Well thanks for trying. Although I understand the exposure issue and
how I got where I am I'm not sure I'm really up to the task of dealing
with the aftermath of my exposure errors. ACR does a good job but I'm
beginning to understand that I'm still a novice driver. And how dumb am
I? I was just thinking today that my 5D (which I've owned for about 3
years) has a perfectly fine spot meter... which I've never used.
Chuck Norcutt
Andrew Gullen wrote:
> 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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