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INTENSITY, DISTANCE


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#1 Quinten Densak

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Posted 26 September 2007 - 03:48 PM

Hi

Is there a formula to tell you how many fc you will get from a fixture at a certain distance? Ive read up on the Inverse square law, and i know that if you chose your stop for a certain asa, you have the fc info on the can.

But suppose i was short of people to move lights around and i wanted to get the lights in the right position ... is there a formula to tell me that if i put a 10k 20 ft from a subject...i will get x fc???



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#2 Michael Nash

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Posted 27 September 2007 - 03:53 PM

It will vary depending on the fixture. You have to consult the "photometerics" for the lamp in question. There are charts in various manuals out there, and on some of the manufacturer's websites.

Other than that, experience teaches you how much exposure you can expect with certain units at different distances. This will get you in the ballpark, and then you use your light meter to find more accurate exposure.
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#3 Chris Keth

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Posted 27 September 2007 - 05:38 PM

The inverse square law is useful to know for some fixtures but it is only accurate for a perfect point source. Even a fresnel is big enough and diffused enough to not follow it perfectly.

Like michael said, most companies can supply you with photometric information. Here's an example from Kino Flo for their blanket light.

There are also generic photometric tables in the back of the American Cinematographer Manual that will get you in the ballpark.

Edited by Chris Keth, 27 September 2007 - 05:42 PM.

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#4 Jon Kukla

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Posted 27 September 2007 - 07:16 PM

The photometrics also need to be used as a very rough guide; bulbs gradually become less efficient and dimmer (and often color shift) the more they are used.
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#5 Chris Keth

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Posted 27 September 2007 - 08:13 PM

The photometrics also need to be used as a very rough guide; bulbs gradually become less efficient and dimmer (and often color shift) the more they are used.


Yup. I remember from cleaning fixtures in school that a dusty fresnel could sometimes cut a whole stop!
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#6 robert duke

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Posted 28 September 2007 - 08:01 AM

Meter each light as you set it up from the same distance lets say 10'. you then know what each fixture does at 10'. then you can use the inverse square law to get closer.

I just worked with a DP who lit a night scene during the day by subtracting the ambient sky from his meter readings as he set up and focused each light during the day. he only spent 3-4 minutes tweaking once the sun went down. GOD it was great. his kungfu was strong.
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#7 Tebbe Schoeningh

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Posted 01 October 2007 - 08:31 AM

his kungfu was strong.


wow, that´s really a strong kung fu. where exactly did he take the readings?
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