The modern use of previsualization has come to mean imposing a particular look on a subject that was determined without much regard to the subject itself. You make the subject look like the previsualization. It's the new, cost-efficient soul of commercial photography and motion pictures. It is arguably a good idea to know what you're going to do BEFORE the $100,000/day shooting session begins. Image makers attempting to preserve unplanned and therefore expensive spontaneity are not finding work. I think Ridley Scott was ruined by the heavy handed instrusion of previz into the story line of "Prometheus" versus "Bladerunner" where previz was mostly limited to the effects scenes.
Ansel knew what his print was going to look like from the moment he first saw the scene, which was his instant of privisualizaion. The look was selected from the small but dramatic vocabulary of what was possible technically, given the conditions of light and subject. What Ansel made out of a scene was often quite different than what the scene looked like to the eye. Deepen the shadows, texturize the clouds, and so on. He always knew where he was going to go with it, which was usually towards a bigger-than-life interpretation. There were few surprises or deviations in post. So in Ansel's case previz was liberating in the strange way that well defined constraints are liberating. He knew there were maybe 2 or 3 interpretations that would glorify the subject to his liking and he had the technical mojo to head straight to them, without wallowing around in keeping the creative possibilities open for later.
One thing's for sure...you can get away with a lot more processing hoopla in b&w than in color, before people start whispering "....Photoshop..." behind your back. Have you noticed? It's not fair. Sorry, I wander.