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How to Control a Large Light Source

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#21 David Mullen ASC

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Posted 09 December 2015 - 08:44 PM

Here's a crude drawing of swapping out the large diffusion frame behind the gap between two flags with a smaller diffusion frame:

soft_flag.jpg

As you can see the subject from their perspective can see more of the large diffusion frame behind the flags and therefore is receiving light from more directions, thus getting a softer light on their face.


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#22 Gregg MacPherson

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Posted 09 December 2015 - 08:51 PM

I think "easier" refers to downscaling everything.  But,  no reasonable person would automatically remove the flags,  so that line of thinking is not so interesting.

 

As you say,  there is a small band of diffussion screen hidden by the flags that contributes something to the subject.  For her face the band is like 8'' wide at the top and 6'' wide at the bottom.  So,  if one was fussy and thinking three dimensionally,  just size the smaller diffussion frame to that.  Or consider the value of that contribution and perhaps ignore it.

 

The fun in a teacup diversion was about whether one could get the same look with a smaller diffusion frame and smaller source.

 

Sorry to Matt if he's dissapointed in that spinoff.


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#23 David Mullen ASC

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Posted 09 December 2015 - 08:56 PM

I don't see why just making everything a little bit smaller is less work, it's the same number of stands, flags, and diffusion frames.  You could make it significantly smaller to make it easier but then it's not the same effect.  To get the same softness you'd have to move everything closer to the subject, and then you'd have a greater fall off in intensity with distance from the source.


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#24 Gregg MacPherson

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Posted 09 December 2015 - 09:15 PM

Our last post crossed in the ether. Your flags in the drawing are a long way away from the diffusion frame, which illustrates your idea very well.

But in Matt's simulation, the top and bottom flags are a bit over 2' from the diffusion screen. Left flag front edge similar? OK the right hand flag front edge is 5 or 6'.

So your drawing illustrates a principal, but not how that principal is applicable to this set, setup. Actualy it's missleading. As I said before, the hidden band on the big diffusion screen at the top and bottom is quite narrow.

Looks to me like the smaller frame would be about half the size so the lamp would be about 1/4 the size. That's not "a bit" smaller.
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#25 Robin R Probyn

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Posted 09 December 2015 - 10:18 PM

But no getting away from the physics .. if its all smaller.. maybe even you could get a short gaffer..  you will have to move it all closer to the subject to get the same softness of light.. which effects your wide shot.. and how much the subject can move around.. so its not the same setup..


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#26 Gregg MacPherson

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Posted 09 December 2015 - 11:10 PM

Robin,

We are talking about the effectively used part of that large screen,  and the wattage that lands on that.  Looking around the border of that effective area,  about half of that large diffusion screen is not being used.

 

Here's a quick drawing that has approximately the proportions of Matt's simulated set/setup.  Lamp on the left,  "girls" face on the right. You can see the parts of the diffusion screen that are not being effectively used.  As a notion,  throw them away.  Or,  better,  leave all alone,  change nothing,  except swap out the source lamp for something about 1/4 the watts,  filling the approxamately matted effective diffusion area.

 

Hopefully that's clear.  I'm hoping that Matt,  who I think probably understands the physics well,  or is at least able to simulate it,  will drop back into his own thread.

 

big diff screen w flags.GIF


Edited by Gregg MacPherson, 09 December 2015 - 11:16 PM.

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#27 David Mullen ASC

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Posted 09 December 2015 - 11:31 PM

I agree that in this case, the flags are a bit too close to the diffusion frame -- on a real set, you'd back the diffusion frame up or the flags out a bit. But the basic principle here is that large diffusion frames need large flags.  I wasn't trying to guess the actual measurements and sizes of everything, I was speaking more about the lesson you are supposed to learn here.


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#28 Gregg MacPherson

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Posted 10 December 2015 - 06:27 AM

I agree that in this case, the flags are a bit too close to the diffusion frame -- on a real set, you'd back the diffusion frame up or the flags out a bit. But the basic principle here is that large diffusion frames need large flags.  I wasn't trying to guess the actual measurements and sizes of everything, I was speaking more about the lesson you are supposed to learn here.

 

I never proposed that the flags were too close.  I wasn't that interested in that here.

Just to be clear,  which lesson,  and given by who?


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