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lumen/ lux/ fc output to T-stops


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#1 Mathew Collins

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Posted 23 May 2017 - 09:56 PM

Hi,

 

I was going thru the photometrics specification given in the following link.

 

http://emea.rosco.co...roduct/silk-210

 

Is there any way to convert the lumen/lux/ fc output given in the light specification into T stops?

 

-Mathew Collins.


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#2 David Mullen ASC

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Posted 23 May 2017 - 10:37 PM

The simple rule to remember is that 100 foot-candles on 100 ASA film gets you a T/2.8 exposure (at 24 fps with a standard 180 degree shutter).

 

From those ratios you can figure out what happens when you change any one of those three, for example, 50 fc at 200 ASA also gets you T/2.8, so does 25 fc at 400 ASA, 12 fc at 800 ASA, etc.

 

There is an actual conversion formula that someone can dig up...


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#3 Phil Rhodes

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Posted 24 May 2017 - 03:22 AM

There is no completely unequivocal conversion, because the calculations rely on a "meter calibration constant" which represents the average observer's idea of a properly-exposed image. For reasons too longwinded to explain, 330 is a common value.

 

With that in mind, the required exposure value for a scene is equal to the binary logarithm of the light level in lux multiplied by ISO over 330.

 

This is complicated by the fact that most calculators don't have a direct way of getting the binary logarithm of a value, but you can get the same result by dividing the natural logarithm of that value by the natural logarithm of 2.

 

Assume 1000 lux and ISO 100:

 

log2(1000 * 100 / 330) = 8.24

 

Similarly, the effective exposure value of a particular camera configuration is equal to the binary logarithm of the aperture squared over the shutter time in seconds:

 

log2(2.82/(1/48)) = 8.56

 

Thus 1/48s at f/2.8 is about a third of a stop overexposed in an incident light of 1000 lux at ISO 100.

 

P


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