[MF3D.FORUM:1506] Re: flash distances...

[Tom Hubin <thubin@xxxxxxxxx>]

David W. Kesner wrote:
> 
> Bill Glickman writes:
> 
> > I would have to strongly challenge this David.  I would like to
> > know how you came up with this....I am very curious...
> 
> Two ways - first I was told this by experienced photographers,
> but more importantly it has proved itself in my own personal
> field trials. Of course in my case we are talking about
> distance in the hundreds of feet. I have shot "big room"
> images where the camera was set up at one end of a room and
> multiple flashes were set up from 10 feet to one hundred feet
> away from the camera. As long as the flash to subject was
> within the desired range for the fstop of the camera the
> exposure was right on. The flash at 10 feet from the camera
> was also 10 feet from the subject and the flash that was 100
> feet from the camera was also 10 feet from the subject and
> both subjects were exposed correctly.
> 
> This might not hold true for very great distances, but I have
> not tried that. I am not arguing with you at all, and I defer
> to your expertise. I am just an amatuer and have no schooling
> in photography or lighting or any of the theory behind it. All
> my information comes from what I have been told (which may or
> may not be correct) and what I have done myself.
> 
> David W. Kesner
> Boise, Idaho, USA
> drdave@xxxxxxxxxx

Hello David and Bill,

Actually, this makes perfect sense. The exposure on the film depends
only on the light leaving the subject (toward the camera), the fStop of
the imaging lens, and the exposure time. It does not depend on the
camera to subject distance or the imaging focal length.

If you double the distance to the camera then you collect one fourth of
the light. But you also image your subject over one fourth the area. The
two effects cancel each other so that the light per square mm on the
film is constant for any camera to subject distance.

If you double the focal length and keep the same fStop then you collect
four times the light. But you also image over four times the area. So
again, no change in film exposure.

So distance and focal length don't matter. But fStop and exposure time
do matter. And of course, the amount of light radiated from the source.

Tom Hubin
thubin@xxxxxxxxx

If you