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Calculation the amount of light for a certain F-Stop


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#1 Jonathan Steil

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Posted 01 October 2015 - 05:24 PM

Hi together,

 

in preparation for an upcoming commercial shoot, i came across this forum and i am looking forward to a vivid exchange of knowledge and experience.

 

The setting of the commercial we plan, will be a modern art museum. We'd like to create a high-key look and will have to light for certain F-Stop due to heavy incorporation of vfx.

 

As we have no experience of that sort we can refer to, i'd like to know how you would plan the amount of light needed.

 

Let's say you want to shoot at a ISO of 850, 50fps, a shutter speed of 1/100 and a f-stop of 16. And you have an area of 100 square meters to cover. What do you think, how much light do we need? Is there any way to calculate it?

 

We'd like to bounce from outside the set into an overhead ceiling.

 

I'm grateful for any comment or advice.

 

jonathan 

 

 


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#2 David Mullen ASC

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Posted 01 October 2015 - 05:30 PM

The quick rule is that you need 100 foot candles to get an f/2.8 at 100 ASA at 24 fps / 180 degree shutter.
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#3 Gregg MacPherson

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Posted 01 October 2015 - 06:06 PM

Feels a bit like cheating,  but this may be useful. 

http://calc.arri.de/calculator

 

You'll also find some tables online for most of the common lights.  Unless you are really close,  and I don't think so,  the inverse square law is true.  2 x the distance means 1/(2^2)=1/4 the intensity,  so you can extend whatever data you find.


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#4 Albion Hockney

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Posted 01 October 2015 - 06:29 PM

the arri photo metric calc is a good place to start. Bouncing light drops the intensity and quickens the fall off a lot. if people are close to the bounce 2 stops is kinda my rule of thumb but if they are farther away it can be a lot more light loss.

 

the material you bounce into will also effect your stop loss ...ie ultra bounce vs muslim etc.

 

you are certainly in a big lights sorta category if you are thinking daylight you are in territory like several 4K-12k HMI's I'd say. Maybe an M90 from each of the 4 corners of set into 20x20 Ultra bounce.


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