A few years ago, James Gurney, a celebrated artist and author, stood before his easel to paint a deli in Poughkeepsie. Surveying the scene before him, he was immediately overwhelmed with literally millions of details. People strolled by. Insects fluttered overhead. Signs poked out from the store and up from the street. Every tree had about 200,000 leaves. “How am I going get all this down onto a 9 by 12 panel in a matter of hours?” he wondered, despite having confronted this conundrum countless times before.

The task was a visual one: translate the three dimensional scene that your eyes and your brain compile into a flat picture that makes you—and other viewers—re-experience something similar. Your eyes play tricks on you. Your brain plays tricks on you. As an artist, Gurney knew, you have to trick them back—or what you produce won’t look anything like what you thought you saw.

Although all good visual artists need to understand perception, Gurney has taken the study more seriously than most. He writes a blog, Gurney Journey, that includes numerous posts on vision and how it works. (He has also illustrated articles for Scientific American. For a video showing his process for painting realistic dinosaur scenes, see Illustrating 'Dinosaur Death Trap.') “Painting is a record of one person’s subjective visual experience of the world,” Gurney told me. Artists, he explained, want to convey that experience. But doing so often means, in a very real sense, not trusting their own eyes.

When Pink Is Bluish Gray

One of the ways our eyes can’t be trusted concerns color. Our visual systems manipulate tones and hues in nonobvious ways. In what scientists call color constancy, we perceive an object’s hue as constant no matter if it is brightly lit, cast in shadow, curved or illuminated by colored light. So a school bus looks yellow even in shadow or dented in a way that locally changes its shade. In the case of a sunlit man in black holding a white paper cast in shadow, we see his suit as black and the paper as white no matter how objectively bright or dark these objects are. Beneath our awareness, we adjust for the lighting.

But such subconscious adjustments may lead an artist astray, if he or she is unaware of them. In the case of the man with the paper, a beginner is likely to paint as his conscious brain sees, so that the coat is rendered in very dark tones and the paper quite white. An artist who knows about color constancy, however, will know that “white in shadow can very often be darker than black in sunlight,” Gurney says. To see this for yourself, look at the checkerboard. The squares marked 1 and 2 are actually the same tone. Our eyes adjust for the shadow such that we see the square that we think is illuminated (1) as much darker than the one that seems to be in shadow (2). As this illusion demonstrates, this adjustment is almost impossible to override. “Colors don’t exist as an objective reality,” Gurney says. “Color is something that happens in the brain.”

Painters need to be aware of this subjectivity, he says, because it makes a difference in how they paint. Here’s another striking example. Let’s say an artist wants to paint a pale-skinned figure standing outside at dusk or in a flame-lit interior. Looking at the figure, his skin would appear pinkish, because we know Caucasian skin to be pink. But cast in a bluish light, it isn’t pink at all, Gurney says. It is bluish gray. As a result, a painter must mix grays and blues in that environment to make the skin appear pink. To see how dramatically our eyes adjust for a color cast in a picture, see the colored cubes above. Although the square denoted in the red picture looks blue or (or to some, green or “cyan”) and the one in the green picture, red, Gurney rendered both using exactly the same color mixture. Here, a skilled artist uses an identical mix of paints in the two contexts to produce what appear to be two very different colors. You need to know how color context affects perception, says Gurney, “to get the color you want in a subjective color environment.”

The Benefits of Blur

Our eyes and brain also continually give us a false sense of focus. When we look out at a scene, we are constructing a sharp image of just a small part of it—the spot at or near the center of our gaze. The cells that detect light here, called cones, are responsible for all high-resolution vision. By contrast, objects on the periphery appear fuzzy because the light-detectors there, called rods, are not tuned to visual details. Nevertheless, as our gaze naturally shifts, our brains combine the various points of focus to construct a detailed mental picture of a large swath of visual territory.

Using this mental picture as a guide, novice artists tend to draw or paint an entire canvas in high definition. But because not everything is in focus when we actually see, softening the edges in the background of a painting actually makes it seem more real, says Gurney. It gives the image greater depth. That sense of reality and immediacy is Gurney’s goal. “I want to better understand how my eyes and brain work so that I can create a record of my observations,” he says.

In deciding what to paint with precision, Gurney also takes into account what people like to look at. Gurney has always wondered where viewers spent the most time looking in his pictures. He brought several of his paintings to Greg Edwards, who is the founder of Eyetools, an eye tracking company in San Francisco. Edwards tracked the eye movements of 15 viewers as they looked at Gurney’s art to discover which parts their eyes landed on most. Above, one of Gurney’s paintings is accompanied by the same painting overlaid with an eye-tracking heatmap. The red color denotes the areas where almost 100 percent of people gazed, followed by decreasing percentages in the orange and yellow. The blue and darker colors mark where hardly anyone looked. The results jibe with the notion that people are drawn to faces—and to human figures, in general. Knowing what captures people's interest helps tell Gurney what to spend the most time rendering in fine detail. Such eye-tracking work justifies the extraordinary attention some artists pay to painting faces. Some famous artists such as the American portrait painter John Singer Sargent reportedly repainted visages repeatedly to fix, say, the lighting, expression, or direction of gaze.

Underlying the rationale for eye tracking is the notion that attention is central to seeing. You don’t see what you don’t focus on, perceptually and conceptually. When Gurney sat down to paint that deli, he could not paint everything in front of him. He had to choose what to paint with precision and what to simplify, to reflect what he felt best represented the scene and what stood out to him. But despite seeing too much, he also saw too little: after an hour and a half of absorbing the sights, he suddenly noticed some telephone wires. It was if they had just suddenly appeared in the scene. “But I didn’t paint them in,” he recalls. “The scene was busy enough, and it was time for lunch.”