As Anne Baldassari has pointed out in her essay on Picasso, Painting in the Mirror of the Photograph, the dark mirror, as used to describe the source of an artist's inspiration, was more than a literary metaphor. Artists from the Renaissance into the nineteenth century routinely kept a mirror made of dark glass at hand in their studios.  Looking into it reduced the chromatic scale of the scene to be painted.  As the dark glass stripped a scene to its essentials of  light and shadow, just so the black & white photograph reduced its subject to a similar succession of monochromatic values. 

It was William Henry Fox Talbot's appreciation of this property of tonal reduction that led him – while viewing an image through a camera lucida – to intuit the essential property of photography.  He wrote in the preface to The Pencil of Nature that "the picture, divested of the ideas which accompany it, and considered only in its ultimate nature, is but a succession or variety of stronger lights thrown upon one part of the paper, and of deeper shadows upon another."  Once Talbot had grasped that the essence of photography lay in the accurate reproduction of ever deepening shades of grey, he was able to attempt a "scientific" solution to the recording of an image through the use of nineteenth century chemistry.  In this, Talbot had an incredible amount of assistance from Herschel, considered the greatest British scientist of the nineteenth century.  It was Herschel, in fact, who invented the cyanotype.

Perhaps the best way to describe the difference between the "straight" f64 school of photography and the pictorialist is to say that the first demands an artist be true to the medium while the second requires he be true to his own vision.

Pictorialism originated as a late nineteenth century phenomenon whose goal was to elevate the status of photography to that of art.  Camera Works reproduced many pictorialist masterpieces in photogravure.  Later, the movement fell out of fashion as Stieglitz, Strand and Weston embraced straight photography as the only truly acceptable form for the medium. Nevertheless, in its heyday, pictorialism produced a number of powerful "artistic" images.  Though some were heavily manipulated,  others were absolutely purist in their use of platinum and cyanotype.  Influenced by Impressionism and japonisme and characterized by soft focus, the best succeeded in creating a dreamlike perspective.  It's only been in recent years that the works of William Mortensen, Clarence White and Karl Struss have begun to receive long overdue recognition.