Who says portraits for pay have to be bad?

PROFESSIONAL portrait painters are generally viewed as second-class citizens by the art world at large. It's perfectly all right for an Andy Warhol or Alice Neel to paint portraits. They, after all, are ''expressing themselves'' through this kind of work. But to do it for a living, to paint portraits to a client's specifications, strikes most people in the art world as unworthy of the high calling that is art.

I've never understood why. Some of the world's greatest paintings were commissioned portraits. Titian, Velazquez, Holbein, Rembrandt, Hals, Goya, to name only a few, did portraits to suit their clients. Some (El Greco and Rubens) may have been more individualistic than others, and others still may have seen portraiture as a lesser aspect of their art. But all were very much aware that unless they did what their patrons or clients wanted, there would be no money for food and clothing.

These artists had one great advantage, however. They and their patrons and clients shared a common artistic tradition. A beginning artist didn't have to shop around among dozens of different styles for a suitable one. Nor, failing that, did he or she feel obligated to invent a new one.

Art was a common language shared by artist and patron alike. The latter knew precisely what he or she wanted, and the former provided it to order. What resulted always bore the artist's personal stamp, of course, but it nevertheless shared a common ''family resemblance'' with everything else painted at that time and in that place.

All that changed, however, with our recent, more romantic notion that the artist is a unique and very special individual superior to society as a whole. And that his or her art must articulate and ''express'' this uniqueness - or find itself denied the status of true art.

Seen this way, anyone's attempt to modify or redirect an artist's near-sacred ''individuality'' must result in something other than art - a hack illustration, perhaps, or an academic portrait, but certainly not ''real'' art.

With this in mind, it is easy to see why commissioned work has such a low standing in art today. (Although, oddly enough, it doesn't in architecture.) And why professional portraiture in particular is seen as a contaminated art form at best, and the lowest form of expedient commercialism at worst.

Professional portrait painters see things differently. While most will probably agree that some of their fellow professionals are hacks grinding out one characterless likeness after another, they will also insist that the best of their peers are concerned artists whose intention is to probe and to reveal character as well as to produce ''speaking'' likenesses.

Having seen a large number of contemporary portraits recently, I would have to agree. If one sets aside the obvious hacks (and they existed in the time of Holbein and Hals as well as today), one is left with a number of professional portraitists who are good painters in the simplest and best sense of that term. They draw well and imaginatively, handle brush and paint with flair, sensitivity , and conviction, and do their best to paint pictures that are at least as interesting as paintings as they are successful as portraits.

Among them is an artist who only recently decided to become a professional portrait painter, but who has already produced a number of excellent commissioned protraits, incuding several of presidents and deans of colleges, law schools, and universities.

Sarah Swenson sees the relationship between artist and client as challenging and satisfying, and not at all damaging to her art. In fact, as she writes: ''The freedom of style in painting today has some advantages but it is not without its drawbacks. Deciding what to paint can be a real drain on one's creative energies. In portraiture, particularly in the commissioned portrait, much is given at the outset, yet, strange as it may seem, these constraints provide a structure which can in fact free one's painterly powers to operate more effectively.''

She also likes the businesslike nature of portrait commissions and the fact that she doesn't have to depend upon art dealers to market her work. Once contacted by a client, and after discussions about how and where her subject is to be painted, Swenson decides upon the work's composition, color scheme, and overall treatment. Preliminary sketches are made, and several black-and-white photographs are taken as reference material. Once that is set, work on the canvas begins.

Her style is lean, discreet, and precise. Although she admires the painterly flair of John Singer Sargent, her own portraits are less fluid and more dependent upon pure drawing than his. Her ability to identify with her subjects is remarkable - with the result that they come across in their portraits as highly individual human beings.

Although most of her portraits are of adults, she does occasionally paint children. ''Lucas and Laura Congdon,'' for instance, was commissioned by the children's grandmother when they were 41/2 and 21/2 respectively. Since Lucas enjoys playing the violin, it was decided to show him holding it - a decision Swenson used to good compositional advantage.

Another decision was to make this an indoor rather than an outdoor portrait as originally planned, and to use a large chair as the connecting device between brother and sister.

The result is a beautifully painted double portrait of children that is neither sweet nor sentimental. Young as they are, these children are real people. Careful study will also reveal how sensitively this picture has been composed, and how exquisitely every line, tone, and form interrelates with every other nuance or detail, and with the composition as a whole.