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60-Hz safe fps / shutter


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#1 Miguel Bunster Claudet

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Posted 19 February 2010 - 05:51 PM

Hi,
Its been a while since I worked in 50hz worldas apposed to US 60Hz
I am shooting a project overseas on 35mm and we are planning to do 23.98 - 24 - 30 & 36 fps.
I generally carry around all my info, manuals etc but not this time :( .
What are the safe speeds / shutter combination for working under 60hz at this frame rates or the formula. Somehow I have a blank spot! And will be working with magnetic / electronic HMI and shooting tvs under fluorecents.
thank you!

Edited by Miguel Bunster Claudet, 19 February 2010 - 05:52 PM.

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#2 John Sprung

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Posted 20 February 2010 - 12:41 AM

Hi,
Its been a while since I worked in 50hz worldas apposed to US 60Hz
I am shooting a project overseas on 35mm and we are planning to do 23.98 - 24 - 30 & 36 fps.
I generally carry around all my info, manuals etc but not this time :( .
What are the safe speeds / shutter combination for working under 60hz at this frame rates or the formula. Somehow I have a blank spot! And will be working with magnetic / electronic HMI and shooting tvs under fluorecents.
thank you!


OK, there are two ways to do this. You get two pulses of light for every cycle of the AC power, one on the positive half cycle, the other on the negative. So, at 50 Hz, you have 100 light pulses per second.

One way to go is to work at frame rates that give you a whole number of light pulses over the whole pulldown and exposure cycle of the camera. At 100 flashes per second, you could go 100 fps and get one flash per frame, 50 fps and get two flashes per frame, etc:

100/1 = 100, 100/2 = 50, 100/3 = 33.3333, 100/4 = 25, 100/5 = 20, 100/6 = 16.66667, etc.

That's easy to figure out, and lets you use any shutter angle you want. But it restricts you to those specific frame rates.

The other way is to use whatever frame rate you want, but for each frame rate, you are restricted to shutter angles that produce exposures of a whole number of light pulses. What you want is:

(1/framerate)x(shutterangle/360) = (wholenumber)/(2xFrequency)

So, for 24 fps, you could use:

(1/24)x(angle/360) = 2/100

Doing the math,

angle = 360x(2/100)x(24/1) = 172.8

For 1/100, you'd get 86.4 degrees, probably too small to be useful. For 3/100, it would be 259.2, which isn't available on film cameras.

You can do the math for the other frame rates.





-- J.S.
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#3 Miguel Bunster Claudet

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Posted 20 February 2010 - 12:57 PM

thank you...and once you put it so obvious! :blink:
Thanks John!
m
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