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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