Lighting

Large Sources and Falloff (1 reply and 4 comments)

Joseph C
1 week ago
Joseph C 1 week ago

Hello everyone, just wanted to give a quick thanks for everything this website offers! To get to my point, I'm having a very hard time trying to understand a certain aspect of light falloff and large light sources. I would usually never ask a question here that has been answered elsewhere on the internet but for this particular question I keep finding conflicting answers. I figured that this would be the best place to ask. 

Say we are shooting in a pitch-black room with nothing for the light to bounce off of. I have one light pointing at the subject- if I increase the surface area of my light source without changing its distance from subject (adjusting power of light to correct difference in exposure), will the falloff be affected? I understand that increasing apparent size (moving light closer) will speed up falloff but I assumed that was because the light is physically being moved closer to the subject. If NOTHING else changes except for the actual surface area size of the light source (besides increasing the power of the light to compensate for exposure), is falloff changed? Some people have told me that you can't use the inverse square law for large sources at all since it only applies to point sources while others have told me that it kind of works but doesn't follow the law exactly. Most people who tell me that the source's surface area affects falloff seem to agree that the larger the source, the slower the falloff. Is this because a larger source at the same distance merely 'wraps' around the subject more, tricking them into thinking that the falloff has changed? I would really appreciate some clarification from the veterans. I would love to test this myself but I don't have access to equipment this week.

Thank you so much for your time!

Joseph C.

dmullenasc
1 week ago
dmullenasc 1 week ago

I think the fall-off of exposure over distance would be close to the same whether it was a large or a small soft source, or a hard source -- it's really about the distance, the fall-off is more gradual when you are farther from the light.

Limbo
1 week ago

Hi David, I have another related question. If I strike a light from far away to a big diffusion. Does the fall off distance factor count from the original light or the diffusion?

J. Roxbrough
1 week ago

Not David obviously, but the diffusion becomes the source. So the diffusion to subject is correct rather than the original light to subject.

Joseph C
1 week ago

Thank you very much, Mr. Mullen! I can finally lay that headache to rest.

dmullenasc
1 week ago

Yes the diffusion is the source in terms of factoring fall-off. However if the diffusion is very thin and porous, there is an odd mix of hard & soft and the “source” is sort of a mix of the two.

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