Artists copy, or draw, paint, and sculpt from other artists’ work, so they can learn their craft. You can learn a lot about an artist through what he chooses to copy. And you can learn even more about him by comparing the copy to the original — an act that can help you to gain insight into both artists simultaneously. That act of comparison is particularly telling in “Lucian Freud: The Painter’s Etchings,” an exhibition of 68 etchings, 21 paintings, and five drawings that opens Sunday at the Museum of Modern Art.

The age-old tradition of copying — making the same moves through the composition and, in turn, unearthing first-hand the artwork’s spatial relationships, forms, rhythms, structure, and metaphors (the story of the work) — is the most direct and essential way to gain understanding not only about a particular artwork but about the tradition of art.

Included in the MoMA show, which was organized by Starr Figura, are two copies by Mr. Freud — a print and an oil painting — of Jean-Baptiste-Siméon Chardin’s “The Young Schoolmistress” (1740), a picture in which a schoolmistress and a child, engrossed in the lesson, lean over a book on a cabinet. In his copies Mr. Freud has zeroed in on the lesson book and on the faces and hands of teacher and student: He has cropped Chardin’s painting substantially. Missing from Mr. Freud’s etching “After Chardin” (2000) are much of the schoolmistress’s body, the cabinet’s face and doors, and much of the space surrounding the figures. In Mr. Freud’s act of focusing in on what matters to himself, he has denied everything that matters to Chardin. In the end, Mr. Freud’s copies are merely exercises in manual dexterity that have almost nothing to do with the original. Not only are Chardin’s metaphors regarding teaching, learning, and the bridge to adulthood from childhood absent in Mr. Freud’s translation, but also seemingly lost, or ignored, are what can be learned through the act of copying itself. In cutting himself off from the lessons of Chardin, Mr. Freud becomes a figure painter cut off from the tradition of figure painting.

The British painter Lucian Freud, who was born in Berlin in 1922, is the grandson of Sigmund Freud, a fact that has led some critics to suggest that in his paintings lingers a deep psychological insight. A reserved Expressionist, Mr. Freud has been called the greatest living realist painter, and he is something of a patriarch among figurative artists. He is especially popular in the camp of painters who believe that when it comes to portraying the nude, traditional notions about beauty in art (seen in today’s climate as sexist and false) must be disregarded in the name of a warts-and-all realism — as if this pursuit leads to a higher moral truth.

The fact that the world is ugly is not a surprise. And art — in the pursuit of truth — must deal with ugliness. But art — in the pursuit of art — must transform ugliness through distortion into beauty. The subject of Goya’s “Saturn Devouring One of His Children” is indeed ugly; but the painting progresses naturally, and is beautiful and sublime. Forced exaggerations and distortions — as seen in the work of artists such as John Currin, Cecily Brown, Jenny Saville, and Mr. Freud (no less than the forced beauty and lyricism in the paintings of an artist such as Lisa Yuskavage) — ring false rather than true. Distortion and ugliness in art, if they are not transformed into truthfulness and the sublime, come across not only as a bitter pill for an audience to swallow but also as artistic incompetence.

Mr. Freud, compared to many of his contemporary followers, is talented as a figurative artist; and this show, which intersperses paintings, drawings, and prints and is installed thematically and more or less chronologically, makes clear both his strengths and weaknesses.

The show opens with early works from the 1940s, including the paintings “Woman With a Daffodil” (1945) and “Man With a Thistle (Self-Portrait)” (1946). The paintings, figures set behind tilted-up tabletops, recall early Netherlandish portraits. The self-portrait’s hair is inventive and comically sprout- and sea-like. And the etchings “Chelsea Bun” (1946) and “Rose” (1948) are naturally felt and alive. But by 1948, in the painting “Girl With Leaves,” Mr. Freud has become more of an illustrator than a painter — a lethal shift in approach that has stayed with him to this day.

What I mean here by the distinction between illustrations and painting is that Mr. Freud early on began to pursue, and to be seduced by, marks and effects in his work, rather than building form. And if his work is any indication, he usually cannot tell the difference between the two. Swirling lines and heavy crosshatchings in his prints and the crusty buildup of lumpy paint in his oils suggest a hardwon eccentricity — a workman ethic and an expressionist angst that are immediate and physically present. His most common subjects are distorted, misshapen, seemingly scarred portrait heads with pig noses, and “Naked Portraits” — obese female nudes and naked, sprawling, spread-eagled men lying with dogs or abandoned on couches as if on sinking ships. The pictures suggest that the artist was getting at the heart of existential matters. His marks, however, no matter how agitated or thick, often merely scratch the surface or actually collapse the forms they mean to build or describe — an hypocrisy that is increasingly felt the more impastoed and disturbed the marks.

The show, which is not a traditional retrospective, jumps from the late 1940s to 1972. Mr. Freud gave up etching for nearly 35 years, taking it up again in 1982, and the exhibition, whose focus is the relationship between the artist’s prints and paintings, picks up again with 1970s paintings of the artist’s mother — a subject he also pursued through etching a decade later.

By the 1970s, Mr. Freud had become fully committed to his distorted figures and heavily built-up and encrusted surfaces. And MoMA’s show is filled with portraits, etchings, and paintings, which bear this out. Occasionally the artist achieves solidity and form, but that form usually has to float in a sea of formlessness. This is especially evident in the etchings, in which the white of the paper rarely reads as volume.

But there are moments of clarity. When Mr. Freud is best — most natural and straightforward — his lens is focused on nature or on himself. Most of his works have little light and air (Mr. Freud, a tonal painter, relies considerably on white, brown, and black — colors that tend to constipate his surfaces). The recent etchings “Garden in Winter” (1997–99) and “Painter’s Garden” (2003–04), however, begin to open and to breathe. And the two works titled “Self-Portrait: Reflection,” one a dark, brooding etching from 1996, and the other an oil from 2002, suggest that the artist was beginning to look at the world not through a lens of superficial distortion but through one of honesty and humility.

December 16 through March 10 (53 W. 53rd St., between Fifth and Sixth avenues, 212-708-9400).