“Memling’s Portraits,” a modest grouping of paintings at the Frick Collection, is a charming, stellar show. Each of the show’s 20 paintings – which together represent roughly two-thirds of Hans Memling’s total output as a portraitist – is small. (I surmised, covetingly, that most could fit easily in a backpack.) Yet each work is also solid, clear, unique, and jewel-like. Collectively, like facets of a diamond, they transform our understanding of Memling.

The perfectly executed show also transforms the museum’s subterranean galleries (which, low-ceilinged and devoid of natural light, can sometimes feel cavernous or cramped) into a hallowed space – like that of a sanctuary or a cathedral crypt. This is not a minor feat, considering that only a handful of the works on view are of religious subjects; most are portraits of prosperous bankers, merchants, burghers, and burgesses. It helps that Memling is one of the greatest-ever portrait painters – creating psychologically complex works that rank up there with those of Rembrandt, El Greco, and Velazquez. It helps more that Memling’s achievement was elevating the genre of portraiture to that of devotional painting.

Memling is often considered to be a second-tier Netherlandish painter, beneath such predecessors as Rogier van der Weyden and Jan van Eyck. “Memling’s Portraits” should change that misconception.

German-born Hans Memling (c. 1440-94) studied with Rogier van der Weyden in Brussels and settled in Bruges in 1465 – following in the footsteps of the successful van Eyck and Petrus Christus. He was the most sought-after artist of his day in that trading center, receiving regular commissions for religious works, altarpieces, diptychs, and triptychs, most with portraits of donors (a practice which was fashionable during the 15th century).

On view at the Frick are two works that depict the Virgin with a donor, including “The Virgin and Child with St. Anthony Abbot and a Donor” (1472), a masterful painting of an interior in which the standing St. Anthony appears to enclose or give birth to the kneeling donor and the nearby, though separated, standing Virgin and cradled Christ child clearly occupy a space of their own. The elastic space magically allows the holy pair to hover forward in the painting – simultaneously adjacent to, in front of, and just behind the saint and donor; it also allows them, especially the child, to be protectively enclosed under a red velvet canopy that hangs far away on the distant rear wall of the room.

The majority of the show’s works are straight-ahead portraits. Memling was one of the most influential artists of his day, and his portraits played a key role in the running dialogue between Netherlandish and Italian art. His devout style – with the pensive sitter, light, and landscape – was picked up by artists as diverse as Piero, Ghirlandaio, Raphael, Leonardo, and Bellini. (Memling’s portraits and their maniera devota were particularly prized in Florence, Urbino, and Venice.)

The 15th-century Netherlandish painters were renowned for their depictions of the landscape. Think of the Limbourg Brothers’ spectacular calendar “Les tres riches heures du Duc de Berry” (1413-16), in which so much attention is given to the smallest of elements and acts in the landscape; or, a century later, of the tremendous naturalism in the landscapes of Bruegel. After the invention of oil paint, generally attributed to the van Eyck brothers, atmospheric light could be achieved through the application of glazes.

One of Memling’s chief accomplishments, in combining landscape with portraiture (both considered inferior genres at the time), was to focus painters’ attention on the luminous effects of atmospheric light. Memling softened and humanized his subjects (as well as the hard-edged and chiseled, northern artistic temperament), imbuing them with a calm naturalism and specificity that transformed them from people into individuals. This combined format led directly to Leonardo’s innovations with naturalism and light, evident in the National Gallery’s “Ginevra de’ Benci” (c. 1474) and the “Mona Lisa” (1503-05), the most famous of portraits – both women sit before a landscape.

Memling’s portraits at first feel very similar. The figures, in seven-eighths or three-quarters view, are solemn, humble, and seemingly devout. With their wide eyes and large heads, they even convey a childlike innocence. Each figure, cropped at the chest and shoulders, is firmly wedged into the front plane and bottom edge of the painting, as if he or she had been hammered down and forward into place like a cork in a bottle. Their heads seem to be cramped by the top edge of the canvas, as in “Portrait of a Man in a Red Hat” (c. 1465-70) and “Portrait of an Elderly Couple (Woman)” (c. 1470-75). Sometimes the heads are abruptly severed by their conical or pillbox hats, as in “Portrait of a Young Man” (c. 1480 or later). Or their chests contain the imposing frontality of a Mondrian rectangle of black, white, or red, as in “Portrait of a Young Woman (‘Sibyl’)” (1480).

The figures’ closely cropped hands, prominent and chest-high, feel disembodied. In the paintings “Portrait of a Man With an Arrow” (c. 1475-80) and “Portrait of a Man With a Spotted Fur Collar” (c. 1475), there is no room for the fig ures’ arms or elbows. It is as if the hands belong to someone else; or as if the hands had been awkwardly pressed, like botanical specimens, into the nonexistent space between the torso and front plane of the canvas.

Similarly mesmerizing are the distinctive range and synthesis of the details in Memling’s portraits – which always serve the portrait as a whole. Not only does Memling convey the qualities of velvet, fur, flesh, foliage, sky, and all kinds of hair – curly, thick, feathery, wispy – he maintains their specificity while elevating them to the iconic: Fur, sky, hair, and velvet are realized to the point of standing in for all fur, all sky, hair, or velvet.

Memling also fused outward naturalism with interior personality. The landscapes, which hang artificially behind their sitters like pull-down screens in a photographer’s studio, are invented. Out of scale with the figures, the countryside views hover like thought balloons; yet they convey light and air, and they open easily into the distance. The landscapes, which are unique to each portrait, feel like extensions of the sitters. Shoulders, heads, hats, and collars become spatial arabesques that merge with streams, hills, and roadways. Tufts of hair billow into foliage and puffs of clouds. Veils pass like mist. Eyes, focused inward, drop back and pierce the sky – hovering like birds; scaling hilltops.

Memling, in introducing the landscape into portraiture, did not merely provide serene or bucolic settings for his sitters. He extended, as he revealed, his subjects’ psychology. In merging genres, Memling transformed the proverbial window on the world into that of a window into man. He paints the landscape of the mind.

New York has a choice grouping of Memlings (the Frick, Metropolitan Museum, and Morgan Library combined own eight masterpieces). But an artist’s full gifts are difficult to ascertain piecemeal. The Frick exhibition allows for side-by-side comparison, which shows Memling’s range, humility, and inventiveness, as well as his ability to convey within a fairly narrow format the rich and various complexities of numerous individuals.

“Memling’s Portraits” is a gleaming precious gem of a show. For the first time in its history, the Frick has hung banners on Fifth Avenue – and I cannot imagine a more worthy exhibition.

Until December 31 (1 E. 70th Street between Fifth and Madison Avenues, 212-288-0700).