ramping

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  • ramping

    I was wondering how exposure works if you want to ramp, for example from 24fps to like, 50-60fps, or more. Since you'll be losing light as you give less exposure to each frame, how does one smoothly compensate for this?
    don't forget the story.

  • #2
    You need to calculate the loss of exposure from the faster frame rate and compensate either with an adjustable electronic shutter or by changing the f-stop. There are add-on devices that change the f-stop automatically as you ramp the camera. If you want the shutter angle to change instead as you ramp, you need to use a camera that allows this, like an Arri-435 or Arri-535A.

    If you change the shutter angle to compensate, you change the look of the motion blur and if you change the f-stop, you change the depth of field.

    Nowadays, it is common to just shoot at the faster frame rate and do the ramp to or from normal speed in post, although this looks similar to changing the shutter angle.

    For example, let's say that you wanted to ramp from 24 fps to 96 fps, a loss of two f-stops of exposure. You set the shutter angle to 45 degrees at 24 fps and open it up to 180 degrees by the time it gets to 96 fps. This keeps the per-frame exposure TIME consistent (1/192 of a second) even as the frame rate changes, but you get the strobing from shooting 24 fps at a 45 degree angle.

    Now if you simply shot at 96 fps with a 180 degree shutter and pulled out frames in post to get 24 fps, your per-frame exposure time would still be 1/192 no matter what, exactly as if had shot at 24 fps with a 45 degree shutter angle. Although nowadays there are software tricks that can add back normal blur.
    David Mullen, ASC
    Los Angeles

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    • #3
      David,

      thank you very much for the comprehensive explanation. That's very interesting.
      don't forget the story.

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      • #4
        You heard it from a pro. I'm reading your Cinematography book you did with Malkiewicz. Its a great book. I learned to shoot 16 with it.

        -Jon

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