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Understanding footcandles/LUX and calculating exposure


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#1 Justin Carrig

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Posted 31 August 2012 - 11:38 PM

Hey guys,

I have what is probably a very basic question regarding footcandles/lux.

I'd like to know how to calculate required exposure based on a light's given footcandles/lux information using a fixed iso and shutter speed and variable aperture.

For example, if I have a digital camera set at 800 ASA and 1/48 shutter and my light puts out 44 footcandles at 6ft, and the light is exactly 6ft from my subject, what aperture is required to properly expose this subject? There is most probably a basic calculator out there, if anyone can point me to one that'd be great.

I basically want to know because I have an upcoming project with an option of lights that I will not have experience with before the shoot, so I'd like to know in advance their capabilities.

Cheers!

Edited by Justin Carrig, 31 August 2012 - 11:40 PM.

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#2 Ari Davidson

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Posted 02 September 2012 - 06:00 PM

Hey guys,

I have what is probably a very basic question regarding footcandles/lux.

I'd like to know how to calculate required exposure based on a light's given footcandles/lux information using a fixed iso and shutter speed and variable aperture.

For example, if I have a digital camera set at 800 ASA and 1/48 shutter and my light puts out 44 footcandles at 6ft, and the light is exactly 6ft from my subject, what aperture is required to properly expose this subject? There is most probably a basic calculator out there, if anyone can point me to one that'd be great.

I basically want to know because I have an upcoming project with an option of lights that I will not have experience with before the shoot, so I'd like to know in advance their capabilities.

Cheers!


Are they film lights? Most manufacturers list this data on their website.
Do you have a light meter?
See here for some information regarding Lumens/Watt: http://en.wikipedia....minous_efficacy
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#3 Tom Jensen

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Posted 03 September 2012 - 12:29 AM

Don't waste your time. Everyone works in stops. But, at 100 ASA, you need 100 foot candles to get a 2.8
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#4 rsellars

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Posted 14 September 2012 - 07:43 PM

Justin, there is probably a smartphone app that has this type of info. But I'm old school. I used to use a chart published in the older ASC manuals to make quick footcandle calculations or use the back of my analogue Spectra meter. But you can figure it out in your head using the info that Tom provided. I'm terrible with math, so I just count on my fingers if necessary. At ASA (ISO) 100, 100 footcandles is f2.8. First, round off your 44 fc to 50 fc. So, 50 footcandles is half of 100 - that means at ISO 100, 50 fc is f2. Double ISO 100 to 200 and you get f2.8. Double 200 to 400 you get f4. Double 400 to 800 you get f5.6. Your actual footcandles was 44, not 50. Open up approximately an 1/4 of a stop from f5.6, and you have the correct exposure.
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#5 Mike Lary

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Posted 14 September 2012 - 09:43 PM

Hey Justin,

If you have the cash to spare (I think it's around $30), you should look at PCam for mobile. You can quickly calculate exposure based on light intensity and distance. It has a slew of other features as well, more than you'll probably ever use. It's real handy for planning and for quick reference on set, and it's a solid app that's been around for years.
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#6 Justin Carrig

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Posted 18 September 2012 - 09:56 AM

Thanking you all for the great suggestions and info, perfect stuff. :)
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#7 Travis Gray

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Posted 18 September 2012 - 10:18 AM

Cinecalc Pro is another decent app. Accurate too (but it's just calculations, so, I'd hope it's accurate). Good library of lights with photometrics in the database as well, so you can kinda plan everything out with distances and wattage, etc. I'll use it a bunch prior to a shoot to get an idea based on what I think will be going on, and then meter on set, but at least I go in with a bit of a plan.
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#8 Justin W. King

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Posted 18 September 2012 - 03:03 PM

Hey guys,

I have what is probably a very basic question regarding footcandles/lux.

I'd like to know how to calculate required exposure based on a light's given footcandles/lux information using a fixed iso and shutter speed and variable aperture.

For example, if I have a digital camera set at 800 ASA and 1/48 shutter and my light puts out 44 footcandles at 6ft, and the light is exactly 6ft from my subject, what aperture is required to properly expose this subject? There is most probably a basic calculator out there, if anyone can point me to one that'd be great.

I basically want to know because I have an upcoming project with an option of lights that I will not have experience with before the shoot, so I'd like to know in advance their capabilities.

Cheers!

fstop chart.jpg
Courtesy of the Set Lighting Technician Handbook

Edited by Justin W. King, 18 September 2012 - 03:04 PM.

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Willys Widgets

Abel Cine

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Media Blackout - Custom Cables and AKS

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Rig Wheels Passport

Gamma Ray Digital Inc

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