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  1. #1
    Poohblah's Avatar
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    why doesn't frame spacing increase as film winds on?

    i've been wondering this for a while. basically, the premise of my question is that, as film winds on to the take-up spool, the effective diameter of the take-up spool increases because there is more film built up around the take-up spool. so, by the laws of geometry, if the angle that the take-up spool rotates through as one frame advances is constant, then the frame spacing should increase as more film is wound on.

    why, then, does this not actually occur?

  2. #2

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    It does occur on film without sprocket holes!
    2F/2F

    "Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."

    - Rob Tyner (1944 - 1991)

  3. #3
    DWThomas's Avatar
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    Many cameras figure out the amount of film motion from an idler roller next to the supply spool. The spool diameter has no effect there. My Perkeo II (120 film) works that way. Other cameras have some sort of magic cam mechanism to adjust the rotation as the film spool diameter changes: I believe that's what my Bronica backs do (but haven't ripped one open yet).

    DaveT

  4. #4
    Ian Grant's Avatar
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    Older cameras had a magic window so you could just line up the frame number printed on the back of the film. Come to think of it so do some modern Panoramic cameras

    Ian

  5. #5
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    Your winding mech is counting sprocket holes. That is the constant.
    He is no fool who gives what he cannot keep..to gain that which he cannot lose. Jim Elliot, 1949

    http://tonopahpictures.0catch.com

  6. #6
    Ian Grant's Avatar
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    Quote Originally Posted by jimgalli View Post
    Your winding mech is counting sprocket holes. That is the constant.
    On 120 film ?????? It'll wear out before it finds one

    Ian

  7. #7
    nsouto's Avatar
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    on 35mm, it's sprocket holes.
    on the pentax 6x7 it's a rubber roller before the take up spool.
    on the 6X6 Arax (Pentacon six offshot), same.
    on the rb67, I don't know but I'll find out: you got me thinking now and it won't go away...
    Cheers
    Noons (Nuno Souto)
    Gallery here

  8. #8
    jimgalli's Avatar
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    Quote Originally Posted by Ian Grant View Post
    On 120 film ?????? It'll wear out before it finds one

    Ian
    The question was in the 35mm Cameras section. But you're right. With my Mamiya Universal 6X9 the spacing did change as the film progressed from one side to the other.
    He is no fool who gives what he cannot keep..to gain that which he cannot lose. Jim Elliot, 1949

    http://tonopahpictures.0catch.com

  9. #9

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    Quote Originally Posted by jimgalli View Post
    The question was in the 35mm Cameras section. But you're right. With my Mamiya Universal 6X9 the spacing did change as the film progressed from one side to the other.
    ...which is why I thought this was common to all MF cameras. The Super 23 was my first MF.
    2F/2F

    "Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."

    - Rob Tyner (1944 - 1991)

  10. #10
    AgX
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    Pooblah,

    The solution to this problem is very, very simple:

    The spacing between the frames is controlled by the transport mechanism via the sprocket wheel.
    Its gear drive is designed that way that one full stroke of the transport lever will turn the wheel as much as it needs for 1-frame+spacing.
    The take up spindle is geared to this mechanism that way that one full stroke of the transport lever will rotate it that much to accept 1-frame+spacing.

    As you realized, that would lead to problems as with advancing film the diameter of the roll of film on the transport wheel and thus its outer circumference will rize, which means that it would need to get more film per stroke than the sprocked wheel would deliever.

    The solution is a friction drive inserted in the link between transport mechanism and the take up spindle.

    As simple as that.

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