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Inverse square law...again! (1 reply and 1 comment)

nickyrudolph
2 months ago
nickyrudolph 2 months ago

Hey all, i had a question regarding the inverse square law that i cant seem to find the answer to. I have read through all the other posts regarding it on this forum so I hope I'm not being repetitive. Regarding fall off, I'm trying to figure out, does fall off/ contrast ratio increase when the light source is closer to the subject because the lit side is getting brighter? So we end up compensating for the closeness by stopping down, pushing the shadow side even deeper into underexposure? Or does it have to do with how the light actually falls. So if the source was say 10 feet from subject, at say 200 foot candles, then we move the source closer and wire the light down to 200 foot candles again, would the light still fall off quicker? Hope this makes sense, thanks!

dmullenasc
2 months ago
dmullenasc 2 months ago

You're mixing up exposure, lighting ratio, and rate of fall-off over distance a little.  Fall-off of the key light increases when the subject is closer to the source.  So if the subject moves closer and farther from the source, the rate of change in intensity will be faster in closer distances, more gradual with farther sources.

That may or may not affect lighting contrast ratio a little only in the sense that the hottest spots of the face may be a bit hotter compared to the transitional area into shadow, but that tends to only be noticeable when a face is extremely close to the source.  Otherwise the ratio of the lit side of the face compared to the shadow side could be the same whether or not the source is closer or farther, assuming the softness is matched in both scenarios.

But if a subject moves closer to the source and gets brighter and you stop down to compensate, yes, you'd have more contrast if there is a consistent overall level of fill in the room from a different source because the fill level isn't changing... but your key level is increasing, so the ratio is changing.

If the source is 200 fc at 10 feet away versus a 200 fc key that is 2 feet away, then the closer source would have a faster fall-off over distance. Let's say in both cases, the actor leans a foot closer and then a foot farther... so with the light that is 10 feet away, the actor could be 9 feet away or 11 feet away at some point.  Compare that to if the source is 2 feet away -- so then the actor could end up being 1 foot away to 3 feet away. You'd see a more obvious change in brightness in the second case as the actor moved closer or further.  Moving from 2 feet away to 1 foot away is literally moving half the distance to the source... compared to moving from 10 feet away to 9 feet away.

I think you're also mixing up the term "fall-off" that describes the rate of intensity change over distance for the key light versus the way that the key light "falls" or transitions into shadow on a face.

nickyrudolph
2 months ago

David! You made it too easy to understand now, thanks so much

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