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Changing Shutter Speed with change of Frame Rate


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#1 Ashley Barron

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Posted 08 February 2008 - 06:14 PM

Hi,
I'm about to shoot something all in 18fps and was wondering what do I need to do with the shutter speed to compensate. I calculated that I need to go at 1/36 of a sec but how do I change that? (Also shooting on an Arri ST).
Thank you,
Ashley.
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#2 David Mullen ASC

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Posted 08 February 2008 - 06:50 PM

Hi,
I'm about to shoot something all in 18fps and was wondering what do I need to do with the shutter speed to compensate. I calculated that I need to go at 1/36 of a sec but how do I change that? (Also shooting on an Arri ST).
Thank you,
Ashley.


You can't. You can't "ramp" in-camera anyway with an Arri-ST except crudely I guess, turning the knob on a variable speed motor. You'd have to adjust the f-stop to compensate but it would be hard to manually get in sync with the change on the motor.

The Arri-ST has a fixed shutter angle -- don't know what it is, I'm assuming 180 degrees. So if you shoot at 24 fps, then the shutter speed is 1/48 and if you shoot at 36 fps, it is 1/72, etc.

To match a change in frame rate in-camera (during a shot) with an adjustment in shutter angle, you'd need a camera that can do this, that has an electronically controlled variable shutter (like an Arri-435 or Arri-535, let's say). Otherwise, you have to tie the motor in with a controller for the f-stop on the lens, which I don't see as possible either with an Arri-ST -- the little Arri-mount lenses that it uses probably don't take a conventional rod and follow-focus unit not to mention a Preston-type remote controller.

If your camera has a variable speed motor, you can of course shoot shots at different speeds.

You could also try an in-camera speed change and fix the exposure problem in post color-correction, though you'd need the right type of color-correction system.
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#3 Ashley Barron

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Posted 08 February 2008 - 08:07 PM

Otherwise, you have to tie the motor in with a controller for the f-stop on the lens, which I don't see as possible either with an Arri-ST -- the little Arri-mount lenses that it uses probably don't take a conventional rod and follow-focus unit not to mention a Preston-type remote controller.


Thanks very much for the prompt reply! Also,does the ST require focusing with measuring tape or can it be done through the lens? If so, what is the best way to do this? I'm aware of all the various formulas in the manual (with hyperfocal distance etc.) but I've seen focus pullers just extend the measuring tape to the point needed to be in focus and just move to that ft. reading on the lens.
Is that a possible method or should the hyperfocal distance be calculated and then the circle of confusion etc.?
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#4 David Mullen ASC

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Posted 08 February 2008 - 10:41 PM

You should focus the lens on what you want to be in focus, either by eye or by tape measure, whatever works better. Hyperfocal distances, etc. are more for situations where you either can't follow focus during the shot (like running fast with the camera handheld) or when you're trying to hold a split-focus (two subjects that where one is slightly closer to the camera than the other, where there's a possibility that depth of field will carry both). Generally it's best to actually set the focus on what you want to be sharp.
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