Advertise on Bikeforums.net



User Tag List

Results 1 to 24 of 24
  1. #1
    me have long head tube TallRider's Avatar
    Join Date
    Oct 2005
    Location
    Chapel Hill, NC
    Posts
    4,097
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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.

  2. #2
    Senior Member
    Join Date
    Jan 2013
    Location
    High Plains
    My Bikes
    old clunker
    Posts
    1,028
    Mentioned
    9 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TallRider View Post
    . . .
    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.
    Last edited by AnkleWork; 04-01-14 at 06:16 PM.

  3. #3
    Senior Member
    Join Date
    Dec 2009
    Posts
    8,299
    Mentioned
    58 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TallRider View Post
    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.
    April Fool?
    Robert

    My hero: "Tar-Baby ain't sayin' nuthin'..." (Joel Chandler Harris, Uncle Remus")

  4. #4
    Senior Member
    Join Date
    Dec 2009
    Posts
    8,299
    Mentioned
    58 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by AnkleWork View Post
    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.
    Robert

    My hero: "Tar-Baby ain't sayin' nuthin'..." (Joel Chandler Harris, Uncle Remus")

  5. #5
    Senior Member
    Join Date
    Aug 2005
    Location
    Pittsburgh, PA
    My Bikes
    '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
    Posts
    25,989
    Mentioned
    10 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by rpenmanparker View Post
    April Fool?
    Not at all. His description as to how "photo finish" cameras work is correct.

  6. #6
    Senior Member
    Join Date
    Dec 2009
    Posts
    8,299
    Mentioned
    58 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by HillRider View Post
    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.
    Robert

    My hero: "Tar-Baby ain't sayin' nuthin'..." (Joel Chandler Harris, Uncle Remus")

  7. #7
    It's MY mountain DiabloScott's Avatar
    Join Date
    Sep 2006
    Location
    Mt.Diablo
    Posts
    5,609
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by HillRider View Post
    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.
    http://diabloscott.blogspot.com/

  8. #8
    Senior Member
    Join Date
    Aug 2005
    Location
    Pittsburgh, PA
    My Bikes
    '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
    Posts
    25,989
    Mentioned
    10 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by DiabloScott View Post
    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.

  9. #9
    Senior Member Looigi's Avatar
    Join Date
    Dec 2010
    Posts
    7,588
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by DiabloScott View Post
    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
    Ride more. Fret less.

  10. #10
    Senior Member
    Join Date
    Feb 2009
    Posts
    4,112
    Mentioned
    5 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by HillRider View Post
    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.

  11. #11
    Senior Member
    Join Date
    Aug 2005
    Location
    Pittsburgh, PA
    My Bikes
    '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
    Posts
    25,989
    Mentioned
    10 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by davidad View Post
    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.

  12. #12
    Senior Member
    Join Date
    Dec 2009
    Posts
    8,299
    Mentioned
    58 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Looigi View Post
    I would imagine it depends upon the ratio of the slit width to the total width of the sensor/film. Narrower slit = more distortion.
    Robert

    My hero: "Tar-Baby ain't sayin' nuthin'..." (Joel Chandler Harris, Uncle Remus")

  13. #13
    Senior Member
    Join Date
    Sep 2012
    Location
    SW Missouri
    My Bikes
    specalized sirrus
    Posts
    107
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    What is up with the guy at the top who has the seat in his stomach.

  14. #14
    Senior Member kv501's Avatar
    Join Date
    Oct 2011
    Posts
    800
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by ragtoplvr View Post
    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.
    Quote Originally Posted by Retro Grouch View Post
    Are you aware that this is a pedal bike forum?

  15. #15
    It's MY mountain DiabloScott's Avatar
    Join Date
    Sep 2006
    Location
    Mt.Diablo
    Posts
    5,609
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by kv501 View Post
    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.

    http://diabloscott.blogspot.com/

  16. #16
    Senior Member
    Join Date
    Sep 2012
    Location
    SW Missouri
    My Bikes
    specalized sirrus
    Posts
    107
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by kv501 View Post
    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.

  17. #17
    afraid of whales Mr IGH's Avatar
    Join Date
    Apr 2009
    Location
    Front Range, CO
    Posts
    2,662
    Mentioned
    2 Post(s)
    Tagged
    0 Thread(s)
    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.
    IGH's, Dyno Hubs, LED lights and old frames

  18. #18
    Senior Member Homebrew01's Avatar
    Join Date
    Jul 2004
    Location
    Ffld Cnty Connecticut
    My Bikes
    Old Steelies I made, Old Cannondales
    Posts
    15,966
    Mentioned
    7 Post(s)
    Tagged
    1 Thread(s)
    Quote Originally Posted by ragtoplvr View Post
    What is up with the guy at the top who has the seat in his stomach.
    The term is "Throwing his bike"
    Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike

  19. #19
    It's MY mountain DiabloScott's Avatar
    Join Date
    Sep 2006
    Location
    Mt.Diablo
    Posts
    5,609
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Mr IGH View Post
    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)

    Last edited by DiabloScott; 04-05-14 at 12:36 PM.
    http://diabloscott.blogspot.com/

  20. #20
    Senior Member
    Join Date
    Aug 2005
    Location
    Pittsburgh, PA
    My Bikes
    '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
    Posts
    25,989
    Mentioned
    10 Post(s)
    Tagged
    0 Thread(s)
    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.

  21. #21
    Senior Member rydabent's Avatar
    Join Date
    Apr 2010
    Location
    Lincoln Ne
    My Bikes
    RANS Stratus TerraTrike Cruiser
    Posts
    4,093
    Mentioned
    5 Post(s)
    Tagged
    0 Thread(s)
    Simply camera aberration Kind of like the wheels turning backward on the stage coach in old western movies.

  22. #22
    don't try this at home. rm -rf's Avatar
    Join Date
    Jan 2006
    Location
    N. KY
    Posts
    2,740
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    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.

    Attached Images Attached Images
    Last edited by rm -rf; 04-06-14 at 09:19 AM.

  23. #23
    Senior Member Homebrew01's Avatar
    Join Date
    Jul 2004
    Location
    Ffld Cnty Connecticut
    My Bikes
    Old Steelies I made, Old Cannondales
    Posts
    15,966
    Mentioned
    7 Post(s)
    Tagged
    1 Thread(s)
    Quote Originally Posted by rm -rf View Post
    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.
    Last edited by Homebrew01; 04-06-14 at 12:55 PM.
    Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike

  24. #24
    It's MY mountain DiabloScott's Avatar
    Join Date
    Sep 2006
    Location
    Mt.Diablo
    Posts
    5,609
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Homebrew01 View Post
    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"
    http://diabloscott.blogspot.com/

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •