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shutter speed question....


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#1 Peter Anderson

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Posted 05 January 2007 - 05:41 AM

What's the shutter speed of a camera with a shutter angle of 180 degrees running at:

9
18
24
&
54fps

Various different web resources claim different amounts. For 24fps i've read 1/36, 1/40 and recently 1/50.
I've currently been rating at 1/40 using an external light meter.

If someone can present a comprehensive equation to work this out it would be much appreciated.

Edited by Prokopi, 05 January 2007 - 05:42 AM.

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#2 Mark Dunn

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Posted 05 January 2007 - 06:22 AM

A 180 deg. shutter is open for half the time, so at 18fps, it's 1/36 second and so on.
For different shutter angles the shutter speed would be 1/ (F(360/A)) sec., where F is the framing rate and A the shutter angle.
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#3 Peter Anderson

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Posted 05 January 2007 - 07:40 AM

A 180 deg. shutter is open for half the time, so at 18fps, it's 1/36 second and so on.
For different shutter angles the shutter speed would be 1/ (F(360/A)) sec., where F is the framing rate and A the shutter angle.


Duh! i feel like a complete moron now. that makes complete sense.

Thanks
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#4 Alessandro Machi

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Posted 05 January 2007 - 08:11 AM

A 180 deg. shutter is open for half the time, so at 18fps, it's 1/36 second and so on.
For different shutter angles the shutter speed would be 1/ (F(360/A)) sec., where F is the framing rate and A the shutter angle.


Is that considered the exact time or an approximate time? Is there an issue with the actual refresh rate of the shutter moving across the aperture and the film coming to still position? Is there a slight loss in time to make sure both are perfectly set and in sync with each other, or does all this happen while the shutter is dark and so it is in fact an exact 1/36 for an 18 fps with a 180 degree angle?

Additionally, the light being split to the viewfinder should be factored in at around 1/3 to 1/2 stop loss of light.
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#5 Nick Mulder

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Posted 05 January 2007 - 01:43 PM

Is that considered the exact time or an approximate time?


It should be exact. (Well, depending on the possibly dodgy mechanics of a variable shutter it should still be 'exact' enough for setting exposure)

Is there an issue with the actual refresh rate of the shutter moving across the aperture and the film coming to still position?Is there a slight loss in time to make sure both are perfectly set and in sync with each other, or does all this happen while the shutter is dark and so it is in fact an exact 1/36 for an 18 fps with a 180 degree angle?


It should all happen when it is dark and the shutter angle will take all film movement into account.

Additionally, the light being split to the viewfinder should be factored in at around 1/3 to 1/2 stop loss of light.


But only if this is the method of reflex viewfinding on your model camera, a mirror shutter will not have this issue.

This info is correct for cameras that have been designed and looked after correctly, its possible that there are some crappers out there that have been mistimed or have faulty shutters... You'll probably notice you have vertical streaks in the picture before any overexposure if there is a problem...
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#6 David Mullen ASC

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Posted 05 January 2007 - 05:37 PM

If movie camera shutters weren't fairly accurate, we'd have more problems with flicker and whatnot -- anyway, any variations from exactly 1/48th of a second at 24 fps with a 180 degree shutter is probably smaller than a light meter can be manually offset to compensate for (less than 1/3 of a stop) and less than would be visible to the eye. I wouldn't be surprised that the lenses and perhaps even density from processing are greater variables to final exposure/density results -- not to mention human error in exposing.

If the camera is running exactly at 24 fps and a half-circle disk spins once around during that time, then the gate is open to light 50% of the time. I don't think the extra exposure lost as the disk moves to its most open position and then back to its closed position is significant.
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#7 K Borowski

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Posted 05 January 2007 - 05:51 PM

If movie camera shutters weren't fairly accurate, we'd have more problems with flicker and whatnot -- anyway, any variations from exactly 1/48th of a second at 24 fps with a 180 degree shutter is probably smaller than a light meter can be manually offset to compensate for (less than 1/3 of a stop) and less than would be visible to the eye. I wouldn't be surprised that the lenses and perhaps even density from processing are greater variables to final exposure/density results -- not to mention human error in exposing.

If the camera is running exactly at 24 fps and a half-circle disk spins once around during that time, then the gate is open to light 50% of the time. I don't think the extra exposure lost as the disk moves to its most open position and then back to its closed position is significant.


Well, I'm sure there are a lot of discrepancies that creep up with shutter speeds and older equipment, especially equipment that isn't maintained, but being that movie cameras are run off of electrical current or, more and more rarely these days, off of clockwork motors, the error in timing should be constant from frame to frame to frame , save maybe at the beginning of a shot as the camera is gaining speed.

You can certainly still see pulsing or strobing in highlights, often in skies, from some sort of irregularity I think is due to processing in modern films. There's a variation in density or uneven development within a single frame that accounts for this, I think. This probably comes up in release printing more than with developing the negatives, as I'm sure release print standards today have reachd an all-time low.
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