I think this refers to Henri Cartier Bresson's famous 'decisive moment' in photography. In his case, of course, it was a single exposure, and your artist is doing something a bit different, but the visual moment is significant for her as it was very famously for him. All students of photography will know of his work.

You might even want to translate 'bestimmt' with something approaching 'decisive'.

In 1952, Cartier-Bresson published his book Images à la sauvette, whose English edition was titled The Decisive Moment. It included a portfolio of 126 of his photos from the East and the West. The book's cover was drawn by Henri Matisse. For his 4,500-word philosophical preface, Cartier-Bresson took his keynote text from the 17th century Cardinal de Retz: "Il n'y a rien dans ce monde qui n'ait un moment decisif" ("There is nothing in this world that does not have a decisive moment"). Cartier-Bresson applied this to his photographic style. He said: " "Photographier: c'est dans un même instant et en une fraction de seconde reconnaître un fait et l'organisation rigoureuse de formes perçues visuellement qui expriment et signifient ce fait.
http://en.wikipedia.org/wiki/Henri_Cartier-Bresson#The_Decis...



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Note added at 1 hr (2009-04-07 16:52:59 GMT)
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Of all the masters of the camera who passed away this year, none was as influential or renowned as Henri Cartier-Bresson, among the towering figures of 20th-century photography.

By 1932, at age 24, Cartier-Bresson had begun to devise a whole new manner of shooting pictures. He displayed an intuitive knack for choosing "the decisive moment," as it came to be called, that instant when a shutter click can suspend an event within the eye and heart of the beholder, an exhilarating confluence of observer and observed. His lyrical, loose, ingeniously composed images were a revelation. Previously, most photographers had used clunky, stationary cameras. They were like Romantic poets who looked back at time, recording the melancholy of a moment's having passed. H.C.B.'s images, many plucked from the everyday whirl of his beloved Paris, had the power and poetry of Zen and particle physics--smashing the atom of the present, bottling its spark, and generating flashes of life and light.
http://www.digitaljournalist.org/issue0412/friend.html

[Take a look at this article for the most famous of these decisive moments - beautiful!]


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Note added at 1 day5 hrs (2009-04-08 20:51:36 GMT) Post-grading
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Thanks, mill, for the points and the kind comments!