At 01:34 2/15/02, Joe Gwinn wrote:
As the object moves away from the lens, less light from the object falls
on the lens, but the resulting image is smaller, and the effects exactly
cancel.
The specific effects:
Brightness is inversely proportional to the square of the distance (e.g. 3X
as far is 1/9th the brightness). Area from which light is gathered is
proportional to the square of the distance (e.g. 3X as far is 9X the area
from which light is gathered).
I wrote a special strobe guide number and aperture calculator for use wth
any film speed, with up to ten strobes of different guide number ratings,
placed at different distances from the subject. It requires the GN rating
for each strobe (at ISO 100). Works in feet or meters, provided the rated
GN units match the strobe to subject distance units (i.e. if the GN is ISO
100 in meters, then distances must be entered in meters; if GN is ISO 100
in feet, the distances must be entered in feet).
http://johnlind.tripod.com/science/StrobeCalc/
Wrote it in Quattro Pro and exported it to Excel also. The Excel version
may require some reformatting of cells, etc. I don't have Excel here to
test it with. If your browser tries to open the file into a browser
window, then right click on it and save the file.
-- John
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