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#1 G McMahon

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Posted 11 July 2006 - 10:16 AM

Hello all,

What am I missing out on by not using or understanding foot candles? I have searched through past forums chasing related foot candle articles. Is it useful with working with new lighting fixtures to understand their throw in comparison to other units, in the past I have been looking at throws of lights based on 500 asa, 10m distance, 180 shutter, 1/50th sec. Does implementing the foot candle approach help take the maths out?

Also for continuity of the look, do you lot try to maintain a consistent T- stop when shooting. When you move the camera around for a different set up, do you move lights back and forth to keep shooting at the same stop? And to what degree, you keep all interiors at ?x? T-stop and exteriors at ?y? T-stop?

Thanks all,

By the way, are we only allocated a certain amount of questions a month? (This one doesn?t count if so)
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#2 Brian Drysdale

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Posted 11 July 2006 - 11:05 AM

Using foot candles is a good way of planning out a lighting plot or the size of lights you require. All you need to know is the number of foot candles required for a particular stop using a film rated at a certain ASA.

Knowing the lighting throw requirements of a location/set and the transmission losses of the gels enables you to pick the lights you may need from the tables supplied by the light manufacturers.

Keeping the same stop helps the photographic continuity of a scene. The stop is dependent on the placement/light output of the lights, not on how close/far the camera is from the subject. If you move the camera and the lighting stays the same you keep the same stop. However, you may want to adjust the fill lighting to maintain the same contrast ratio and tweak the other lights for best effect. ND gels or scrims can be used to control the output of the lights.

How much you want to use the same stop through out a film depends on how you want to tell the story, but keeping it the same in each scene helps tie things together. Although, sometimes you do need extra depth of field on a particular shot (E.G. an extreme CU), so this can be a judgement call.
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#3 Chris Keth

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Posted 11 July 2006 - 11:30 AM

Knowing the lighting throw requirements of a location/set and the transmission losses of the gels enables you to pick the lights you may need from the tables supplied by the light manufacturers.



This is the best reason to at least understand footcandle measurements, in my book.

You're not Garrett McMahon from RIT, by chance, are you? :huh:
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#4 David Mullen ASC

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Posted 11 July 2006 - 11:34 AM

In a way, footcandles was a way of talking to a gaffer that didn't require that they know the stock you were using or how you were going to rate it, etc. -- you could tell them to light the set with a key of 50 fc, for example. And it's easier to talk about key to fill ratios when you say something like "key at 60 fc and fill at 15 fc".

But the truth is that not a lot of people work in footcandles anymore, not with all the digital light meters out there giving readings in f-stops. Now we usually tell the gaffer the ASA rating and the f-stop we want for the key and how many stops under for fill.

With modern soft lighting styles, it's a lot harder to guess what the light level will be based on the photometric data of the hard light before it is softened.

We generally at least try to shoot the coverage within a scene at the same f-stop. This makes it easier to maintain the key-to-fill ratio and not alter the ambience of the room, nor the depth of field characteristics (although just switching focal lengths does that.) It's not too hard since one often uses the same units for the different set-ups, so keeping to the same levels naturally happens. Some DP's will generally stick to one overall f-stop for a whole movie -- within reason -- to create a consistent tone, optical quality, etc.
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#5 John Pytlak RIP

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Posted 11 July 2006 - 02:38 PM

Footcandles or lux actually measure the light falling on the scene. They were the primary calibrations used for light meters. You adjust exposure time (frame rate and/or shutter opening) and lens opening (f/stop or T-stop) to expose the film with the correct amount of light for the film's exposure rating. Here is a simplified table:

http://www.kodak.com....shtml#incident

Here is how it all "fits together" to give proper exposure:

http://www.kodak.com...structure.shtml
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#6 G McMahon

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Posted 12 July 2006 - 12:58 AM

This is the best reason to at least understand footcandle measurements, in my book.

You're not Garrett McMahon from RIT, by chance, are you? :huh:


No, I'm not Garrett. The G is for Graeme. I have been wondering though, if I was to change my name to something more European would I get more work. Graeme doesn't sound very bohemian.
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