British artist David Hockney has a theory. In his book, “Secret Knowledge,” he writes that artists can be divided into two camps, the “lens-based tradition” and the “eyeballers”: those who traced their works, and those who did not.

He lays out a sweeping, startling case that Caravaggio and Vermeer traced images from a camera obscura; that Ingres’ draftsmanship, long the pride of France, was not just his own; and that these greats’ descendants are modern-day photo tracers like Andy Warhol and Chuck Close.

Hockney’s theory has led to a flurry of media coverage, from the The New York Times to Newsweek, most of it insinuating that the masters are “cheaters” and “copycats.” But the opinion of most historians and critics, even those who disagree with the theory, is that using projected images to paint is simply part of the trade.

Thomas Eakins, the 19th century American artist whose work is widely admired for its realism, is among those artists who used projected images to help his draftsmanship. His tracing was furtive — the practice was frowned on at the time — and there was little direct evidence that he did so.

But a major retrospective of Eakins’ work in Philadelphia places photographs he took, which surfaced only recently, next to paintings with surprisingly identical figures or backgrounds.

Yale has four Eakins paintings hanging in its galleries, and many more in storage.

But rather than changing opinions about Eakins, the revelations may simply deepen historians’ knowledge of how painters work.

“If you look at Eakins’ paintings, he is using certain elements of the painting to play off other elements,” said Jules Prown, an art history professor emeritus. “[Photography] is a tool used to play off other elements.”

Eakins’ use of photography is, in some ways, not surprising.

“Eakins was one of [America’s] key artists, but also a wonderful photographer,” said Jock Reynolds, the director of the Yale University Art Gallery. “This is an artist who really explored.”

Eakins worked in almost every media available to him — including sculpture — and once made detailed, photographic studies of horses’ motion. Projecting photographic images on the canvas, Reynolds said, was simply another way for Eakins to explore his subjects.

“Now is a time when people see influences working back and forth between [photography and painting],” Reynolds said.

Even photography, Reynolds said, is not purely objective.

“A photograph isn’t reality: there’s focus, framing, light,” he said.

For all the uproar about the importance of Hockney’s theory, and what it might suggest about the talent and validity of some old masters, there is a sense that the question is largely academic.

“Eakins used the underpinnings of photography to transform an original photographic image into an [artistic statement],” Reynolds said.