I must have misunderstood and thought you were still looking for
clarification of the exposure issue. Sorry.
Andrew
On Jan 5, 2009, at 20:34, Chuck Norcutt wrote:
> Andrew, I'm not sure you have followed all of this thread. We were
> well
> beyond the point of my trying to photograph people on stage and working
> up some silly hypothesis about distance and the inverse square law. I
> was clearly in error there and realized it within the first couple of
> responses to my initial query. What I was trying to understand beyond
> that point, however, was the physics of why an extended light source
> does not behave the same as a point source. Andrew F. was intrigued by
> the same question. But somewhere in the back of my brain I dredged up
> some incomplete memory of extended vs point light source so did a
> Google
> search and found a perfectly fine reference book.
>
> The reference book I pointed out (and which you obviously read as well)
> gave what I believe to be a perfectly adequate explanation, a very
> simple diagram and was perfectly understandable without any references
> to integrals. No cameras or lenses are required in the explanation
> because we see it with our eyes and light meters. I still have no idea
> why you consider the lens and camera here to be important to the
> answer.
> The physical laws governing extended light sources have presumably
> been in operation since the origin of the universe when no cameras were
> around to observe and record it. As far as I'm concerned I adequately
> answered my own question. You seem to be continuing with trying to
> answer something that I don't think I asked. I'm still trying to
> understand what that is.
>
> Chuck Norcutt
>
>
> Andrew Gullen wrote:
>> Sorry if the previous attempt didn't hit the mark. Permit me to try
>> again.
>>
>> First: When we're talking about general illumination of something
>> (e.g.
>> a grey card), an extended light source, when you're close, does *not*
>> behave like a point source. This is because as you pull away from the
>> surface you start "seeing" more of the source at a higher angle, so as
>> you pull away the spread of the light is compensated for by bigger
>> contributions from more of the source. For an infinite plane, if you
>> do
>> the integral then as with an electric field you should see *no falloff
>> at all*. This will be true whether the plane is emitting or reflecting
>> light (assuming it reflects equally at all angles).
>>
>> But: People on stage are too far away for such effects to be
>> significant - essentially we already see the whole surface. And anyway
>> we're not interested in how their reflected light is illuminating
>> something - we're going to focus that light into an image.
>>
>> The reason a lens is relevant is that it forms an image; it ideally
>> takes all the light that falls on its front element from a point on
>> the
>> subject and directs it to a single point in the image, and does so for
>> all points that are in the field of view and in focus (let's assume
>> everything is in focus for the moment). That means a pixel (or
>> whatever) is not illuminated by the whole extended source, as it would
>> be without the lens - it's illuminated by all the light that was
>> reflected from a very small area of the subject and hit the front
>> element (I wish I could draw here).
>>
>> Now: You're absolutely right that the light from the object is
>> spreading out and therefore falls off with the inverse square law.
>> What
>> happens if you double your distance to the subject, for example? The
>> amount of light hitting the front element drops fourfold. But the
>> image
>> has shrunk to half dimensions, and the pixel is now getting light from
>> an area on the subject that is twice as high and twice as wide, thus
>> four times the area. That increases the amount of light fourfold,
>> exactly compensating for the falloff.
>>
>> How'd I do that time?
>>
>> Finally, you're right about the experiment. Perhaps I misread - I
>> wasn't sure you'd accepted the conclusion, so I was trying to
>> establish
>> that camera-subject distance does not affect exposure.
>>
>> HTH,
>> Andrew
>>
>> On Jan 5, 2009, at 12:55, Chuck Norcutt wrote:
>>
>>> I'm afraid I don't understand you and Ken bringing the action of the
>>> lens into this discussion as I don't see the relevance. This started
>>> as
>>> trying to understand why an extended light source didn't behave as a
>>> point source and lose light according to the inverse square law. I
>>> thought that the link I had posted adequately explained the physics
>>> of
>>> that without reference to cameras or lenses. After all, the lens can
>>> only work with what's impinging on its surface and has no part in how
>>> or
>>> in what intensity the light arrives. Then, what happens between
>>> front
>>> element and film/sensor is immaterial since it's the same regardless
>>> of
>>> the light source or light intensity.
>>>
>>> Finally, I don't see the relevance of the experiment you propose.
>>> All
>>> it can do is show what we all know to be true but does nothing to
>>> elucidate why that is so. But maybe if the dimensions of that card
>>> are
>>> only 5% or less of the distance between card and camera we'd see that
>>> it
>>> begins to behave as a point source. But that info came from my own
>>> reference.
>>>
>>> What am I missing?
>>>
>>> Chuck Norcutt
>>>
>>>
>>> Andrew Gullen wrote:
>>>> Sorry, I should have addressed that too.
>>>>
>>>> You are correct that line and plane sources have different falloff
>>>> of
>>>> illumination, like electric fields - but only when you are close
>>>> enough
>>>> that this makes a difference. See page 61 of this reference, where
>>>> it
>>>> says:
>>>>
>>>> However, as a practical matter, whenever the longest dimension
>>>> of
>>>> the surface
>>>> of an emitting source is less than 1/20 of the distance from
>>>> which
>>>> the light is
>>>> being measured, it is usually acceptable to treat it as a point
>>>> source.
