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+sour
>> ce%22+light+intensity&source=web&ots=LIVAzSfvOh&sig=v8i03Qz7Eg4N1g2_lE9
>> 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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