In the mid-1960s, frustrated by the limitations of color printing technology, Bullock returned to making black-and-white photographs, continuing to expand his vision to create innovative images that reflected his philosophical nature. Differentiating what he termed "reality", the visible and the known, from "existence", the underlying truth of things, he was ceaseless in his attempts to expand his own faculties of perception and understanding so he could come ever closer in his experiences to the essence of things. Finding the means to more fully evoke that essence was also a key part of his quest. Although he included several different alternative processes (extremely long time exposures, multiple images, up-side-down and negative printing) in his repertoire of techniques, each was always used in the service of symbolizing new ways of relating to and knowing the world. He wrote,