Combining Sources with Diffusion/Bounce (2 replies and 2 comments)
Hello, first time posting around here. I was wondering if there was any way to calculate how much diffusion is needed in order to make multiple sources combine into one? That is, to cast a single shadow.
I asked this in class a couple of weeks ago while discussing how to calculate exposure with various modifiers and the best my professor could do was recommend I ask here.
Thank you in advance, Henry.
That kind of depends on how far the diffusion is from the light sources, how large those sources, the distance between each individual light and, also, the distance of the whole thing from the subject. I'm sure a Brian Greene or a Roger Penrose could make a stab at the math but it is beyond me.
A Dino lamp of 24 individual bulbs appears as if a single source at 20' or so without the need for diffusion. If you have no access to such a lamp, I suggest you take a row of 6 x 40 watt bulbs set at 9" apart and try holding different diffusion in front of them. One other consideration is the surface the light is falling onto. If it is a complex surface, such as a face, the feeling of the 'multiple source' will be harder to spot than it would be on a smooth white wall.
Thank you for the response. It seems like the type of thing to best be handled on a case-by-case basis with actual testing. With that, I'll be off to the races, cheers and thanks again.
There's no hard "line" where you either have a single shadow or multiple shadows as you diffuse the multi-bank source more and more, it's really a matter of how much your particular shot shows the multiple shadows. For example, the African landscapes in "2001" shot on stage with front projection backgrounds created soft daylight from hundreds of bulbs in the ceiling -- but it is very hard to spot any multiple shadows, yet the bulbs were not diffused. But I suppose an insert of a hand hovering over flat smooth ground might make the shadow patterns more clear.
Also the distance of the multi-bank light source is a factor, in other words, the relative distance between each hard source from the perspective of the subject affects how obvious any multiple shadows are. You take a 24-light Dino undiffused but put it a city bock away, the light will mainly seem like a single source with a single shadow. But put that 24-light unit just five feet from the subject and the multiple shadows could be very obvious.
And when you say "multiple sources", it matters how close each source is to each other if you are trying to blend them.
That's a very good point about shot design. I was thinking theoretically when I should've been thinking practically. Speaking of practicality, I think I should get some tests going with the rigs I have. This topic's math seems like it could take much longer than just giving it a shot. Thank you for your insightful answer.