I accept what you're saying but it is not an explanation in a physics sense.
Chuck Norcutt
C.H.Ling wrote:
> 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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>
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