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shutter angle and overexposure


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#1 darryl walthall

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Posted 21 July 2006 - 04:23 PM

I like to overexpose negative stock 2/3 stops sometimes. If I close my shutter angle to 90 degrees, how do I factor in the 2/3 stops over exposure? whats the equation ?
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#2 Stephen Williams

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Posted 21 July 2006 - 04:32 PM

I like to overexpose negative stock 2/3 stops sometimes. If I close my shutter angle to 90 degrees, how do I factor in the 2/3 stops over exposure? whats the equation ?


Hi,

180 degrees to 90 is one stop. 1 2/3 stops is what you are looking for.

Stephen
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#3 darryl walthall

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Posted 21 July 2006 - 04:55 PM

so does a 90 degree shutter cause flicker with HMI lighting?
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#4 timHealy

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Posted 21 July 2006 - 06:28 PM

by itself not neccessarily.

are you using magnetic or flicker free ballasts and what will be your frame rate? See the link to David comment about exposure vs flicker below.

for magnetic ballasts
http://www.panavisio...kerfree60Hz.htm
http://www.panavisio...kerfree50Hz.htm

flicker free ballasts are generally flicker free from 20 -30 frames per second in silent mode, but this shoulds be checked with different manufacturers. In the flicker free mode you can shoot at most frames rates and shutter angles.

David Mullen had some helpful advice here:
http://www.cinematog....php?t6927.html
Also helpful
http://www.cinematog..._Time_Lapse.htm

an interesting side note: In the Harry Box book, he describes a method to shoot flicker free with magnetic ballasts by using more than one light. Say if you are using three phase power. If you put three lights into a bounce or through a frame, and plug each into a different phase, the three varying phases and cycles will balance out and you would be able to shoot at any frame rate with any shutter angle .... theoretically. I have not tried it but it sounds like it should work. Obviously test test test.

Best

Tim

also may be helpful
http://www.panavisio...terOpenings.htm

Edited by heel_e, 21 July 2006 - 06:31 PM.

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#5 Daniel Stigler

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Posted 22 July 2006 - 12:17 AM

so does a 90 degree shutter cause flicker with HMI lighting?


Tell your gaffer to check the HMIs with a flickermeter when getting the gear. I have set cameras to all kinds of combinations of shutter angles and running speeds (and ramps) and there never were any problems with flickerfree ballasts.
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#6 timHealy

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Posted 22 July 2006 - 02:00 AM

Tell your gaffer to check the HMIs with a flickermeter when getting the gear. I have set cameras to all kinds of combinations of shutter angles and running speeds (and ramps) and there never were any problems with flickerfree ballasts.


a flicker meter!? what the hell is that?

I think you're pulling our legs. Unless you are talking about a meter that checks cycles which I have always understood that they don't work properly with flicker free ballasts.

Best

Tim
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#7 Daniel Stigler

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Posted 22 July 2006 - 02:34 AM

a flicker meter!? what the hell is that?

I think you're pulling our legs. Unless you are talking about a meter that checks cycles which I have always understood that they don't work properly with flicker free ballasts.

Best

Tim



I'm deadserious.
http://www.thedopsho....cfm?itemID=211
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#8 timHealy

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Posted 22 July 2006 - 12:23 PM

I'm deadserious.
http://www.thedopsho....cfm?itemID=211



ok so how does this work and what does it mean?: "it will also read percentage of flicker - which other frequency meters do not."

Best

Tim
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