"F for Fake," now at the K-B Janus 1, purports to be a roguishly deceptive dissertion on the theme of artistic charlatanism and fakery by Orson Welles, who now seems to cast himself as a venerably prince of fakers in exile.

Such a pose may be psychologically necessary to Welles at this stage of his career, which resembles a magnificent ruin, but diminishes the genuine originality of his best work and reflects not so much the facts and about himself or artists in general as the defense mechanisms that evolve out of failure. It's apparent that Welles perceives himself as a has-been when he repeats such worn self-deprecating jokes as the following: "I started at the top and have been working my way down ever since."

Welles has been fidding with/agonizing over a number of unfinished movie projects for the past several years. "F for Fake," which made its debut at the 1975 New York Film Festival, is the first project he has finished in almost a decade. A considerable chunk of footage incorporated into this 70-minute micellany originated with another filmmaker, French documentarian Francois Reichenbach, who appears in "F for Fake" as Welles crony and collaborator.

Reichenbach has shot TV documentaries about many celebrities, Welles included. In the early '70s he shot extensive interviews with the late art forger Elmyr de Hory and his biographer, Clifford Irving, on the island of Ibiza, and I recall seeing a great deal of material now collected in "F for Fake" on a Tv documentary perhaps a year or so before Irving became famous.

Having used the footage for conventional documentary purposes. Reichenbach evidently lent it to Welles to doodle with after Irving's Howard Hughes caper became known.

The result is a curios, unsatisfactory pastiche of documentary tidbits acquired from Reichenbach and speculative filler supplied by Welles himself, who appears prowling around in his Felliniesque hat and cape, performing a couple of magic tricks and mostly pontificating about himself, Hughes, Irving, de Hory and the nature of art and illusion in the editing room or a the dinner table.

Unfortunately, time has enhanced both the documentary and human interest of the original interview material while tarnishing Welles' sometimes dextorous but essentially fatuous interpolations and commentary.

For example, it's amusing to step into the time machine and hear Irving describe de Hory as "a modern folk hero acting out larcenous impulses the rest of us repress." However, Welles goes on to romanticize de Hory's career as an art forger to degree that would probably appall Irving, who finds the idea of fooling "expers" appealing - but never onfuses a confidence game with a benediciton.

De Hory recently committed suicide, apparently because he feared the possibility of extradition to France and another prison term. It was dreadfully naive of Welles to swallow the carefree and even arrogant personality Reichenbach encountered on Ibiza a few years ago. It's especially embarrasing to hear Welles proclaim. "What will Elmyr do now that he's out of prison? Give another party!"

There are cautionary notes in Irvinng's remarks about de Hory that might have kept Welles from going overboard, even in 1972, but perhaps they came too close to his own defenses to be heeded. For example, when Irving says, "He had developed a fiction about his life and to tear down that fiction would destroy his life," the statement could apply to the Welles we see in "F for Fake."

Welles' speculation don't clarify the difference between authenticity and fakery, between originality and imitation. What's worse, they don't even obscure the difference in an entertaining or revealing way. In his desperation to prove something he probably doesn't believe in Welles falls back on garrulity and false humility. Aided by his protege, Oja Kodar, he pretends to fox us with a shaggy dog art forgery story. Waxing sanctimonious, he repeats the old Ingmar Bergman chestnut about standing inawe of Chartres, "the premiere work of man's genius and it's without a signature." This form the American filmmaker with perhaps the most identifiable signature of them all! Oh, well. Welles' reputation is secure, even against the onslaught of his own misapprehensions and self-deceptions.