Formula for Calculating Shutter Angle Exposure Compensation

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#1 Steven Wyatt

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Posted 30 November 2008 - 11:57 AM

Hi everyone, any heads up on this?
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#2 Stephen Williams

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Posted 30 November 2008 - 12:11 PM

Hi everyone, any heads up on this?

180 degrees is normal
90 is 1 stop Under
45 is 2 stops under
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#3 Laurent Andrieux

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Posted 30 November 2008 - 01:10 PM

General formula is t=A/(C.360) where t is the exposure time, A the shutter angle in degrees and C the camera frame frequency (24 or 30 fps for instance).

Your exposure compensation depends on the calculated exposure time, using the reciprocal relation between exp. time and T stops.

Sorry it's a very quick answer, but it may fit your question. Does it ?

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#4 Steven Wyatt

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Posted 30 November 2008 - 06:52 PM

cool, am still abit confused as to how the calculation illustrates how many stops you need to compensate for. Do you think you could elaborate on this a little bit further. I'm abit slow!

Thanks
Steve
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#5 Laurent Andrieux

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Posted 30 November 2008 - 07:03 PM

The shutter angle changes the exposure time, that is all.

Then, refer to Stephen's post : Normal aperture is 180°. It's exactly half of the whole cycle (the film will be tracked to the next image in the remaining time).

If you operate at 24 fps, setting your shutter at 180° makes your exposure be half of the whole cycling time, ie 1/48 s

If you close the shutter to 90°, then the exposure time is devided by two and then becomes 1/96 s, therefore, as Stephen says, you loose 1 stop, etc.

Dig it ?

May be you are wondering about a special aperture, like 144° ?

Or is it that you want to close the aperture for a special effect, reducing the time exposure, and wonder what effect will occure on the exposure ?

Is it that you are a student and don't understand something in your course ?

Best,
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#6 Steven Wyatt

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Posted 01 December 2008 - 03:53 AM

Hi

Thanks very much Laurent Andrieux! I just needed some clarification on the subject, its brilliant.

Regards,
Steven
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#7 Juliana

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Posted 01 December 2008 - 09:53 AM

Hi all...
I have a doubt on this subject...
I have to change shutter angle from 180o to 90o AND the frame rate from 24 to 96 fps. I know that my frame rate change makes me open 2 stops. And the angle change makes me open 1 stop, right? My doubt is: as i am doing both simultaneously, the alterations in the diaphragm are cumulative? Should I open 3 stops or only 2 solve my problem?

Thank you very much!!!!!!!!

The shutter angle changes the exposure time, that is all.

Then, refer to Stephen's post : Normal aperture is 180°. It's exactly half of the whole cycle (the film will be tracked to the next image in the remaining time).

If you operate at 24 fps, setting your shutter at 180° makes your exposure be half of the whole cycling time, ie 1/48 s

If you close the shutter to 90°, then the exposure time is devided by two and then becomes 1/96 s, therefore, as Stephen says, you loose 1 stop, etc.

Dig it ?

May be you are wondering about a special aperture, like 144° ?

Or is it that you want to close the aperture for a special effect, reducing the time exposure, and wonder what effect will occure on the exposure ?

Is it that you are a student and don't understand something in your course ?

Best,

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#8 Ian Cooper

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Posted 01 December 2008 - 11:47 AM

Imagine you're not doing them simultaneously to begin with. Imagine you're changing from from 180 deg to 90 deg but keeping the frame rate at 24fps. The compensation is simply open 1 stop.

Now having made this compensation and set the lens, you subsequently want to also change from 24fps to 96fps. This change now needs 2 stops opening up compensation (in addtion to the 1 stop you've already got from altering the shutter angle previously).

The total compensation from 180degrees at 24fps to 90 degrees at 96 fps is opening up by 3 stops, but hopefully I've been able to show why this is the case.
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#9 Simon Wyss

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Posted 01 December 2008 - 11:50 AM

Three minutes more time of preparing my post . . .Yes, correct, each alteration in the basic light controls calls for compensation. You change the shutter angle from 180 degrees to 90, the iris diaphragm has to be opened by one full stop. You increase the frame rate from 24 a second to 96, the iris has to be opened another two stops. In case you'd set a light filter between film and object, that will have to be taken into account, too. The last point to consider is when you pull out the lens in order to shoot macro down to 1:1, then correction must be made for the inherent light loss but I think this subject can be left out for the moment.

To retain a certain diaphragm opening you will have to either increase the illumination of the object or replace the raw stock by one of higher sensitivity.

I am sure you are aware of the influence of the lens diaphragm on depth of field. With a reflex camera this can be observed by the price of a darker or brighter viewfinder image.

Edited by Simon Wyss, 01 December 2008 - 11:51 AM.

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#10 Glen Alexander

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Posted 01 December 2008 - 01:48 PM

I am sure you are aware of the influence of the lens diaphragm on depth of field. With a reflex camera this can be observed by the price of a darker or brighter viewfinder image.

i find the vignetting can be nice effect to pull the viewer into the space, willing or unwilling.
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#11 Stephen Williams

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Posted 01 December 2008 - 02:06 PM

i find the vignetting can be nice effect to pull the viewer into the space, willing or unwilling.

I don't underatand what that has to do with what Simon was saying.
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#12 Paul Bruening

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Posted 01 December 2008 - 02:20 PM

I don't understand what that has to do with what Simon was saying.

I'm a little baffled as well. Wasn't he referring to the iris change darkening or lightening the viewfinder image? The vignetting remark, in and of itself, was a correct remark. Not so applicable to the question, though.

But, then again, I'm one of the worst thread redirectors here. So, I should just shut up.
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#13 Simon Wyss

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Posted 01 December 2008 - 04:39 PM

Paul (of age unknown)
You don't need to blame yourself. When you're right, you're right.

Remember that German Hausfrau in Woody Allen's I Forgot the Title: "Whenn your numberh is upp, your numberh is upp !"
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#14 Paul Bruening

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Posted 01 December 2008 - 10:07 PM

Paul (of age unknown)

I'm only three and a half years old.
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#15 Hal Smith

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Posted 02 December 2008 - 09:55 AM

Doubling or halving anything that affects exposure is a one stop change. Anything that changes exposure by 1.4 (actually 1.414... the square root of 2) is a 1/2 stop change. For instance: going from a film with a rating of 100 to a film with a rating of 200 is doubling sensitivity and therefore would require halving exposure which means setting one f-stop more...or...one-half the shutter angle...or...doubling the frame rate...or...ND3 gel, etc.
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#16 Simon Wyss

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Posted 02 December 2008 - 11:00 AM

Was it Everyone Says I Love You ?
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