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Andrew McKee
Bounced Sources
on Jul 17, 2011 at 2:07:45 pm

With a direct source if you move your subject twice as far from the light you loose one stop of illumination, move them twice as near and you gain one stop. How does this apply with bounced sources? Do you measure to the bounce, then to the light? Or does the bounce become your source position for these types of calculations?

Andrew McKee
Editor/Colourist
Avid Certified Trainer - MC5.5
Apple Certified Trainer - FCP7
Pixelwizard.net


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Todd Terry
Re: Bounced Sources
on Jul 17, 2011 at 4:05:46 pm

The bounce surface becomes your lighting source. You can ignore the actual instrument... for calculation purposes the bounce card (or whatever surface you are using) is now effectively the lighting instrument.

T2

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Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: Bounced Sources
on Jul 17, 2011 at 5:16:45 pm

The math/physics is wrong. The inverse square law applies to all waves, such as light and sound: The intensity changes as the inverse square of the distance.

Intensity = 1/(relative distance change)squared

Example: the light is at 10 feet. You move it to 20 feet. The distance is now 2 x what it was. The inverse of 2 is 1/2. Square that, you get 1/4. The light's intensity has been reduced to 1/4 of what it was. Each time you reduce the light by 1/2 of its former intensity, you get a change of one stop. Here we have reduced the light by 1/2 and again 1/2 to 1/4, so the light loss is TWO stops.

Equally, if you moved the light from 10' to 5', it is now at 1/2 the distance it was. The inverse of 1/2 is 2. Square the 2 and you get 4. The light intensity is now 4 times what it was. That is TWO stops.

On a simple, practical level, the inverse-square law means that when you move the light a small amount you get a relatively large change in the light's intensity. Sometimes that works to your advantage, and sometimes not. Example: You have two people sitting side-by-side on a couch. Your key (direct, diffused, or bounced) is 4-5 feet away coming in from one side. The person closer to the key will be substantially "hotter" than the person further away. If you could move the key to say 15', the relative difference in their light intensities would be mostly negligible. But you need a strong light to be able to do that; otherwise the light's intensity on both will be too low for a proper exposure. If you can't move the key you would have to fly a grip scrim, probably a double or even triple, so that the scrim(s) cut the light on the nearer guy but leaves the far one in the clear. With a soft key coming in quite sideways placing the scrim this way can be tricky.

Moving the key so that it is next to the camera and at the same distance from both guys produces even illumination that is utterly flat and uninteresting.

Rick Wise
director of photography
San Francisco Bay Area
part-time instructor lighting/camera
Academy of Art University/Film and Video (grad school)
http://www.RickWiseDP.com


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Andrew McKee
Re: Bounced Sources
on Jul 22, 2011 at 2:19:31 pm

Thanks for your help guys. I suspected this was the case but just wanted to clarify in my own head. I suppose the benefit of bounce is that you can often get a bounce much further from the subject than the light itself, thus reducing the effect you are describing of small differences in distance, affecting exposure quite a bit.

Cheers.

Andrew McKee
Editor/Colourist
Avid Certified Trainer - MC5.5
Apple Certified Trainer - FCP7
Pixelwizard.net


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