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Shutter angle + Filter calculations


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#1 Kirk Sade

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Posted 03 March 2008 - 05:44 PM

Hello,
I'm shooting outside during the day with a 500asa Tung stock, I have a 85b correction and want to shoot it with a 45 degree angle...so to set my lightmeter's ISO, is this calculations correct?

-2 stops comp for shutter, brings me down from 500 to 125
-2/3 stop (which I'm going to put as 1 full stop) for 85b which brings me down from 125 to 64 ISO

Is 64 asa correct? thanks a bunch in advance...
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#2 Saul Rodgar

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Posted 03 March 2008 - 11:30 PM

Hello,
I'm shooting outside during the day with a 500asa Tung stock, I have a 85b correction and want to shoot it with a 45 degree angle...so to set my lightmeter's ISO, is this calculations correct?

-2 stops comp for shutter, brings me down from 500 to 125
-2/3 stop (which I'm going to put as 1 full stop) for 85b which brings me down from 125 to 64 ISO

Is 64 asa correct? thanks a bunch in advance...


That sounds right . . .
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#3 Michael Nash

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Posted 03 March 2008 - 11:46 PM

That sounds right . . .


Yes.
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#4 Chris Keth

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Posted 04 March 2008 - 12:25 AM

Absolutely correct. Why are you using a full stop as your 85B correction? Just to build in some healthy overexposure?
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#5 Michael Nash

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Posted 04 March 2008 - 12:40 AM

85 "B" has bit more density than a straight 85, so one full stop compensation rounds up for safety.
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#6 Ken Minehan

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Posted 04 March 2008 - 02:33 AM

i have never shot at any other shutter angle other than 180 deg, so i am interested in finding out the compensation when shooting at different shutter angles.

when you shoot at:
90 deg is it 1 stop of compensation?
45 deg is 2 stops of compensation?
is this right?

regards
ken minehan
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#7 rohtash chandel

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Posted 04 March 2008 - 05:31 AM

i have never shot at any other shutter angle other than 180 deg, so i am interested in finding out the compensation when shooting at different shutter angles.

when you shoot at:
90 deg is it 1 stop of compensation?
45 deg is 2 stops of compensation?
is this right?

regards
ken minehan

the best way to understand this is to understand exposure time
exposure time can be calculated by dividing shutter opening with the product of 360 and fps you are shooting at
thus
exposure time= shutter opening/ 360 x1/ fps
thus when you shoot with 180 shutter opening at 24fps you get
exposure time = 180/360X1/24
=1/2x1/24
=1/48
so now when you close your shutter to have 90 degree opening you get
exposure time = 90/360x1/24
= 1/4x1/24
= 1/96
which is half the time a frame gets at 180 degree
so in oder to make it normal exposure, film would require double amount of light thn it requires with 180 degree opening
2x1/96=1/48
thus you need to give one stop compensation; as it doubles the amount of light reaching the film
hope it helps
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#8 Kirk Sade

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Posted 04 March 2008 - 10:50 AM

Absolutely correct. Why are you using a full stop as your 85B correction? Just to build in some healthy overexposure?


That's correct.
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#9 DJ Kast

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Posted 11 March 2008 - 08:23 PM

What is the corellation between Stop Compensation and ASA. By that I mean, you started with and ASA of 500 and knew that you needed to compensate 3 stops, how did you get to 64? Is it just 500 to 200 to 125 to 64? Thanks for the clarification.

Edited by DJ Kast, 11 March 2008 - 08:27 PM.

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#10 Chris Keth

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Posted 11 March 2008 - 08:57 PM

What is the corellation between Stop Compensation and ASA. By that I mean, you started with and ASA of 500 and knew that you needed to compensate 3 stops, how did you get to 64? Is it just 500 to 200 to 125 to 64? Thanks for the clarification.


ASA (ISO; Exposure index - all the same numbers) go on a scale by one-third stops. Halving or doubling the ISO numbers is a change of one stop. So, to go 3 stops from 500 is 64.

500
-1 stop change>
250
-1 stop change->
125
-1 stop change->
64.
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