“I really thought what I would do, if I ever learned to draw properly, was I would try to change the world,” the British artist Ralph Steadman explains in “For No Good Reason,” Charlie Paul’s documentary about him, opening Friday.

So in 1970 Mr. Steadman, his first marriage imploding, jetted to New York, where he planned to take a thousand photographs that would inspire the illustrations he couldn’t seem to get quite right.

As he explains in the film, speaking of his early work: “There was an arrogance missing, there was a wildness missing, there was a rawness missing. It lacked that bite I needed, that real ferocious bite, the thing that would make it noticeable.”

Then he received instructions to meet Hunter S. Thompson, not long after his infiltration of the Hells Angels.