From: Carl Weese (cweese@earthlink.net)
Date: 04/27/00-03:48:20 PM Z
At risk of slipping off topic, there are several clear examples of this
lack of inverse-square falloff familiar to all commercial photographers.
A small electronic flash unit used at distances of several meters shows
quite perfect inverse square falloff--that's why guide numbers actually
work quite well, if you can stand the look of direct flash. In a small
room with lots of reflection, the 'source' begins to include the ceiling
and walls and falloff begins to be less than inverse-square would predict.
However, broad-source lights ('softboxes', 'lightbanks', etc) are
generally used very close to subjects (portraits, still-life) and often
the surface of the lightbox is bigger than the subject, used at a
distance that is smaller than the width of the box. Within this range,
the box can be moved over a wide array of positions including doubling
or halving of nominal distance from the subject, with enormous variation
of esthetic effect--modeling of subject shape, play of light and open
shade on the surfaces--with almost no effect on exposure even with
critical chrome film.
The proximity of a softbox to still life subject is nothing compared to
the tubes-to-print distances we're discussing here with UV sources. I've
just begun using a large industrial vacuum frame which places the print
just about twice as far from the tubes as a simple frame which fits into
the 'pizza oven' box design, but my exposures have lengthened by only
25-30%, which I attribute mainly to the much thicker glass on the
monster vac frame. Moving from 2" to 4" distance from a UV source over
2' square doesn't have a major effect on exposure times.
---Carl
-- Website with online galleries and workshop information at: http://home.earthlink.net/~cweese/
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