Most light source or reflected light is divergence light source, parallel
light source is very rare except the guided laser beam. Any divergence light
source apply the inverse square law, the sky is an exception since the area
is just too big compare with the distance to the earth. Consider if you have
a 100" lightbox and the distance between the lightbox and object is just 2",
when you increase the distance to 4" the light intensity will be more or
less the same.
You are still confusing about light source and object, the truth is:
- the light intensity will drop only when increasing the distance between
light source and object.
There is no difference if you are looking at a reflected light (the object)
or the light source, the intensity on their surface does not drop with your
viewing distance. The size does vary but not the intensity, the intensity
will drop only when there is particles in between your view and the object.
C.H.Ling
----- Original Message -----
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
>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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