Richly detailed biography of movie pioneer D.W. Griffith; D.W. Griffith: An American Life, by Richard Schickel. New York: Simon & Schuster. 672 pp. $24.95.

Everyone agrees that D.W. Griffith was the first great genius of the movies. But not many geniuses have needed so many excuses and apologies to prop up their reputations.

His gift for cinema can't be questioned. From the moment he joined the film world in 1908, after failing to make his name on the stage, he found it natural to think in terms of imagery and editing. He jotted the scenario of his first picture under five headings: ''heart, interest, drama, danger, comedy, rescue'' - the movies in a nutshell! What remained was to tinker with the technology of the new medium - inventing its vocabulary, refining its possibilities, and wowing the public in the process.

Unfortunately, most of his talent and insight came to a screeching halt in other areas of experience. Though he was far more than a technician, Griffith was never quite an artist in the fullest sense of the word. He was too self-absorbed, too limited in his outlook. And his movies all suffered from this , as Richard Schickel's exhaustive biography shows.

Exhibit A is the first Griffith blockbuster, the 1915 ''Birth of a Nation.'' The family scenes are sweet; the Civil War episodes are thrilling; the filmmaking is revolutionary - but it all leads to an appalling climax, wherein the Ku Klux Klan delivers the helpless, downtrodden South from the clutches of stupid, lascivious blacks.

And if this weren't bad enough in itself, Griffith never got the point of the outraged protests (often led by the NAACP) against the picture. Years later he still ranted that ''Birth'' corrected ''unfair and prejudiced'' histories pushed by Northerners. Even in his withdrawn, grumbling old age, it never dawned on the Kentucky-born director that his critics might have been onto something. How strange for a man who prided himself on his artistic sensibility and knack for capturing wide audiences.

Nor was the ''Birth'' debacle an isolated event. Griffith's life was full of odd gaps between thought and behavior, idea and achievement. Though he blazed the trails of a new art, his own cultural life was backward-looking, stuck on Victorian attitudes and sentiments. Though he idealized women, his two marriages ended unhappily, and his romances revolved obsessively around teen-age girls.

He saw filmmaking as the delicate ''science of photographing thought,'' yet coached performers into such twitchy, exaggerated acting that an early review complained of his ''bounding'' heroines. Realism was one of his strong points, yet, as Schickel notes, it wasn't ''poetic'' or ''spectacular'' enough to suit his ambition. He was a cool professional, yet he spent years vainly trying to make a superstar of Carol Dempster, partly because he identified with her looks. ''You don't know what it is to have a big nose,'' the great director whined to Lillian Gish.

A seasoned author and Time magazine film critic, Schickel explores such conundrums with intelligence and sympathy. Along the way he chronicles the early history of American cinema, from one-reelers (Griffith directed a new picture every two days for the Biograph studio) through talkies.

He also links Griffith's story with a critical evaluation of his films. This is a tricky job, since modern appraisals of these flawed masterpieces are fogged by everything from the passage of time to a general apathy about silent movies among younger critics. Schickel's thoughtful views are especially useful since (as he notes) some ''received opinions'' now current can be traced to newspaper reviews published decades ago and never questioned since.

To most moviegoers who stumble on them today, Griffith's works look funny, like quaint museum pieces. Yet many contain images, gestures, feats of bravura filmmaking, and moments of quiet humanity that still have power to charm and touch. And their influence has been profound, affecting everyone from a European filmmaker like Werner Herzog - who told me Griffith was ''the Shakespeare of the cinema'' - to an American pundit like Pauline Kael, who once pronounced the wildly uneven ''Intolerance'' to be ''the greatest movie ever made.''

In his richly detailed though sometimes carelessly written book, Schickel puts the movies and their maker into fresh perspective, without sensationalizing or touching up either subject. It's a major contribution to film history and criticism alike.