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Bernhard Walzl

172.8 degree vs. 180 degree Shutter

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Hello!

I came across the question, why in America a shutter angle of 180 degrees is used when shot on 24fps, but in European countries it is 172.8 degrees.

I would say, the 60/50Hz power supply causes this difference. But when I do the maths it doesn't work out:

So 172.8 degrees equals a shutter speed of 1/50. This matches the 50Hz.
But 180 degrees equals a shutter speed of 1/48. In order to have 60Hz, it should be 144 degrees.

Can somebody explain this difference?

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Hello!

 

I came across the question, why in America a shutter angle of 180° is used when shot on 24fps, but in European countries it is 172.8°.

 

I would say, the 60/50Hz power supply causes this difference. But when I do the maths it doesn't work out:

 

So 172.8° equals a shutter speed of 1/50. This matches the 50Hz.

But 180° equals a shutter speed of 1/48. In order to have 60Hz, it should be 144°.

 

Can somebody explain this difference?

 

Hi,

 

I've used a 172.8 degree shutter opening while shooting 24 FPS in Europe.

 

It allows you to shoot non-flicker-free HMI's, Flos and other sources that are powered at 50Hz and not get any strobing or flicker at 24FPS. Normally in Europe, you would run 25FPS at a 180 degree shutter to lock-out the flicker issues there. The 172.8 shutter angle gives you that same "shutter-speed," thus the same result when you are working on a production (such as American films that project at 24FPS) that requires a 24FPS running speed.

 

Hope that makes sense!

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Kapabel

 

Your math's wrong:

 

172.8/360 is the ratio of time the shutter is open.

 

1/24 seconds is the time elapsed for one frame.

 

Dividing 172.8 by 360 then dividing by 24.0Hz (AKA cycles per second) gives you .020 seconds which is the time for two complete on/off cycles of the HMI's light in 50Hz countries (1/50Hz = .02 seconds).

 

If your shutter is open for exactly the same time as exactly one or more complete half cycles of the power, there's no flicker

 

PS: The rules of this Forum require your real full name as your user name (like mine, "Hal Smith"). Go to "Your Profile" and change your user name to what your name actually is.

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Hi Kapabel,

 

I'm making a wild estimation but are you applying for the KODAK Cinematography Masterclass in Brisbane? I'm making the assumption from the date and nature of your question. Because I too am slightly baffled at this question.

 

I understand why one would shoot 172.8 for a 50Hz country. Mathematically it makes sense. However, 24fps at 180degrees for 60Hz doesn't. Unless I am missing something which I assume I am. Does anyone know a definitive answer to this? Why do Americans shoot 24fps at 180 when it mathematically doesn't synch with their power supply?

 

Thanks

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You can see here that strangely 24 fps is safe for 60 hz at any shutter angle. So probably they just use a 180 degree shutter for convenience.

http://www.panavision.co.uk/services/filming-speeds-60hz.asp

 

I used to shoot at 24 fps when filming fluorescents in the US, even though the rest of the production was 25fps. BTW The shutter angle was 180 degrees.

 

However, for filming a US CRT television you use 144 degrees, although, you're only filming one of the fields.

 

You can see the same chart for 50 hz with the 172.8 shutter angle at 24fps. http://www.panavision.co.uk/services/filming-speeds-50hz.asp

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I start shooting tomorrow in a 50hz country at 24 fps.

 

Should we shoot 24fps or 23.98? (red camera)

 

Please reply quickly :)

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I start shooting tomorrow in a 50hz country at 24 fps.

 

Should we shoot 24fps or 23.98? (red camera)

 

Please reply quickly :)

 

Depends on your post people's requirements, but for post in the US 23.98 would be usual.

Edited by Brian Drysdale

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Post can't tell the difference between 24.00 and 23.98, provided that all cameras and sound are on the same side of the fence between "point zero zero" and "point nine something".

 

 

 

 

-- J.S.

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