselves backward. They give themselves the appearance of stomaching their seats. From their point of view, however, like the rider on the train who doesn't sense that he is moving and gets the feeling that the station is moving as the train pulls away from it.

From the poor dumb cyclist's perspective, while it is he who is actually moving backward, his eyes tell his fabulously insufficient brains that the bicycle is moving forward. No dude! The bike isn't moving further forward, you are moving further back.

Last edited by YWoodman; 08-22-15 at 03:05 PM. Reason: clarification
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