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formulae for working out shutter exposure


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#1 Allyn Laing

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Posted 18 July 2006 - 12:38 AM

Hello,

I am wondering if there is a mathematical equation for working out shutter times and angles

for example if a shutter is set to 180 degrees and the camera is running at 24 fps then the exposure is 1/48th of a second, but say we increase the speed to 40fps we are exposing the film for 1/80th of a second how do i arrive at this conclusion what is the formulae for working this out, with different shutter angles and frame speeds. I have seen the charts but i think this would be good knowledge to have.

Warm regards,

Allyn

Edited by Allyn Laing, 18 July 2006 - 12:41 AM.

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#2 David Mullen ASC

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Posted 18 July 2006 - 12:52 AM

We just had this discussion in another thread a couple of days ago...

If a full-circle is 360 degrees, then 180 degrees is half that because 180/360 = 1/2.

And if you run the camera at 24 fps, then a frame is pulled down every 1/24th of a second.

So the time the shutter is open at 180 degrees is 1/2 x 1/24 = 1/48.

So if the shutter is 45 degrees and the frame rate is 60 fps, it would be:

45/360 (which is 1/8) x 1/60 = 1/480

So I guess you could write this as Shutter Angle / 360 x 1 / FPS = Shutter Speed
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#3 Allyn Laing

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Posted 22 July 2006 - 02:11 AM

Thankyou David, it makes complete sense now

Allyn
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#4 Adam Frisch FSF

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Posted 22 July 2006 - 04:42 AM

But as for exposure compensating, if someone strolled up to me and said the shutter speed's at 1/150 I wouldn't have a clue how much to compensate. What I have to do is silently count in my own head down from 1/50....

I also tend to not compensate the film speed setting for ND filters, Polarisers and 85 and such because I found out you then change filters and forget to re-adjust the setting, screwing up your exposures. I measure and get a 'clean' reading and then silently count through all the light stealing filters/processes until I reach my lens stop. It keeps my head om straight and avoids big messy cock-ups.
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