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
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|