>>>>
>>>> But anyway, this is relevant only when considering gross
>>>> illumination
>>>> -
>>>> as when you light a reflector to illuminate a subject, or use a
>>>> softbox, and you're only concerned with *how much light in total* is
>>>> falling on an area. It's not relevant when you focus an image of
>>>> something, because in that case the contributions from each little
>>>> area
>>>> are not summed but fall on different parts of the film/sensor. As
>>>> Ken
>>>> just said. Extended light sources are a red herring in this
>>>> discussion.
>>>>
>>>> But words are cheap - try an experiment!
>>>> - Use a camera where you can lock ISO, focal length, aperture,
>>>> shutter
>>>> speed and white balance.
>>>> (An OM-1 with film and a fixed lens would be good. :-) )
>>>> - Set up a small lit object in an otherwise dark space, e.g. a
>>>> card
>>>> lit with a flashlight (torch)
>>>> - Determine a correct exposure by incident metering, spot
>>>> metering,
>>>> or
>>>> trial and error.
>>>> - Take a sequence of shots ranging from close to far.
>>>> - In all shots, though the object's size will vary it will be
>>>> properly
>>>> exposed.
>>>> (I'm assuming you'll actually use a digital camera. Don't use
>>>> color
>>>> print film as your photofinisher
>>>> will adjust and invalidate everything. Slide would be OK.)
>>>>
>>>> You can also see this in everyday shooting, though. We don't change
>>>> exposure when varying distance to the subject (except for macro,
>>>> which
>>>> is another topic). Sunny 16, for example, holds no matter how far
>>>> you
>>>> are.
>>>>
>>>> Manual exposure would be excruciating if this were not so - you'd
>>>> have
>>>> to adjust every time you changed distance.
>>>>
>>>> It does take some time to get one's head around this - I remember.
>>>>
>>>> Andrew
>>>>
>>>> On Jan 5, 2009, at 9:30, Chuck Norcutt wrote:
>>>>
>>>>> The memory is weak but not wrong. I knew it had something to do
>>>>> with
>>>>> point vs. extended light sources. Read pages 60 and 62 of:
>>>>> Perception of the Visual Environment By Ronald G. Boothe and note
>>>>> the
>>>>> distinction between "intensity" (point source) and "luminance"
>>>>> (extended
>>>>> source) Page 63 goes on to discuss luminance from reflection.
>>>>> <http://books.google.com/books?
>>>>> id=rCBuW7u6qhsC&pg=PA60&lpg=PA60&dq=%22point+source%22+%22extended+
>>>>> so
>>>>> ur
>>>>> ce%22+light+intensity&source=web&ots=LIVAzSfvOh&sig=v8i03Qz7Eg4N1g2
>>>>> _l
>>>>> E9
>>>>> XiJG_Wd0&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPA60,M1>
>>>>>
>>>>> Chuck Norcutt
>>>>>
>>>>>
>>>>> Andrew Gullen wrote:
>>>>>> Hi -
>>>>>>
>>>>>> Ian has the right answer here.
>>>>>>
>>>>>> There is no difference between "source" light and reflected light.
>>>>>> The
>>>>>> reflected light from a person on stage that falls on a given area
>>>>>> (like
>>>>>> the front element of your lens, or your cornea) does indeed fall
>>>>>> off
>>>>>> with the square of the distance. But the area of the formed image
>>>>>> also
>>>>>> goes down with the square, so everything balances out.
>>>>>>
>>>>>> Note that if you double your distance (and cut the light
>>>>>> fourfold),
>>>>>> but
>>>>>> go for a lens with twice the focal length to keep the image size
>>>>>> the
>>>>>> same, you need to double the diameter of the front element (I'm
>>>>>> approximating a bit here) and thus quadruple the area of the front
>>>>>> element, in order to gather enough light to maintain the
>>>>>> illumination
>>>>>> of the film/sensor. But that's just keeping the same f-stop (focal
>>>>>> length divided by diameter). It's lovely that the physics and math
>>>>>> of
>>>>>> optics make photography so simple, except when we stop to think
>>>>>> about
>>>>>> it. :-)
>>>>>>
>>>>>> Andrew
>>>>>>
>>>>>> On Jan 4, 2009, at 13:53, Ian Nichols wrote:
>>>>>>> Right answer, but I think your maths is a bit out - moving from 4
>>>>>>> feet
>>>>>>> to 8 feet, the image fills 25% of the viewfinder (it's an area,
>>>>>>> not a
>>>>>>> length) and the light from the subject has decreased by a factor
>>>>>>> of
>>>>>>> 4.
>>>>>>> So 1/4 of the light gets focused onto 1/4 of the area, hence
>>>>>>> same
>>>>>>> brightness
>>>>> --
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>>
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