# Measuring Light Output

exposure preproduction film cinematography footcandle lux light measure

3 replies to this topic

### #1 Jacob Mitchell

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Posted 12 September 2016 - 06:59 PM

Hi All!

As a student, each projects means a way to stretch myself further. If I'm not trying something new (or a little bit scared) on a project then I feel like I'm not taking steps forward.

Lately I've been doing pretty in-depth floor plans and 2D pre-vis, and it has been working wonders for every project. However, I'm still working to get exactly what I want in my head to paper, so that on set my images are not a surprise.

Is there a way I can tell how many foot-candles a certain light will output? I'm wanting to essentially know exactly how powerful a light I'll need to expose to a pre-determined f-stop. I'm familiarizing myself with inverse square law, so all I'd need is an output at any one distance and could work from there.

Is this just a knowledge gained from experience? Or is there a practical way to determine output?

Thanks,

Jake Mitchell

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### #2 Jacob Mitchell

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Posted 12 September 2016 - 07:08 PM

Here's an awesome spreadsheet of different measurements, spot and flood, at 20 feet.

http://wolfcrow.com/...sComparison.jpg

Edited by Jacob Mitchell, 12 September 2016 - 07:09 PM.

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### #3 John E Clark

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Posted 13 September 2016 - 11:22 AM

Hi All!

As a student, each projects means a way to stretch myself further. If I'm not trying something new (or a little bit scared) on a project then I feel like I'm not taking steps forward.

Lately I've been doing pretty in-depth floor plans and 2D pre-vis, and it has been working wonders for every project. However, I'm still working to get exactly what I want in my head to paper, so that on set my images are not a surprise.

Is there a way I can tell how many foot-candles a certain light will output? I'm wanting to essentially know exactly how powerful a light I'll need to expose to a pre-determined f-stop. I'm familiarizing myself with inverse square law, so all I'd need is an output at any one distance and could work from there.

Is this just a knowledge gained from experience? Or is there a practical way to determine output?

Thanks,

Jake Mitchell

ARRI has an on line calculator and an iPhone App (perhaps a Android one as well...) which lists their light products' output at various distances.

The actual lux/footcandles delivered by an arbitrary light depends on a number of factors, reflector shape, lensing if any, etc. Many off brand light producers only list the wattage of the lamp itself, which doesn't directly give 'how much light is falling on the subject'.

I'd suggest getting a light meter that reads out lux/fc, and finding a rental house that has a selection of various lights and measure them.

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Posted 13 September 2016 - 10:43 PM

To be honest; I normally ballpark off of the Arri Estimates for most things-- the issue of moving a light a little closer or using a scrim it in are pretty academic if you ask me. I haven't seen the arri photometrics as an android app, but i did save the page to my homescreen, so i have it whenever (data connectivity permitting).

With time, though, you'll just kinda know.

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