From: Art Chakalis (achakali@gcfn.org)
Date: 12/12/01-09:36:57 AM Z
Daren,
You are correct, the inverse square law applies to all light sources (all
photon's are created equally). But the application of the law to a light
box is not as simple at it might first seem. In fact, you cannot use the
formula for a light box at the distances we use them as you must consider
each tube as a separate light source.
In a light box we are working with a bank of light tubes. Some light
striking a negative comes from tubes that are very close and yet some
light is coming from a number of tubes that are far away; either directly
at sharp angles to the negative or by way of bouncing off a reflector.
Using the 4" versus 2" distance example, the inverse square law predicts a
4x energy change factor. But what about the light that was already
traveling 10" and is now traveling 8", its factor is only 1.56x. We need
to add its effect to the nearest tube. But what about the light that was
already traveling . . . . . and don't forget this tube over here . . .
Though the light that has traveled the longer distance is weaker, it's
incremental reduction in energy is less than the light that traveled the
shortest path. If you were to calculate the energy change from moving all
of the tubes in the light box I am certain the numbers would agree with
the real world. Think about it . . . when the box gets far enough away
the distance change from any one tube begins to approach the same number
for all of the tubes which is why the formula can be used (its close
enough for friends) once the box is at a long distance away.
No laws of physics have been violated here, at least in this time and
space . . . to bad as it could have been pretty exciting.
Sincerely, Art
Art Chakalis
Columbus, Ohio, USA
On Tue, 11 Dec 2001, Daren wrote:
> Sandy,
>
> If what you have just stated is true then when shooting with my softbox
> the inverse square law shouldn't apply?? That's absurd!! And what about
> flourecent banks used for lighting?? Are these not effected by inverse
> square law??? Something is more than a bit fishy here.
>
> Light falls off at a regular rate regardless of relative distance and
> size of light source to subject (point source vs. soft source). The
> idea that a soft or broad source will create light in any way different
> than a point source is wrong. Light is light, we obviously have
> different wave legnths in the elctromagnetic spectrum, but a photon is
> . . .
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