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+source%22+light+intensity&source=web&ots=LIVAzSfvOh&sig=v8i03Qz7Eg4N1g2_lE9XiJG_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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