I think that part of your answer is that there are point sources and
there are point sources. Relative to a bare bulb your soft box from a
meter away ain't much of a point source. But relative to the sky it's a
pretty sharp point source.
As to the rest no one has yet fully expounded on the Physics of Light
101 but I do think it's tied up in what I was calling point and extended
light sources. A perfectly reflecting mirror can redirect a collimated
beam of light without causing it to diverge in the way beams do from a
point light source. A bare wall is a less perfect mirror which tends to
reflect the light without causing it to diverge. Your subjects are even
less perfect mirrors. They probably cause some divergence but not
enough to get us to the inverse square law. Anyhow, that's what I think
and I'll happily await any different and better explanation.
Chuck Norcutt
Andrew Fildes wrote:
> I'm having trouble with this one as well.
> In the studio, if I use a soft box, it's hardly a 'point source' but
> its output is governed by the inverse square law (thank goodness, as
> i have a couple of old non-adjustable heads).
> Again, I have noticed that if i move the camera position well back
> and nothing else, from an evenly lit subject such as a person against
> a backdrop with a similar tone to skin, then there seems to be a drop
> in the light reaching the camera.
> This made sense to me - why should there be any difference in the
> behavior of light emitted and light reflected? - photons are photons.
> Never really bothered to check it - I just check the histogram every
> time I change a set up significantly - but it seems to make sense.
> But then of course, using an incident light meter makes nonsense of
> all that too.
> Bugger - I'm confused.
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 05/01/2009, at 5:19 AM, WayneS wrote:
>
>> The difference is not about the subject being the light source
>> but rather the subject is illuminated, hence its intensity is what it
>> is. Just because the photographer is further from the subject
>> does not change its illumination.
>
--
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